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INTRODUCTION
The Pselaphinae is a globally distributed and very
diverse subfamily of the beetle family Staphylinidae,
comprising 9267 described species (A.F. Newton, pers.
comm.). They are small predators (0.5–5.5 mm in body
length) with a characteristic appearance. They possess
compound eyes with only a few ommatidia. The mouth-
parts are prognathous, often with prominent maxillary
palps and strong mandibles, indicative of a predatory life
style. Saprophagy is recorded from some species of the
genera Batrisodes,Claviger and Adranes, but not myco-
or phytophagy (Thayer, 2005).
Pselaphinae are most species-rich and diverse in the
tropics, but also abundant in temperate regions. They usu-
ally occur in moist forest leaf litter or debris, or moist
mosses at the margin of water bodies. Some pselaphines
display unusual biological adaptations (e.g., myrme-
cophily), which sometimes also involve interesting
behavioural features (especially remarkable in Claviger
testaceus: Cammaerts, 1991).
Research on pselaphines has mostly focused on their
systematics and geographical distribution, whereas
studies on their behaviour (De Marzo, 1985, 1986, 1988;
Engelmann, 1956; Poggi, 1990) and ecology (Reichle,
1967) are scarce. Furthermore, there are only a few gen-
eral morphological studies on this group (e.g., Chandler,
2001; Jeannel, 1950; Sabella et al., 1998; Thayer, 2005).
Most of the morphological data on Pselaphinae are gen-
eral taxonomic morphological descriptions, often with
notes on the conspicuous structures used for
identification.
The aim of this publication is to extend the knowledge
on the behaviour and morphology of Pselaphinae by
examining several Central European species. Detailed
information on the way that adult pselaphines catch and
handle their prey, and comparative descriptions of the
morphology of their sensory organs and mouthparts are
provided. It was examined whether different species-
specific strategies or structures are used in this process.
Although the assembled information is limited, we
attempt to establish correlations between certain aspects
Eur. J. Entomol. 105: 889–907, 2008
http://www.eje.cz/scripts/viewabstract.php?abstract=1411
ISSN 1210-5759 (print), 1802-8829 (online)
Predatory behaviour of some Central European pselaphine beetles
(Coleoptera: Staphylinidae: Pselaphinae) with descriptions of relevant
morphological features of their heads
ANDREA SCHOMANN1, KERSTIN AFFLERBACH2 and OLIVER BETZ3
1Natural History Museum of Denmark/University of Copenhagen, Zoological Museum, Universitetsparken 15, DK-2100
Copenhagen, Denmark; e-mail: aschomann@snm.ku.dk; andrea.schomann@web.de
2Eichenstraße 8, 56305 Puderbach, Germany; e-mail: kerstin.afflerbach@web.de
3Universität Tübingen, Zoologisches Institut, Abteilung für Evolutionsbiologie der Invertebraten, Auf der Morgenstelle 28E,
D-72076 Tübingen, Germany; e-mail: oliver.betz@uni-tuebingen.de
Key words. Staphylinidae, Pselaphinae, Brachygluta,Bryaxis,Bythinus,Pselaphus,Rybaxis,Tyrus, predatory behaviour,
prey-capture, morphology, head, antenna, mouthparts, maxillary palp, sensillum, sensory organ
Abstract. The Pselaphinae is a large subfamily of staphylinid beetles with a characteristic habitus and small body size. Detailed
morphological and behavioural studies on these beetles are scarce. In this study, specimens of Bryaxis puncticollis (Denny, 1825),
Bryaxis bulbifer (Reichenbach, 1816), Bythinus burrelli (Denny, 1825), Brachygluta fossulata (Reichenbach, 1816), Rybaxis longi-
cornis (Leach, 1817), Pselaphus heisei (Herbst, 1792) and Tyrus mucronatus (Panzer, 1803), all collected in Northern Germany,
have been examined with regard to their sensory organs (eyes and antennae), mouthparts and method of capturing prey. Scanning
electron microscope studies revealed sex-specific differences in the numbers of ommatidia in Bryaxis puncticollis. A multitude of
different sensilla on the antennae and great differences in the shape of the mouthparts were observed and peculiarities of the
antennae and maxillary palps (e.g., the segment-like appendage) were examined using scanning and transmission electron micros-
copy. The prey-capture behaviour of these species is described in detail for the first time based on laboratory experiments using Het-
eromurus nitidus (Templeton, 1835) (Collembola) as prey. This behaviour seems to be tribe specific, ranging from simple seizure
with the mandibles (e.g., Rybaxis longicornis, tribe Brachyglutini) to the employment of raptorial legs (Tyrus mucronatus, tribe Tyr-
ini). The two Bryaxis species (tribe Bythinini) even employ their apparently sticky maxillary palps to capture prey. The assumption
that a viscous secretion is used by these species is supported by the finding of glandular structures in the interior of their maxillary
palps. Prey-capture is preceded by a complicated preparation phase in most of the species and followed by a sequence of prey-
handling movements that seem to be adapted to restrain prey such as Collembola. In simple prey-choice experiments the beetles of
several species preferred small prey, irrespective of their own body size. In these experiments, Bryaxis bulbifer and Brachygluta fos-
sulata were more successful in capturing prey than Bryaxis puncticollis and Pselaphus heisei. This might be related to their different
sensory equipment and different methods of capturing prey.
889
of behaviour and particular morphological traits. The
observations and conclusions are discussed in the context
of earlier observations made by Engelmann (1956).
MATERIAL AND METHODS
Adult male and female beetles were collected in wet habitats
mostly around the city of Kiel (Schleswig-Holstein, Northern
Germany) by searching the ground litter at the margins of water
bodies using a Reitter’s sieve or “sifter”.
The animals were transferred in small containers (4.5 cm in
diameter) with a floor of moist plaster of Paris, and kept at
14–20°C in an outdoor shelter. The insects were maintained on
a diet of small collembolans [Heteromurus nitidus (Templeton,
1835)]. The photoperiodic conditions were that of natural day
length.
The specimens examined (in all 144) belong to 7 species in 6
genera, representing four tribes in two supertribes (Table 1).
The smallest were Bryaxis puncticollis,Bryaxis bulbifer and
Bythinus burrelli, all around 1.3 mm in length (from base of
labrum to abdominal tip). These have maxillary palps with a
voluminous fourth segment. Brachygluta fossulata (ca. 1.8 mm)
and Rybaxis longicornis* (ca. 2.0 mm) have less well developed
maxillary palps compared to the previously mentioned species.
Tyrus mucronatus, a large pselaphine beetle of about 2.3 mm,
which is found beneath bark of dead trees, has comparatively
small maxillary palps. Finally, Pselaphus heisei (1.8 mm long)
has extremely long maxillary palps with a slender and termi-
nally clubbed fourth segment. These species will be referred to
by abbreviations in the following text: Bp = Bryaxis
puncticollis,Bb = Bryaxis bulbifer,Bf = Brachygluta fossulata,
Rl = Rybaxis longicornis,Ph = Pselaphus heisei,Tm = Tyrus
mucronatus.
Prey-capture behaviour
Observations in the laboratory: The beetles were placed in a
cuvette or a small labyrinth of plaster of Paris and were pre-
sented with different-sized specimens of Heteromurus nitidus.
The behavioural responses were filmed at 25 frames/s with con-
ventional digital video cameras (Sony Digital Handycam, DCR-
TRV345E, and Sony Handycam, DCR-HC17E) fitted with
close-up lenses (hama Nah (Close-Up) +1, +2, and +4). Light
was provided by cold-light illuminators (Schott KL105B).
Actual data were obtained by single frame analysis of the video
recording.
Evaluation of the preferred prey size and the prey-capture
success: Beetles that had been starved for two days were placed
in small 5 by 5 cm containers with a base of moist plaster of
Paris under natural light conditions, along with defined numbers
and size classes of Heteromurus nitidus for two hours [presented
prey-size classes: (I) d 0.6 mm, (II) 0.6–0.9 mm, (III) 0.9–1.2
mm, and (IV) >1.2 mm]. To determine prey-capture success, the
absolute number of Collembola caught during these two hours
was recorded, and the proportion of the size classes in this catch
was used to determine the preferred prey size. In order to avoid
any external disturbances, this was conducted as a “blackbox”
experiment, i.e., captured Collembola were not replaced. How-
ever, since only in a small percentage of these experiments all
the Collembola of one size-class were captured, the preference
for a specific prey type could be clearly established.
Statistical analysis: If multiple data sets per specimen were
available, species-specific grand means were calculated, sum-
marizing the mean values for several individuals. All means and
standard deviations refer to these grand means. There was only
a single specimen of Tm available for behavioural observations
(marked as “one individual” in the tables), so that in this case
only the means for this specimen together with their standard
deviations are given.
