ArticlePDF Available

Description and phylogenetic interpretation of chromatophore migration from larval air sacs to adult structures in some Chaoboridae (Diptera)

Authors:
  • California State Collection of Arthropods
  • Royal BC Museum

Abstract and Figures

During development, many chromatophores on the air sacs of larvae of Chaoborus Lichtenstein disperse to the tracheal trunks and throughout the body of the pupae. In male pupae, chromatophores on the posterior air sacs move to the developing testes and vasa deferentia and some become the adventitious spotting previously reported for adults of Chaoborus. In larvae of Mochlonyx Loew, chromatophores have a similar development pattern, but in female pupae some also surround the spermathecae. Larvae of Eucorethra Underwood have chromatophores scattered throughout much of the body but it is uncertain whether these are homologous to those of Chaoborus and Mochlonyx. Outgroup comparisons show that the migration of chromatophores from the larval air sacs to the adult male testes and vasa deferentia is a synapomorphy of Chaoborinae. The presence of pigmented fat body on the larval testes in many Culicidae, Eucorethra, and Mochlonyx is plesiomorphic, and the transparent larval testes in Chaoborus are a synapomorphy of the genus. The dark adult testes in Mochlonyx are derived from pigmented larval fat body and chromatophores from the larval air sacs, and this is proposed as an intermediate evolutionary state. It is likely that the chromatophores surrounding the testes of pupae of Chaoborinae provide protection against ultraviolet radiation, but further study is needed.
Content may be subject to copyright.
Description and phylogenetic interpr etation of
chr omatophore migration fr om larval air sacs to
adult structur es in some Chaoboridae (Diptera)
Christopher J. Borkent
1
Department of Natural Resource Sciences, Macdonald Campus, McGill University,
21 111 Lakeshore Road, Sainte Anne de Bellevue, Quebec, Canada H9X 3V9
Art Borkent
691–8th Avenue SE, Salmon Arm, British Columbia, Canada V1E 2C2, Research associate of
the Royal British Columbia Museum, 675 Belleville Street, Victoria, British Columbia,
Canada V8W 9W2, American Museum of Natural History, Central Park West, New York,
NY 10024-5192, United States of America, and Instituto Nacional de Biodiversidad,
P.O. Box 22-3100, Santo Domingo de Heredia, Costa Rica
Abstract—During development, many chromatophores on the air sacs of larvae of Chaoborus
Lichtenstein disperse to the tracheal trunks and throughout the body of the pupae. In male pupae,
chromatophores on the posterior air sacs move to the developing testes and vasa deferentia and
some become the adventitious spotting previously reported for adults of Chaoborus.Inlarvaeof
Mochlonyx Loew, chromatophores have a similar development pattern, but in female pupae some
also surround the spermathecae. Larvae of Eucorethra Underwood have chromatophores scattered
throughout much of the body but it is uncertain whether these are homologous to those of
Chaoborus and Mochlonyx. Outgroup comparisons show that the migration of chromatophores
from the larval air sacs to the adult male testes and vasa deferentia is a synapomorphy of
Chaoborinae. The presence of pigmented fat body on the larval testes in many Culicidae, Eucor ethra,
and Mochlonyx is plesiomorphic, and the transparent larval testes in Chaoborus are a synapomorphy
of the genus. The dark adult testes in Mochlonyx are derived from pigmented larval fat body and
chromatophores from the larval air sacs, and this is proposed as an intermediate evolutionary state.
It is likely that the chromatophores surrounding the testes of pupae of Chaoborinae provide protection
against ultraviolet radiation, but further study is needed.
Borkent and Borkent640
Résumé—Pendant le développement, beaucoup de chromatophores sur les sacs d’air des larves de
Chaoborus Lichtenstein dispersent vers les troncs trachéaux de la pupe et dans tout le corps. Dans
les pupes mâles, ceux des sacs d’air postérieurs se déplacent vers les testicules en développement
et le canal déférent. Certains des chromatophores de viennent la tache adventice précédemment rap
-
portée pour des adultes de Chaoborus. Les larves de Mochlonyx Loew possèdent un patron similaire
de développement, alors que certains de ceux retrouvés dans les pupes femelles entourent égale
-
ment les spermathèques. Les larves d’Eucorethra Underwood ont des chromatophores dispersés à
travers la majorité de leur corps, mais il est incertain qu’ils soient homologues à ceux de Chaobo
-
rus et Mochlonyx. Les comparaisons de groupes externes prouvent que la migration des chromato
-
phores des sacs d’air larvaires jusqu’aux testicules et au canal déférent du mâle adulte est une
synapomorphie des Chaoborinae. La présence de masse adipeuse pigmentée sur les testicules lar
-
vaires chez beaucoup de Culicidae, Eucorethra,etMochlonyx est une plésiomorphie, alors que les
testicules larvaires transparents de Chaoborus sont une synapomorphie du genre. Les testicules
foncés chez l’adulte de Mochlonyx sont dérivés de masse adipeuse larvaire pigmentée et des chro
-
matophores des sacs d’air larvaires, et ceux-ci sont proposés comme état évolutif intermédiaire. Il
est probable que les chromatophores entourant les testicules des pupes de Chaoborinae assurent la