All the statistical analyses were performed with SPSS 11.0
(SPSS Inc., Chicago), generally employing univariate analyses
of variance (ANOVA) followed by pairwise comparisons
employing Student t-tests with a Bonferroni correction.
Morphology
Sensory organs (i.e., the compound eyes and sensilla on the
antennae and maxillary palps) and the mouthparts were exam-
ined using scanning and transmission electron microscopy.
For scanning electron microscopy (SEM), the antennae, the
maxillary palps, and the mouthparts were removed from the
890
(+) maxillary
palps
–(+) number
of ommatidia
–+mucronatus
(Panzer, 1803)
Tyrus
Aubé, 1833
Tyrini
(Pselaphitae)
+++++heisei
Herbst, 1792
Pselaphus
Herbst, 1792
Pselaphini
(Pselaphitae)
+++–+longicornis
(Leach, 1817)
Rybaxis
Saulcy, 1876
+++++fossulata
(Reichenbach, 1816)
Brachygluta
Thomson, 1859
Brachyglutini
(Goniaceritae)
–(+) structural
peculiarities
–––burrelli
Denny, 1825
Bythinus
Leach, 1817
+++++bulbifer
(Reichenbach, 1816)
+++++
puncticollis
(Denny, 1825)
Bryaxis
Kugelann, 1794
Bythinini
(Goniaceritae)
Morphology
of mouthparts
General
morphology
of antennae
Morphology
of eyes
Prey-capture
success and prey
size preference
Prey-capture
behaviour
SpeciesGenusTribe
(Supertribe)
TABLE 1. Pselaphine species investigated in the present study (classification into tribes according to Löbl & Besuchet, 2004) (+ =
studied, – = not studied, (+) = partly studied).
* According to Besuchet (1989) R. laminata is a synonym of R. longicornis. However, a more recent publication (Hansen et al.,
1999) considers them to be two separate species; this is based on male genitalia. In the present study all the specimens were deter-
mined using Besuchet (1974, 1989), and therefore treated as R. longicornis.
head and attached at their base to SEM stubs by double-sided
sticky carbon tape. The objects were then left to dry in a desic-
cator. Afterwards, the stubs were sputter-coated with gold
(Bal-Tec Sputter Coater SCD 050) and examined in a LEO S
402 SEM.
In order to count the ommatidia and measure their diameters,
the heads of the beetles were cut in half longitudinally. Each
half was then attached to SEM stubs, so that the eye faced
upwards. Additionally, the corneae of some individuals were
detached, photographed under a light microscope and measured
with the help of the software TPSdig (Rohlf, 2004).
For transmission electron microscopy (TEM), individuals
were fixed in glutaraldehyde (2.5% solution in 0.1M cacodylate
buffer, pH 7.4) and osmium tetroxide (1% solution in buffer; 2
h), gradually dehydrated in isopropanol and propylene oxide,
and embedded in agar (Agar-100 Resin Kit, Plano). Serial semi-
thin and ultra-thin sections were cut with a diamond knife on a
Reichert Ultracut S. The semi-thin sections (0.5 µm) were then
stained with Richardson solution (Robinson et al., 1985),
whereas the ultra-thin sections (60 nm) were stained with lead
citrate (Reynolds) and uranyl acetate (Robinson et al., 1985),
and examined using a Philips TEM 208 S. Semi-thin sections
were examined using an axioscope (Zeiss) and digitally photo-
graphed (Zeiss Axiocam).
Body lengths were measured on dead or stunned individuals
by using a binocular and a stage micrometer (Wild, Heerbrugg,
Switzerland, No. 310345).
RESULTS
Behavioural observations
Prey-capture
The prey-capture behaviour consisted of several steps:
(i) searching behaviour, (ii) detection of prey, (iii)
approach towards the prey, (iv) prey-capture and finally
(v) prey-handling in which the prey is oriented for
feeding (species in Table 1). However, this pattern can
vary depending on the species, the specific situation and
motivation of the individual, with steps omitted or per-
formed in a different way. To facilitate the understanding
of the following descriptions pre-defined body angles
(Fig. 1) and postures (Figs 2–5) are given.
Searching behaviour (Fig. 2A):
The labyrinth was searched for prey using slight lateral
swinging movements of mainly the head (angle d in Fig.
1B; only slightly accompanied by the prothorax, angle f
in Fig. 1B) in Bb,Bp,Bf and Rl. Ph beetles did not move
their head or prothorax laterally relative to their abdomen.
Tm beetles moved the whole body axis laterally, plus
additional lateral head movements of about 10° to either
side. In Ph and Tm, the movements performed while
searching for prey were slower than in the other species
(Table 2).
Depending on the species, the antennae are spread at
different angles with respect to the median axis (angle e
in Fig. 1B). Usually, they are horizontally (angle e in Fig.
1B) and vertically (angle c in Fig. 1A) swung in a
pendulum-like manner (Ph moved them only slightly ver-
tically). On account of the slow and often irregular
antennal movement in the two Bryaxis,Ph and Tm
beetles, it was almost impossible to measure the frequen-
cies of movement. This was only possible in Bf and Rl,
which move their antennae much faster (up to 11Hz in Bf
and 12.5Hz in Rl, in the vertical direction) in a highly
regular manner (Table 2). For the Bryaxis species we can
only state that they move their antennae faster than Ph
and Tm.Rl and Bf also use their antennae for testing
irregularities in the surface of the ground.
891
Fig. 2. Predatory behaviour of Bryaxis puncticollis prior to the strike. (A) Search for prey. (B) Detection of prey. For further
explanation see text.
Fig. 1. Body angles referred to in the text and in the tables. (A) Lateral view: (a) body angle [= angle between longitudinal body
axis (i.e., the line from the abdominal tip to the elytral humerus) and the ground]; (b) head angle (= angle of the head with respect to
the longitudinal body axis); (c) antennal angle [= vertical angle (proximal part) of the antenna with respect to the ground]. (B) Dorsal
view: (d) head angle (= angle of the median axis of the head with respect to the median axis of the rest of the body); (e) antennal
angle [= horizontal angle of the antenna (proximal part) with respect to the median axis of the head]; (f) pronotum angle [= angle of
the median axis of the pronotum with respect to the median axis of the rest of the body – this angle is usually small; here 2° ( arrow)].
The head is bent downward at different angles relative
to the longitudinal body axis in a species-specific manner
(angle b in Fig. 1A; Table 2).
The maxillary palps are retracted during searching.
They may perform only slight movements and may occa-
sionally be protracted. Bp and Tm usually perform rapid
vibrating movements with the last palpomere during this
time. Specimens of Ph sporadically stop moving and test
the ground slowly with their maxillary palps.
All these movements result in different areas being
searched (“width of search path” in Table 2). In contrast
to the other species, specimens of Ph pursue a slightly
meandering path of differing widths.
Prey detection:
The tactile detection of prey takes place with only a
slight contact with the antennae (Fig. 2B; regularly
observed in Bp,Rl and Tm) or other parts of the body
(observed in Rl). Bb,Ph and Bf beetles change their
behaviour immediately prior to contact, so that they seem
to detect the prey by olfactory or vibrational cues, the
latter possibly transmitted via the substrate or the air.
Approach and preparation of predatory strike:
Approach: Bp,Bb, Bf and Tm visibly slow down when
approaching their prey. Tm even stops for a moment after
the first contact with the prey and moves closer to the
ground. In contrast Rl specimens rush forward with a
velocity of up to 8 body lengths per second and, if neces-
sary, pursue moving prey for 0.12 to 0.24 s (n = 2) at
about 5.5 body lengths per second. Rl and Tm may turn
towards the prey when it is located lateral to the body axis
of the beetle. The prey is generally located between the
antennae, with one or even both antennal tips often
pointing directly towards the prey. The antennae are now
moved more cautiously. Specimens of Bb,Bf and Tm
recurrently seem to touch the prey lightly with the tips of
their antennae, which might help them to adjust them-
selves better with respect to the position of the prey. The
maxillary palps are usually retracted in Tm, and slightly
or more strongly protracted in the other species.
Preparation for the predatory strike (for all species
except Tm whose behaviour will be described in the fol-
lowing section): The beetles perform an upwardly
directed movement with their fore bodies prior to the
strike (angle a in Fig 1A; Fig. 3A). In the two Bryaxis
species and Rl the body was moved upwards after having
approached the prey. The latter species occasionally made
this movement as it rapidly turned towards the prey.
Specimens of Bf raise their fore body (by about 10°) after
detecting prey, hold this posture as they approach the
prey and complete the movement while making the last
step. Ph always performs this upward movement while
approaching the prey, and thus needs an extra second for
preparation, which is about three to five times as long as
for the other species. In both Bryaxis species the hind legs
are often rapidly extended in order to perform a jump-like
forward movement. The different angles of both the body
and head are listed in Table 3. The head is brought into a
more prognathous position immediately prior to the strike
in all the species. The antennae are usually held diago-
nally forward and approximately parallel to the ground;
only Bp (Fig. 3A) and sometimes Rl hold their antennae
892
Fig. 4. Preparation for the predatory strike in Tyrus mucronatus. (A) General body posture during this behaviour. (B–C) Driving
of the prey toward the mandibles by means of the antennae. For further explanations see text.
Fig. 3. Prey-capture behaviour in selected Pselaphinae. (A) Upward movement of the body in Bryaxis puncticollis. (B–C) Actual
predatory strike [(B) Bryaxis puncticollis and (C) Brachygluta fossulata]. For further explanations see text.
perpendicularly up. As the beetles raise their body, the
maxillary palps are either pushed continuously forwards
or are, initially, only slowly extended and finally thrust
forwards (occurring in Bb and sometimes in Bp). At the
end of the upward body movement, the maxillary palps
are completely extended. In Rl and sometimes Bf, the
palpal extension might in some cases occur only after the
following downward movement of the body (predatory
strike). Observations on Bf reveal that the beetles open
their mandibles widely at the end of the upward body
movement.
Beetles of Tm perform the upward movement of their
fore body (angle a in Fig. 1A) prior to the strike while
approaching the prey, as do Ph beetles. However, subse-
quent to this approach, they have a unique way of pre-
paring for prey-capture (Fig. 4). Their prothoracic legs
are widely spread laterally, with the tibiae and femora
forming an angle of at least 90° (Fig. 4A). At the same
time, both the tibia and tarsus are extended in a straight
line towards the ground, which is touched only by the
claws or the last tarsomeres (if at all). Lastly, the palps
are slowly protruded and the mandibles widely opened. A
further approach or angular adjustments relative to the
prey might be conducted in this posture, and the beetle
leans forward (thereby reducing the body angle, angle a
in Fig. 1A; see Table 3). Ideally the prey is now located
directly in front of the predator. The antennae are then
brought together, each forming an angle of about 30°
with respect to the median line (angle e in Fig. 1B), and
are held nearly parallel to the ground. They are moved
continuously downwards and inwards, and the prey is fre-
quently touched by the tips of the antennae. This enables
893
5.41 ± 0.6 (n = 7)
maximum 6.4
< 5 (n = 1)
5.64 ± 0.3 (n = 3)
maximum 6.7
5.77 ± 0.3 (n = 3)
maximum 6.2
4.72 ± 0.4 (n = 3)
maximum 5.0
4.11 ± 0.5 (n = 3)
maximum 4.4
Width of search
path (individual
head widths)
(irregular)(irregular)
6.75 ± 1.5 (n = 6;
maximum 12.5)
6.35 ± 2.0 (n = 7;
maximum 11.1)
(irregular)(irregular)
Frequency of
antennal movement
(Hz)
38.5 ± 9 to
57 ± 13 (n = 10)
35 to 60 (n = 1)
30.9 ± 4 to
59.3 ± 1 (n = 3)
21.6 ± 8 to
64.2 ± 12 (n = 3)
36.3 ± 8 to
53.3 ± 6 (n = 3)
35.0 ± 13 to
61.6 ± 14 (n = 3)
Horizontal antennal
angle (range of the
movement) (°)
(angle e in Fig. 1B)
51.8 ± 5 (n = 11)38.8 ± 8 (n = 4)55.8 ± 3 (n = 6)49.6 ± 5 (n = 6)42.5 ± 2 (n = 7)46.6 ± 2 (n = 7)
Vertical head angle
(°) (angle b in Fig.
1A)
9.3 ± 6 (n = 8)0 ± 0 (n = 4)10.8 ± 3 (n = 3)11.3 ± 2 (n = 3)11 ± 4 (n = 3)8.3 ± 3 (n = 3)
Horizontal head
angle (°) (angle d
in Fig. 1B)
0.58 ± 0.3 (n = 15;
range: 0.3–1.1)
0.93 ± 0.5 (n = 4;
range: 0.4–1.9)
1.73 ± 0.4 (n = 6;
range: 1.0–3.6)
1.96 ± 0.6 (n = 7;
range: 1.2–3.7)
1.57 ± 0.6 (n = 7;
range: 1.0–3.6)
1.49 ± 0.7 (n = 7;
range: 0.5–2.5)
Searching velocity
(body lengths/s)
Tyrus mucronatus
(one individual)
Pselaphus heisei
Rybaxis
longicornis
Brachygluta
fossulata
Bryaxis bulbifer
Bryaxis
puncticollis
TABLE 2. Searching behaviour of Pselaphinae. Searching velocities, the head angles (mean angles of deflection of the head hori-
zontally and vertically, Fig. 1), the positions and movement of the antennae (average maximum and minimum angles with respect to
the median axis during movement, and frequency), and the resulting widths of the search path. The numbers represent grand means
[± standard deviations (SD)] and, in some cases, statistical ranges (labelled “range”). In Tyrus mucronatus (one individual), the sta-
tistics refer to repeated observations on one individual. “n” refers to the number of individuals, except in Tyrus mucronatus, where it
is the number of observations on one individual.
Fig. 5. Characteristic prey-handling behaviour illustrated by that of Bryaxis puncticollis. (A) Handling of the prey in an upright
position immediately subsequent to the strike. (B) Holding the prey sandwiched between the tibiae and the femora. (C) Final grasp
(note the upward orientation of the furca of the collembolan).
the beetle to locate the prey precisely and causes the prey
to move gradually towards its mouthparts, often exactly
below the widely opened mandibles (Figs 4B–C). If the
prey has, however, moved on and is no longer within the
reach of the beetle, the antennal inward and downward
movement is continued, until an angle of about 13° to the
median line (angle e in Fig. 1B) and a vertical angle of
–53° with respect to a horizontal line parallel to the
ground (angle c in Fig. 1A) is reached. Otherwise it stops
once the prey is immediately below the widely open man-
dibles. The first complete antennal inward movement
without contacting or driving the prey below the mandi-
bles takes about 2.2 ± 0.2 s (n = 3, measured on one indi-
vidual). The movement can then be repeated several times
(often over only a smaller part of the area), and the time
needed for these repetitions is very variable. If the prey is
already located below the mouthparts, the antennae are
not moved inwards.
In conclusion, the main difference between Tm and the
other species in the preparation for the strike is that Tm
beetles actively drive the prey towards their mandibles,
whereas the other beetles move towards the prey.
Predatory strike:
The predatory strike consists of a rapid downward
movement of the fore body (angle a in Fig. 1A) in most of
the species, and the prey is captured either with the man-
dibles, by means of the apparently sticky maxillary palps,
or, in the case of Tm, by the fore and middle legs. In addi-
tion to the downward movement, Bp,Bf and Ph beetles
bend their heads downwards with respect to the pronotum
(angle b in Fig. 1A). Table 3 lists the different periods of
time recorded for different species for this downward
movement. Specimens of all the species, with the excep-
tion of Tm, stretched out their maxillary palps and placed
them upon the dorsal surface of the prey (Figs 3B–C). In
the two Bryaxis species, this seems to be sufficient to
restrain the prey, although Bp beetles also sometimes
additionally use their mandibles. Ph has an additional
second phase of downward movement, during which they
lower their opened mandibles towards the prey, which is
894
0.055 ± 0.02 (n = 6)
(< 0.04 – 0.08)
0.633 ± 0.24 (n = 3)
(0.16 – 0.80)
0.108 ± 0.07 (n = 4)
(0.04 – 0.24)
0.110 ± 0.04 (n = 4)
(0.08 – 0.16)
0.070 ± 0.03 (n = 4)
(< 0.04 – 0.12)
0.047 ± 0.01 (n = 3)
(< 0.04 – 0.08)
Time for final
strike (s) (mean
± SD; range)
32.9 ± 11 (n = 7)23.0 ± 12 (n = 3)36.2 ± 8 (n = 3)35.0 ± 14 (n = 6)24.2 ± 19 (n = 3)20.8 ± 4 (n = 3)
Head angle (°)
(angle b in Fig.
1A)
Maximum:
30.0 ± 6 (n = 5);
shortly before strike:
10.7 ± 4 (n = 7)
17.5 ± 3 (n = 3)26.9 ± 1 (n = 3)19.3 ± 10 (n = 6)21.6 ± 7 (n = 3)26.0 (n = 2)
Body angle (°)
(angle a in Fig.
1A)
Tyrus mucronatus
(one individual)
Pselaphus
heisei
Rybaxis
longicornis
Brachygluta
fossulata
Bryaxis
bulbifer
Bryaxis
puncticollis
TABLE 3. Prey-capture behaviour of Pselaphinae. Body postures at the end of the upward movement prior to the strike (body angle a
and head angle b in Fig. 1A) and time needed for the subsequent downward movement during the strike (Figs 3 B–C). The numbers
usually represent grand means [± standard deviations (SD)] and statistical ranges (time only, in brackets). For Tyrus mucronatus (one
individual), the statistics presented refer to repeated observations on only one individual (the maximum value is the maximum angle,
as due to subsequent additional forward corrections of its body posture the previous angle is diminished shortly before the strike; for
further information see text). “n” refers to the number of individuals, except for Tyrus mucronatus, where it is the number of observa-
tions on one individual.
115 ± 9
(n = 3)
100
(n = 1)
(see above)
108 ± 18
(n = 5)
94 ± 12
(n = 3)
100
(n = 1)
Head angle (°) (while prey
is held sandwiched with the
forelegs; angle b in Fig. 1A)
6.22 ± 3.9
(n = 5)
1.04
(n = 1)
1.53 ± 0.6 (n = 3,
prey only turned,
not sandwiched)
1.66 ± 1.3
(n = 5)
4.32 ± 4.2
(n = 4)
4.08
(n = 1)
Duration for which the prey
is held sandwiched with the
forelegs (s)
(did not occur)
62.5
(n = 2)
50
(n = 1)
54.4 ± 12
(n = 4)
50.6 ± 12
(n = 4)
63.3 ± 8
(n = 3)
Body angle (°) (upright;
angle a in Fig. 1A)
(did not occur)
1.64
(n = 1)
1.0 (n = 1,
often skipped)
1.97 ± 1.3
(n = 5)
1.26 ± 0.4
(n = 3)
1.44 (n = 1)
Duration of upright posture
(s)
Tyrus mucronatus
(one individual)
Pselaphus
heisei
Rybaxis
longicornis
Brachygluta
fossulata
Bryaxis
bulbifer
Bryaxis
puncticollis
TABLE 4. Prey-handling behaviour of Pselaphinae. Average durations of prey-handling subsequent to the strike [in upright posture
(Fig. 5A) and while holding the prey sandwiched with the forelegs (Fig. 5B)], the corresponding body angles (in the upright
position), and the angles of the head while holding the prey (large angles achieved by simultaneously bending the pronotum down-
wards). The numbers represent grand means [± standard deviations (SD)]. In Tyrus mucronatus (one individual), the statistics refer
to repeated observations on one individual. “n” refers to the number of individuals, except in Tyrus mucronatus, where it is the
number of observations on one individual. For further explanations see text.
finally seized and pressed on the ground. The prey some-
times gets stuck to their maxillary palps and can, in this
way, be lifted towards the mandibles or escape is pre-
vented to a certain degree. Rl and Bf beetles also place
their maxillary palps on the dorsal surface of the prey
(Fig. 3C), even though there is no clear indication of a
sticky surface of the palps. These beetles push the prey
down and then seize it with their mandibles. In contrast to
Ph, beetles of these species perform the whole downward
movement in one quick action. The maxillary palps of Rl
are sometimes placed on either side of the prey, possibly
to hold it in place.
Tm again differs in its method of capturing prey. At the
beginning of the downward movement of the fore body,
the maxillary palps may be widely extended for a short
time. Thereafter, the pro- and mesothoracic tibiae and
tarsi are pulled rapidly inwards, and the beetle crouches
down. Similar to the predatory strike of a mantis, the prey
is seized between the tibia and the trochantero-femoral
spines and ridges of the fore legs and, to a lesser extent,
the middle legs. Lastly, the head is bent downwards and
the prey may also be seized by the mandibles.
In the case of an unsuccessful predatory strike, the two
Bryaxis species, Bf and Rl often search the ground near
their heads using their maxillary palps. They may even
bite the plaster with their mandibles. Ph beetles show a
special searching pattern, which resembles the behaviour
of Tm when preparing to strike. If after the downward
movement the maxillary palps or the mandibles do not
contact the prey, both the maxillary palps and the
895
Fig. 6. Preferred size of the collembolan prey (Heteromurus nitidus). Proportion of each of four prey sizes in the total amount of
prey caught (points = grand means). The error bars mark the standard error (SE). (Size classes: I = d0.6 mm, II = 0.6–0.9 mm, III =
0.9–1.2 mm, IV = >1.2 mm. Sample sizes: Bryaxis puncticollis n = 29, Bryaxis bulbifer n = 8, Brachygluta fossulata n = 18, Pse-
laphus heisei n = 9). Different small letters above the error bars indicate statistically significant interspecific differences (Student
t-test with Bonferroni correction; p < 0.05).
antennal tips are swept over the ground, distally to proxi-
mally. If this is not successful, it may be repeated several
times. Each cycle takes about 1.5 s. This behaviour may
locate nearby prey, often causing it to move towards the
predator. Finally, the maxillary palps are placed on the
dorsal surface of the prey, so that the second phase of the
downward movement (as described above) can take
place, ending in prey capture.
Prey handling and final seizure:
Immediately subsequent to the strike individuals of all
the species except Tm raise their fore bodies in order to
lift the prey off the ground (Fig. 5A). While doing this,
they manipulate the prey with their fore legs (tibiae and
tarsi), while the middle and hind legs ensure a firm
stance. The different periods of time spent in this position
are given in Table 4. The struggling prey may generate
abnormally steep angles of the beetle’s body with respect
to the ground (generally, a maximum angle of 70° is
observed, although in Bp it can be 105°; see Table 4 for
means). Sometimes the beetle is even knocked over. The
prey is held in the mandibles (Bf,Rl, and Ph) or stuck to
at least one maxillary palpus in Bryaxis spp. (sometimes
also in Ph). Even when the prey was seized using the
mandibles, the maxillary palps of Bf and Rl sometimes
remain in contact with the prey. The antennae are
extended anteriorly in Bb and Bf, and seem to form a
mechanical barrier preventing the escape of the prey.
Most species drew the prey towards their ventral side
and sandwiched it between both the tibiae and the femora
of the fore and often the middle legs (depending on prey
size), at the same time lowering the anterior part of their
body (Fig. 5B). They now take a posture similar to that
adopted by Tm beetles after their strike. All species,
including Tm, continue in approximately the same
manner. Further adjustments of the prey are often neces-
sary. To attain this the head is usually bent by about 100°
(Table 4) relative to the longitudinal body axis. Finally a
permanent grasp with the mandibles is achieved. The
grasp of the legs is then relaxed and the prey lifted up in
the mandibles (see Table 4 for the specific durations of
the period the prey is held with the legs).
Rl beetles often omit the first part of the prey-handling
behaviour sequence (i.e., the upright posture) and even
reduce the second part (prey held with the legs), just
turning but not grasping the prey with the forelegs. The
mandibular grasp is supported by pressing the prey to the
ground with the mandibles. In Bp passive prey might only
be manipulated using the maxillary palps and the fore
legs without sandwiching it. In this case the maxillary
palps are only moved away after achieving a firm hold
with the mandibles.
Finally, all the beetles usually hold the collembolan so
that its extended furca points upwards (Fig. 5C), irrespec-
tive of the initial orientation of the prey. This prevents the
prey from harming the predator and makes it more diffi-
cult for the prey to escape. Some beetles do not show a
specific prey-handling behaviour if the prey is exception-
ally passive (Bp) or small (Rl, see above), and as a result
the furca of the collembolan does not necessarily point
upwards.
896
not measured42 (n = 1)Tyrus mucronatus
0.0184 ± 0.005 (n = 20)24 ± 1 (n = 14)Pselaphus heisei
0.0331 ± 0.006 (n = 20)30 ± 2 (n = 11)Rybaxis longicornis
0.0121 ± 0.002 (n = 20)35 ± 2 (n = 11)Brachygluta fossulata
0.0089 ± 0.001 (n = 20)20 ± 3 (n = 8) Bryaxis bulbifer
0.0053 ± 0.002 (n = 20)14 ± 1 (n = 20)Bryaxis puncticollis (female)
0.0074 ± 0.001 (n = 20)23 ± 2 (n = 20)Bryaxis puncticollis (male)
Surface area of the cornea [mm²]Number of ommatidia per eyeSpecies
TABLE 5. Number of ommatidia per eye and the mean surface area of the cornea [mm²] of Pselaphinae. Means ± standard devia-
tions (sex-specific differences in the number of ommatidia were only observed in Bryaxis puncticollis). “n” refers to the number of
individuals in the column “number of ommatidia per eye” and to the number of ommatidia measured in the column “surface area of
the cornea”.
Fig. 7. Mean number of prey captured in two hours. The
points represent grand means of caught collembolans (total
numbers), ignoring prey size. The error bars mark the standard
error (SE). Significantly different groups are marked by dif-
ferent small letters (Student t-test with Bonferroni adjustment, p
= 0.01). Abbreviations: N = sample sizes, Bp = Bryaxis puncti-
collis, Bb = Bryaxis bulbifer, Ph = Pselaphus heisei, Bf = Bra-
chygluta fossulata.
In all species the maxillary palps are not involved in
feeding and are usually held laterally. Even during
feeding the beetles might react to other prey animals
(observed in Bp and Rl). In most cases the antennae, the
maxillary palps, and at least the tarsi of the fore legs are
cleaned after feeding. This might also include self-
grooming of other parts of the body.
Preferred prey size and prey-capture success
All species tested (both Bryaxis,Bf, and Ph; see Table
1) prefer the smallest and the second smallest (<0.9 mm)
of the prey offered (Fig. 6).
Bb and Bf were more successful predators than Bp and
Ph (Fig. 7) with respect to the prey species used. This is
again not related to the size of the predator.
Morphology
Mainly Bp,Bb,Bf, Rl, and Ph were used for the mor-
phological studies (Table 1). Tm and Bythinus burrelli
were only partially studied (eyes resp. antennae).
Sensory organs
As described earlier, pselaphine beetles seem to rely on
tactile and olfactory cues to find prey and orient them-
selves. The following section presents information on the
various sensory organs (eyes, antennae, maxillary palps)
likely to be important for prey-finding.
Eyes:
The compound eyes are composed of only a few omma-
tidia, which are relatively large in size, convex and distin-
guishable at low magnifications (10×) (Fig. 8A). The
range in numbers of ommatidia is small and species-
specific. The males and females of Bp have significantly
different numbers of ommatidia, with 23 in males and 14
in females (Table 5). Such intraspecific differences were
not found in any of the other species in this study.
Pselaphines have apposition ommatidia of the acone
type (Meyer-Rochow, 1999; Fig. 8B), i.e., without a dis-
tinct crystal cone. The corneal lenses (c in Fig. 8B) con-
sist of a multitude of separate layers, which are separated
from each other by unmodified cuticle (cu in Fig. 8B).
897
Fig. 9. (A) Representation of a pselaphine ommatidium, with the different cross sectional planes indicated as a–d. (B) The four
drawings and TEM-micrographs are of cross sections through the rhabdome of Pselaphus heisei at the levels indicated in (A). The
numbers (1–8) in the photographs refer to separate rhabdomeres. Bar = 2 µm.
Fig. 8. Eyes of Pselaphinae. (A) SEM-micrograph of a lateral view of the compound eye of Rybaxis longicornis. The large and
convex ommatidia are clearly visible. Bar = 10 µm. (B) TEM-micrograph of a single ommatidium of Bryaxis puncticollis (axial sec-
tion). Bar = 5 µm. Abbreviations: c = cornea, cu = cuticle, n = nucleus, om = ommatidium, pg = pigment granules, pp = primary pig-
ments, rh = rhabdome, sc = Semper cells.
Pigment granules occur beneath the cornea, appearing as
black spherical components that enclose the rhabdome
(pg in Fig. 8B). Particularly large granules in the apical
regions of the ommatidia (primary pigments, pp in Fig.
8B) occur in the Bryaxis species. Two to three nuclei of
Semper cells (sc in Fig. 8A–B) occur immediately below
the cornea.
The rhabdome is closed (i.e., the rhabdomeres are
fused), and consists of eight rhabdomeres (Fig. 9). A
wedge-shaped rhabdomere (labelled 2 in Fig. 9Ba) is pre-
sent in the distal part of the rhabdome, which extends in a
proximal direction and is surrounded by two other rhab-
domeres (labelled 1 and 3 in Fig. 9Bb). The basal part
contains only three rhabdomeres, two of which occur
along the entire length of the rhabdome (labelled 1 and 4
in Fig. 9Bd). The third rhabdomere (labelled 8 in Fig.
9Bd) does not extend far distally.
898
Fig. 11. Average length of the antennae as a percentage of the
body length [means ± standard deviation (SD)]. Abbreviations:
N = sample sizes, Bp = Bryaxis puncticollis, Bb = Bryaxis bul-
bifer, Bf = Brachygluta fossulata, Rl = Rybaxis longicornis, Ph
=Pselaphus heisei. Different small letters above the error bars
indicate statistically significant interspecific differences (Stu-
dent t-test with Bonferroni adjustment; p < 0.05).
Fig. 12. Representation of the sensilla complement on the
11th antennomere of (A) Bryaxis puncticollis, (B) Brachygluta
fossulata and (C) Pselaphus heisei based on the structural
analysis. Different symbols represent the different types of sen-
silla; their sizes and numbers reflect the relative sizes and the
proportional quantities of sensilla. Large black circles = large
sw-wp sensilla; small black circles = small sw-wp sensilla;
small grey circles = dw-wp sensilla; triangles = long projecting
sensilla; large grey circle = trichobothrium-like sensillum; small
black square = probably simple mechanoreceptive sensilla; long
black bar = crescent-shaped aggregation of sw-wp sensilla.
Abbreviations: dw-wp = double-walled wall pore sensillum,
sw-wp = single-walled wall pore sensillum.
Fig. 10. Schematic drawing of the antennae of (A) Bryaxis puncticollis, (B) Brachygluta fossulata and (C) Pselaphus heisei. Bars
= 100 µm.
Antennae:
The pselaphine antennae, which can be described as
clubbed in the species studied, are composed of 11 anten-
nomeres: a scape, a pedicel and a flagellum of 9 segments
(Fig. 10). The club is composed of the terminal three
antennomeres (9–11), and is especially conspicuous in
Bryaxis species. The relative length of the antennae varies
with body length and is significantly larger in the larger
species, i.e., Bf,Rl and Ph. The antennal length can reach
50% of the body length or more (Fig. 11) in these species.
The absolute length of the antennae ranges from 0.44 mm
(Bp; SD = 0.034, n = 10) to 0.96 mm (Rl; SD = 0.068, n =
8).
SEM studies show that the antennae differ greatly in
the number and types of sensilla present on their surface
(Fig. 12 for a representation, Fig. 13A). Several distinct
types of sensilla revealed in this study are described
below [double- and single-walled wall pore sensilla, fol-
lowing the terminology of Altner (1977) and Steinbrecht
(1997)].
Single-walled wall pore (sw-wp) sensilla (Figs 14A–B):
These large sensilla are found mainly on the most distal
antennomere (11th) and in smaller numbers on the 10th
antennomere. Characteristic features are a blunt tip and a
deflection towards the antennal tip (Fig. 14A). Their
lengths differ between species, the longer being found on
the antennae of the Bryaxis species (53.28 µm; n = 10;
SD = 5.5 µm) and Ph (45.05 µm; n = 8; SD = 8.4 µm),
and the shorter on the antennae of Bf (32.41 µm; n = 5;
SD = 4.7 µm) and Rl (36.48 µm; n = 10; SD = 1.9 µm).
Only small numbers of this type of sensilla were found.
The Bryaxis species have an average of 16 (n = 3; SD =
0.6; Bb) and 17 sw-wp sensilla (n = 7; SD = 0.7; Bp). The
larger species tend to have greater numbers of slightly
less conspicuous sw-wp sensilla: 23 (n = 3; SD = 0.6) in
Bf, 39 (n = 3; SD = 1) in Rl, and 48 (n = 4; SD = 1.2) in
Ph.Bf and Rl have a crescent-shaped aggregation of 7
sw-wp sensilla on the 11th antennomere (Figs 12B,
13B–D; less conspicuous in Rl).
Double-walled wall pore (dw-wp) sensilla (Figs 14A,
C): Of conical shape with the distal part featuring
899
Fig. 14. Sensilla at the antennal tip of Bryaxis puncticollis. (A) SEM-micrograph of both a sw-wp and a dw-wp sensillum
(arrows). Bar = 10 µm. (B) TEM-micrographs of subaxial resp. cross-sections through an sw-wp sensillum in which wall pores are
clearly visible. The pore tubules within the wall pores are distinguishable as thin dark lines (arrows; insert). Bar = 2 µm (main pic-
ture); bar = 0.5 µm (inset). (C) SEM-micrograph of a dw-wp sensillum showing cuticular fingers distally. Bar = 1 µm.
Abbreviations: cf = cuticular fingers, dw-wp = double-walled wall pore sensillum, pt = pore tubules, sw-wp = single-walled wall
pore sensillum, wp = wall pore.
Fig. 13. Overview of the peculiar structures found on the terminal antennomeres. (A) Array of sensilla in Bryaxis bulbifer. (B–C)
Crescent-shaped aggregation of 7 single-walled wall pore (sw-wp) sensilla in Brachygluta fossulata. (D) Flat seta at the tip of the
11th antennomere in Rybaxis longicornis (inset for orientation); the less conspicuous crescent-shaped aggregation of 7 sw-wp sen-
silla is visible in the lower part of the picture. Bars = 10 µm.
cuticular fingers (Fig. 14C). According to Altner (1977)
and Steinbrecht & Gnatzy (1984) this sensillum lacks
pore channels and communicates with its surroundings
through so-called spoke channels (Steinbrecht, 1997) that
are found between the cuticular finger-like structures. The
dw-wp sensilla are characteristically found on the 11th
antennomere. They are much smaller than the sw-wp sen-
silla with an average length of between 8.06 µm in Rl (n
= 5; SD = 0.9) and 12.3 µm in Ph (n = 5; SD = 1.6).
Average numbers vary from only 6 on the antennae of
both Bryaxis species (n = 10; SD = 0.7) to 28 in Ph (n =
4; SD = 1.7).
Long projecting sensilla (Fig. 15): Originate from a
cuticular protuberance. A tubular body is present and the
sensillum is much longer than other sensilla. Characteris-
tically it is bent away from the antennal surface and pos-
sesses a knob-like ending, visible under the SEM at high
magnifications (~70,000×, Fig. 15B). The average lengths
vary from about 54 µm (Bp,Bb and Ph) to 77 µm (Bf), or
even 85 µm (Rl). There are few of them and the number
900
Fig. 17. Peculiar structures on the antennae of selected male pselaphine beetles. (A–B) SEM-micrographs of (A) mesal resp. (B)
ventro-mesal aspects of the pedicellus and antenna respectively of Bythinus burrelli. (C–E) SEM-micrographs of scape and pedicel
of Bryaxis bulbifer. The pores are clearly visible. (D) Ridge-like modification on the pedicel. (E) Stamp-like modification on the
scape. Abbreviations: sc = scape, ped = pedicel, po = pores. Bars = 10 µm.
Fig. 15. (A) SEM-micrograph of the long projecting sensilla
on the antenna of Rybaxis longicornis. Bar = 10 µm. (B) Higher
magnification of the sensillum tip. Bar = 1 µm. Abbreviation:
lps = long projecting sensillum.
Fig. 16. Trichobothrium-like sensilla on the antenna of
Bryaxis puncticollis. (A) SEM-micrograph of a top view of the
base of a single sensillum. Bar = 10 µm. (B) SEM-micrograph
of an apical view of the apex of the antenna showing the trian-
gular arrangement of the trichobothrium-like sensilla (arrows).
Bar = 10 µm. (C) TEM-micrograph of a cross-section of a
trichobothrium-like sensillum showing the rhombic shape of the
cross-section. Bar = 1 µm. Abbreviation: po = pores.
was species-specific in our sample, with the largest
number (16) in Bp.
Trichobothria-like sensilla (Fig. 16): Long and
extremely fine recognizable by the rhombic shape of the
cross-section of their hair shafts and small diameter (Fig.
16C) in ultra-thin sections. There are three of them on the
tip of the 11th antennomere in the Bryaxis species. They
are arranged in a triangle when viewed apically (Fig.
16B). They arise from a cuticular depression on the 11th
antennomere (Fig. 16A). Several pores are present at their
base, which were not investigated further (Fig. 16A).
Peculiar structures on the antennae (Fig. 17): Stamp-
and ridge-like modifications occur on the lower inner side
of both the scape and the pedicel in males of Bb (Figs
17C–E), and a crescent-shaped modification on the
pedicel of males of Bythinus burrelli (Figs 17A–B). SEM
has revealed the presence of pores (Figs 17A, D, E) along
the ridges on the pedicels of both species and on top of
the stamp-like structure in Bb (Fig. 17E).
Furthermore, Rl has a distinct broad flattened seta on
the tip of its 11th antennomere (Fig. 13D); its function is
unknown.
Mouthparts
The following brief descriptions of the mouthparts of
pselaphines generally refer to Bp,Bb,Bf and Ph
(Table 1).
Clypeo-labrum (Fig. 18):
Trapeziform and free, i.e., connected with the clypeus
by a membrane. The anterior margin features two medial
peg-like sensilla (ps in Fig. 18). Several hair-like sensilla
of different lengths can be found on its dorsal and lateral
sides: the long lateral sensilla are bent toward the medial
axis, so that they come in contact with the prey seized by
the mandibles. The epipharynx (investigated in Bf only)
has several disc-like sensilla and two rows of medially
directed scales or trichomes (Fig. 18E).
901
Fig. 19. SEM-micrographs of selected mouthparts of Bryaxis puncticollis. (A) Dorsal aspect of the left mandible. (B) Mesal aspect
of the mola. (C) SEM-micrograph of the left maxilla. Bars = 10 µm. Abbreviations: gal = galea, lac = lacinia, ma = mandible, m =
mola, s = sensillum, st = subapical teeth.
Fig. 18. (A–D) SEM-micrographs of the labrum of (A) Bryaxis puncticollis, (B) Pselaphus heisei, (C) Brachygluta fossulata and
(D) Rybaxis longicornis. Bars = 10 µm. (E) SEM-micrograph of the epipharynx of Brachygluta fossulata. Bar = 20 µm. Abbrevia-
tions: ds = disc-like sensilla (two different types), hs = hair-like sensilla, ps = peg-like sensilla.
Mandibles (Figs 19A–B):
More or less symmetrical. The apex is simple (uniden-
tate) and several subapical teeth are present (st in Fig.
19A). A prostheca is lacking. The mola is distinct and
firmly united to the rest of the mandible. The mola differs
in size from well-developed in the Bryaxis species to
reduced in Ph. It has small nipple-like grinding cones
(Fig. 19B) on its medial surface. The mandible lengths,
measured from the abductor to the tip of the mandible,
vary from 0.137 mm in the Bryaxis species (n = 13; SD =
0.01 mm) to 0.167 mm in Bf (n = 3; SD = 0.01 mm) and
0.207 mm in Rl (n = 1). Electron-micrographic images of
the Bp mandibles have revealed the presence of a single
sensillum on the outer margin (s in Fig. 19A). It is par-
ticularly conspicuous and long (up to 70 µm), and later-
ally flattened. The sensilla present on the mandibles of the
other species (partly visible in Figs 18B–D) are inserted
on the dorsal side and are less distinct and often much
smaller and thinner (Ph has several short sensilla). An
interaction with other mouthparts has not been observed,
and the sensillum is usually positioned relatively close
against the mandible. Its function is unknown.
902
Fig. 21. SEM-micrographs of the maxillary palps of Pselaphinae. The enlarged fourth palpomere and the appendix at its tip is
clearly visible. (A) Bryaxis puncticollis; bar = 100 µm; (B) Bryaxis bulbifer; bar = 10 µm; (C) Brachygluta fossulata; bar = 10 µm;
(D) Rybaxis longicornis; bar = 10 µm; (E) Pselaphus heisei; bar = 10 µm; (F) Tyrus mucronatus; bar = 10 µm.
Fig. 20. SEM photograph of the ventral aspect of the head of
Brachygluta fossulata. Overview of the mouthparts. Bar = 10
µm. Abbreviations: a = appendage, ca = cardo, lab = labrum, lp
= labial palpus, ma = mandible, me = mentum, p1–p4 = maxil-
lary palpomeres 1–4, st = stipes.
Maxillae (Fig. 19C):
The cardo is transverse. The stipes is subdivided into
basi- and mediostipes, the latter forming the base of both
the galea and lacinia. The apices of both are differentiated
into mesal brush-like arrays of curved hair-like trichomes
(Fig. 19C). The maxillary palps are four-segmented (Fig.
20). The external appearance of the palpomeres is at least
genus-specific (Fig. 21). The first palpomere is small
(only Ph beetles have a strongly elongated first pal-
pomere), the second relatively long and the third short.
The fourth palpomere is generally large and varies greatly
in appearance between species (Fig. 21).
Both Bryaxis species seem to capture their prey with
the help of their maxillary palps. SEM analyses of their
fourth palpomere have shown that the dominant type of
setae resembles the adhesive structures that are often
found on insect tarsi, with the distal parts being spoon-
shaped (ssh in Fig. 22). The fourth palpomere of Bp bears
on average 356 (female, n = 4, SD = 14) and 441 (male, n
= 3, SD = 57), respectively, of these setae. Semi-thin sec-
tions of the maxillary palps have revealed the presence of
glandular structures (Fig. 23). The outlet was not visible
but is possibly situated on the apical part of the spoon-
shaped hairs, thus supporting the hypothesis that the bee-
tles capture their prey by means of the adhesive surface of
their palps. Additionally, another type of sensilla was
found (lps in Fig. 22), which resemble the long projecting
sensilla on the antennae.
A common peculiarity of pselaphine maxillary palps is
a segment-like appendage at the tip of the fourth pal-
pomere (Newton & Thayer, 1995; Fig. 24). This
appendage measures 25–35 µm in length and ca. 5 µm in
diameter, and is inserted either in a cuticular depression
or on a cuticular socket (Ph). The distal part of this
appendage features longitudinal striae, which only
become visible at high magnification (7,500 ×). The apex
is oblique (Fig. 24B) and transmission electron micro-
graphs reveal several cuticular cavities and pore tubules
in the cuticle on the slanting mesal side (Fig. 24C). The
space between two cavities is filled with small, almost
rectangular, cuticular structures, which are separated by
small interstices (Fig. 24C). The depth of these cavities
varies between 400 and 800 nm. Receptor lymph cavities
are visible further proximally, as are several dendrites
inside the appendage (Fig 23D). These originate in the
fourth palpomere and extend into the appendage, thus
indicating its sensory function. A tubular body, which is
characteristic of mechanosensilla, was not observed.
Labium (Fig. 25):
The mentum is large and plate-like and separates the
maxillae (Fig. 20). The palps are three-segmented, with
903
Fig. 24. Ultrastructure of the appendage of the fourth palpomere. (A) SEM-micrograph (Rybaxis longicornis). Bar = 10 µm. (B)
Higher magnification of the apex of the appendage (Bryaxis puncticollis). Bar = 1 µm. (C–D) TEM-micrographs of longitudinal sec-
tions through the appendage (Bryaxis puncticollis). Bars = 1 µm. Abbreviations: cc = cuticular cavities, dist = distal, ds = dendritic
sheaths, p4 = fourth maxillary palpomere, prox = proximal, pt = pore tubules, rlc = receptor lymph cavities.
Fig. 22. SEM-photograph of the hair-like structures on the
fourth maxillary palpomere of Bryaxis puncticollis. Bar = 1 µm.
Abbreviations: ssh = spoon-shaped hairs, lps = long projecting
sensilla.
Fig. 23. TEM-micrograph of glandular structures within the
left maxillary palp of Bryaxis bulbifer. The secretory vesicles
are clearly visible. Bar = 2 µm. Abbreviations: cu = cuticula, n
= nucleus, rer = rough endoplasmatic reticulum, sv = secretion
vesicles.
the last palpomere sensillum-like (Fig. 25). At its tip the
prementum bears prominent apical lobes with a fringe of
stout bristles and trichomes (al in Fig. 25). These lobes
result from the fusion of the glossa and paraglossa
forming a synglossa (Nomura, 1991).
DISCUSSION
All the species investigated in this study are distributed
throughout most of Europe (Besuchet, 1974), with all but
Tyrus mucronatus living in grass- and leaf-litter, and in
moss growing close to waterbodies and bogs. Tyrus
mucronatus live under the bark of dead trees, a habitat
where only a small number of species of the subfamily
are found.
Pselaphines (more than 9000 described species world-
wide) are generally predacious as larvae and adults
(Chandler, 2001; Thayer, 2005), the commonest feeding
mode among staphylinids (Thayer, 2005). Pselaphines
were initially reported to feed on earthworms, insect lar-
vae, small flies and especially mites (Park, 1932, 1933,
1942, 1947a, b; Pearce, 1957), whereas in later studies
Collembola were used as the main prey (De Marzo, 1985,
1986, 1988; De Marzo & Vit, 1982; Engelmann, 1956).
Other potential prey animals (mites, ant larvae and Dro-
sophila larvae) were used in this study, but rejected or
only reluctantly ingested. However, all of the beetles
eagerly preyed upon Collembola (Entomobryidae,
whereas the isotomid Folsomia was rejected). Therefore,
the collembolan Heteromurus nitidus was used in the
prey-capture experiments. Hence, recorded here is the
performance of the different beetle species when
attempting to catch a rather elusive prey. They might per-
form differently on other prey types encountered in situa-
tions different from those in our laboratory experiments.
Park (1932, 1933, 1947a) worked especially on those
species facultatively associated with ants, focused on
their feeding behaviour but did not describe their method
of capturing prey, possibly because ant larvae and mites
are easy to locate and not elusive. Park (1947b) includes a
short note indicating that free-living pselaphines mainly
feed on mites, which after capture they press to the
ground using their fore tarsi. De Marzo & Vit (1982)
report Batrisodes occulatus “trying to catch Collembola
with their mandibles”, and that fragments of Collembola
were found in the intestines. A short account of the prey-
capture behaviour of Pselaphus parvus larvae is given by
De Marzo (1988), who reports that they capture their prey
by means of protrusible viscous head organs, with the vis-
cous secretion originating from glands in the head (De
Marzo, 1988). According to DeMarzo (1985, 1988), this
structure is found in all investigated pselaphine larvae;
however, it is absent in larvae of Faronitae (Newton,
1991). The most detailed description of the prey-capture
in adult Pselaphinae to date is given by Engelmann
(1956).
Detection of prey
The pselaphine beetles studied have relatively reduced
eyes, with only a few convex ommatidia. The presence of
large numbers of screening pigment granules between the
ommatidia is found only in scattered taxa among Coleo-
ptera, and within Staphyliniformia only in Micropeplinae
and some Scydmenidae (Caveney, 1986). The Semper
cells are probably the remains of a crystal cone (cf.
Meyer-Rochow, 1999). The reduced eyes and the lack of
a crystal cone are indicative of a life in habitats where
light intensities are low (Lawrence & Britton, 1991), such
as litter and rotting logs. These structures clearly suggest
that the beetles are not visual hunters. Nonetheless, they
respond to light, possibly even ultraviolet light, since
many species in tropical and some in temperate regions
are collected at ultraviolet lights (Wolda & Chandler,
1996). Our observations suggest that Brachygluta fossu-
lata,Rybaxis longicornis, and Tyrus mucronatus, which
have a higher number of ommatidia than other species,
might be able to perceive and follow the movement of
their prey visually. The higher number of ommatidia in
the males of Bryaxis puncticollis is presumably advanta-
geous when searching for females and a rather common
sexual dimorphism known from a number of Pselaphinae
(Chandler, 2001; Jeannel, 1950).
In accordance with their poor visual capabilities, tactile
and chemical cues seem to play a principal role in prey
detection and several types of sensilla may be involved.
Of special importance in the search for prey are the anten-
nae, which have a wide range and multitude of sensilla
(much more so than on the rest of the body). In the spe-
cies studied 42–50% of the antennal sensilla were con-
centrated on the terminal antennomere (Figs 10, 12). The
structural peculiarities of the antennae of Bryaxis bulbifer
and Bythinus burrelli are obviously related to mating, as
they occur only in males. These structures are rarely
located on scapus and pedicel in Pselaphinae, but almost
always on the flagellomeres (Chandler, 2001). Sensilla
described as single- and double-walled wall pore sensilla
have an olfactory function (cf. Altner, 1977; Steinbrecht
& Gnatzy, 1984) in other insects. The position of the
olfactory sensilla at the apex of the antennae allows the
assessment of the direction of the source of olfactory cues
(Skilbeck & Anderson, 1996), and the constant move-
904
Fig. 25. SEM-micrograph of the right labial palpus of Bryaxis
puncticollis. Bar = 10 µm. Abbreviations: al = apical lobe, lp
1–3 = labial palpomere 1–3.
ment of the antennae promotes the perception of chemical
cues according to phasic receptor physiology (e.g.,
Zacharuk, 1985). The antennal tips of the Bryaxis species
are additionally equipped with specific trichobothria-like
sensilla (Fig. 16). They probably function as receptors of
air vibrations. Brachygluta fossulata,Rybaxis longicornis
and Pselaphus heisei lack this type of sensilla. Notwith-
standing, these species were capable of identifying prey,
even without mechanical contact.
Like the antennal movements, the maxillary palps,
vibrated in Bryaxis puncticollis and Tyrus mucronatus,
also seem to be involved in the perception of olfactory
cues. This is also described for Batrisodes globosus,
Euplectus sp. and Bibloplectus sp. (Engelmann, 1956).
Testing the ground with the maxillary palps as observed
in Pselaphus heisei can be interpreted in terms of
searching for gustatory chemical cues of prey.
The fourth maxillary palpomere has a characteristic
appendage (a in Fig. 20; Figs 21, 24) at its distal end, a
synapomorphy for the subfamily (Newton & Thayer,
1995). Thayer (2005) describes this appendage of the
maxillary palps as an “unsclerotized digitiform segment-
like appendage”, whereas Jeannel (1950) and Pearce
(1957) consider it to be a fifth palpomere. Others have
identified it as an ordinary sensillum. The origin of this
structure remains unclear. As it has several cuticular
sheaths it appears to be a compound sensillum formed by
the fusion of several sensilla. That it has a sensory func-
tion can be deduced from the dendrites and dendritic
sheaths within the appendage (Fig. 24). The morphology
suggests a contact chemoreceptor.
The long projecting sensilla (Fig. 15) on the antennae
seem to have a bimodal mechanoreceptive function. They
are characterized by their shape (usually S-curved), with
the most distal part being bent away from the antennal
surface (G. Alberti, pers. comm.). This type of sensilla
lacks lateral wall pores. Instead they have a single pore at
the apex (cf. Ozaki & Tominaga, 1999), which suggests a
gustatory function. We have found similar sensilla on the
fourth maxillary palpomere (between those with spoon-
like apices, Fig. 22), which might have the same modali-
ties. The many small, thin sensilla on the antennae are
probably simple mechanoreceptors for tactile perception.
Tactile contact (generally with the antennae) seems to be
necessary to induce preparation for the predatory strike in
at least Bryaxis puncticollis,Rybaxis longicornis and
Tyrus mucronatus.
Predatory strike
All the beetles investigated slightly touch their prey
during the preparation phase prior to the strike, probably
in order to ascertain its exact location and identify it as
prey using gustatory sensilla. These contacts are usually
so gentle that they do not initiate the escape mechanism
of the Collembola, and therefore the beetles do not have
to strike faster than the prey can initiate the escape (in
Collembola about 26 ms subsequent to the stimulus,
Bauer, 1978).
In all the pselaphines studied there occurred an upward
movement of the body prior to the strike (Fig. 3A). This
might function to accelerate the strike, so that the maxil-
lary palps of the Bryaxis species hit the prey at a rela-
tively high velocity. If any adhesive secretion is involved,
this would result in a better adhesion to the surface of the
prey (Betz & Kölsch, 2004). The role of the palps of Pse-
laphus heisei in prey-capture is not easy to determine,
since ultrastructural examinations revealed no evidence of
an adhesive surface. Hence, in this case the prey might
simply be captured by entanglement among the massive
setae on the maxillary palps (Fig. 21E).
The broad conclusion of this limited comparative study
is that the prey-capture behaviour seems to be similar
within certain tribes of Pselaphinae (classification into
tribes according to Löbl & Besuchet, 2004, and Newton
& Chandler, 1989). The prey-capture behaviour in all the
species studied is substantially different from the rela-
tively simple behaviour described for Batrisodes globosus
(tribe Batrisini), Euplectus sp. (tribe Euplectini), and
Bibloplectus sp. (tribe Trichonychini) by Engelmann
(1956). In these three species the beetles briefly stop their
forward movement, then lunge forward and grasp the col-
lembolan with their mouthparts. Of the species investi-
gated in the present study, the behaviour of the members
of the Brachyglutini is most similar.
Other behavioural features and structures seem to be
more derived compared with this general pattern. (1) The
presumably sticky maxillary palps of the Bryaxis species
(tribe Bythinini) are a specific prey-capture device not
previously described. (2) The behaviour of Tyrus mucro-
natus beetles seems to resemble that of Cedius spinosus
(both species belonging to the tribe Tyrini) described by
Engelmann (1956). The latter species slowly approaches
the prey, “steadies itself upon its meso- and metathoracic
legs, and rears up slightly” before seizing the prey
(Engelmann, 1956). Hence, both of these species capture
their prey by means of their forelegs, sandwiching it
between the femur and tibia, an action supported by the
mouthparts in Cedius spinosus (Engelmann, 1956).
Engelmann (1956) observed similar prey-capture behav-
iour in Tmesiphorus costalis (tribe Tmesiphorini, closely
related to Tyrini). All three species have specific morpho-
logically modified forelegs, which supposedly improve
their predatory strike, i.e., (i) spines on the femur and
tibia in Cedius spinosus, (ii) trochantero-femoral spines
and ridges in Tyrus mucronatus and (iii) stiff brushes of
setae on the femur and tibia in Tmesiphorus costalis.
Such prey-capture behaviour employing raptorial fore
legs appears to be rather unique within the Coleoptera
and has only recently been described for two other groups
of staphylinids, i.e., the genus Philonthus subgenus Ony-
chophilonthus Neresheimer & Wagner (Betz & Mumm,
2001) and Nordus fungicola (Sharp) (Chatzimanolis,
2003).
Complex preparation behaviour prior to the strike and
directing the prey towards the mandibles with the help of
the antennae was not recorded for Cedius and Tmesi-
phorus by Engelmann (1956), possibly because of the
poor technical facilities available at that time.
905
Prey-handling
Another previously undescribed behaviour in pse-
laphines is prey-handling. Only Engelmann (1956)
records that a Bibloplectus beetle held down a large iso-
tomid collembolan with its fore tarsi. The specific behav-
iour described in the present study, which involves the
manipulation of the position of the prey in the
mouthparts, might be a special adaptation for catching
elusive prey such as Collembola.
The maximum prey size manageable by a beetle
depends on the prey’s combativeness and the predator’s
ability to hold and subdue it. As far as prey size is con-
cerned, the two Bryaxis species, Brachygluta fossulata
and Pselaphus heisei were similar, i.e., they preferred
small to large prey (Fig. 6). However, these species dif-
fered significantly in their general prey-capture success
(Fig. 7). The absolute length of their antennae and their
pattern of movement did not seem to be related to prey-
capture success. Apart from its lower strike velocity, the
reason for the lower success in Pselaphus heisei might be
the lower number of mechanoreceptors on the antennae,
possibly resulting in a less accurate location of the prey
and an imprecise strike. Moreover, Pselaphus heisei has
relatively short mandibles (ratio mandible length/body
length, 0.086), which could also decrease its capture suc-
cess. The difference in the capture success of the two
Bryaxis species might be caused by the different adhesive
properties of the maxillary palps.
Based on the behavioural observations on one individ-
ual, Tyrus mucronatus seems to be an extremely effective
predator. Its complex behaviour prior to the strike permits
accurate location of the prey, and its predatory legs allow
the strike to be especially precise and successful.
CONCLUSIONS
This study provides new information on the behaviour
and morphology of a hitherto largely neglected group, the
Pselaphinae. These data improve our knowledge of the
prey-capture behaviour of this very diverse group of bee-
tles and should spur further comparative behavioural
analyses.
The pselaphines studied differ significantly in the array
of sensilla on their sensory organs. In particular, the dif-
ferences in the numbers and types of sensilla on the
antennal tip might correspond to differences in prey-
finding success. Pselaphines do not seem to be visual
hunters, but rather employ chemical and mechanical cues,
which is to be expected of litter-dwelling organisms.
Although they vary in size and have different sized
mandibles, all of them prefer the smallest prey-size
classes offered. Notwithstanding, the beetles differ in
their specific prey-capture strategies. These might be
tribe-specific and are highly elaborate in some species,
leading to increased prey-capture success, for instance the
use of sticky maxillary palps (e.g., Bryaxis bulbifer) or
raptorial legs (Tyrus mucronatus), or certain behavioural
patterns prior to the strike. Beetles of other species simply
use their mandibles, probably the ancestral method of
capturing prey.
Because of their great ecological and morphological
diversity, this subfamily seems to have great potential for
future integrative studies on the evolution of functional
and ecological diversity in soil organisms.
ACKNOWLEDGEMENTS. Our thanks are extended to T.
Bauer who supervised this work, to A. Thomas and F. Anton-
Erxleben for technical assistance with the morphological work
(all at the University of Kiel, Germany), to G. Alberti (Univer-
sity of Greifswald, Germany) who shared his expertise on the
structure and function of arthropod sensilla, and to A. Solodov-
nikov (Zoological Museum Copenhagen, Denmark) for helpful
comments on a previous version of this manuscript. T. Jones
and an anonymous language editor corrected the English of the
manuscript.
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Received February 1, 2008; revised and accepted July 10, 2008
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