protection contre le rayonnement ultraviolet, mais des études plus poussées sont nécessaires.
Can. Entomol. 140: 630–640 (2008) © 2008 Entomological Society of Canada
630
Received 5 June 2008. Accepted 1 September 2008.
1
Corresponding author (e-mail: chris.borkent@mail.mcgill.ca).
doi: 10.4039/n08-049
Introduction
To a rather distressing degree the phylogen
-
etic interpretation of taxa in Diptera is based on
a limited number of characters. In particular, in
-
ternal structures and their development are
poorly understood for most taxa and there is a
need for further comparative studies throughout
the order. In this paper we describe a unique de
-
velopmental pathway for a group of species of
Chaoboridae.
Chaoboridae, the sister group of mosquitoes
(Culicidae), is an ancient group with a fossil re
-
cord going back to the Jurassic (Borkent 1993).
Adults are nonbiting, but larvae are predaceous,
with most using their prehensile antennae to cap
-
ture prey (only Australomochlonyx Freeman has
filter -feeding larvae). The subfamily Chaoborinae
includes five genera: Chaoborus Lichtenstein is
the sister group of the monophyletic Cryophila
Edwards + Australomochlonyx + Promochlonyx
Edwards + Mochlonyx Loew (Sæther 1992).
The larvae of Chaoborinae are the only known
planktonic insects. Each larva has two pairs of
air sacs (one pair each in the thorax and abdom-
inal segment 7) that allow them to float hori-
zontally in the water column and, at least in
Chaoborus, to change their depth in the water
column (Teraguchi 1975). The unique larva of
Cryophila has an additional pair of air sacs in
abdominal segment 6. The presence of the air
sacs is a synapomorphy of Chaoborinae (Sæther
1992).
In many Chaoborus species the larvae under
-
take large diurnal migrations that can encompass
over 200 m vertically (Allison et al. 1996) by
controlling the amount of gas in each of their
elastic air sacs (Teraguchi 1975). Inflation and
deflation of the air sacs appears to be under ner
-
vous control (Wemhöner and Weber 1974) and
is accomplished by diffusion of gases from the
water into the larva and then into the air sacs,
which the larv a expands or contracts using mus
-
cles attached to the air sacs (Teraguchi 1975).
Larvae of Cryophila + Australomochlonyx +
Promochlonyx + Mochlonyx occur only in small
lentic habitats. The reason for their vertical posi
-
tion in the water column is poorly understood.
They float horizontally but rarely or never
(Mochlonyx velutinus (Ruthe); A. Borkert, per
-
sonal observation) go to the surface to obtain air
with their siphons.
The air sacs of chaoborine larvae are either
somewhat kidney-shaped or elongate, with the an
-
terior and posterior ends tapering more sharply.
Those of Mochlonyx, Cryophila,andChaoborus
have dorsal pigment cells that are either light or
medium brown in Mochlonyx and Cryophila and
dark brown and (or) black in Chaoborus.Theair
sacs and the associated pigment cells have been
studied in detail only in Chaoborus (Hadorn and
Frizzi 1949; Gersch 1956; Teraguchi 1972, 1975;
Wemhöner and Weber 1974; Weber and
Grosmann 1988). The chromatophores contract,
and therefore shrink in surface area (leading to
less coverage of the air sacs), when exposed to
light and expand and flatten out in the dark
(Figs. 1E, 1F). The functional significance of this
change in colouration is unknown, although the
suggestion has been made that it provides camou
-
flage, depending on where in the water column a
larva is situated (Weber and Grosmann 1988).
The chromatophores are held on the surface of
the air sacs by an extracellular membrane and are
able to move about in an amoeboid fashion
(Weber and Grosmann 1988). The reaction of the
chromatophores in pupae and adults to light is
unknown.
In this paper we report the results of our
examination of the developmental fate of the
chromatophores on the air sacs from the larval
stage through to the adult stage in Chaoborus
trivittatus (Loew), C. americanus (Johannsen),
and M. velutinus. To interpret outgroup features
we also studied each developmental stage in live
Eucor ethr a underwoodi Underwood and preserved
specimens of unidentified Anopheles Meigen
(Culicidae). Our analysis of the chromatophores,
the pigmentation on the testes of various species
of Chaoboridae, and the form of the male internal
reproductive system is put in a phylogenetic con
-
text and provides additional support for previ
-
ously proposed relationships within and outside
Chaoboridae.
Materials and methods
Adults were reared from field-collected lar
-
vae kept at room temperature, about 18–20 °C,
in Styrofoam coffee cups half-filled with water.
Shortly after the larvae pupated, a light mesh
was secured over the mouth of the cup with an
elastic band.
Live larvae and pupae of C. trivittatus,
C. americanus,andM. velutinus were studied
daily under a Wild M3 dissecting scope and the
development of the chromatophores was re
-
corded. These taxa are particularly amenable to
gross morphological study because larvae and
pupae of all Chaoborus and Mochlonyx species
© 2008 Entomological Society of Canada
Borkent and Borkent 631
are virtually transparent or semi-transparent and
it is possible to examine internal features easily
with dissecting and compound microscopes.
Larvae and pupae of E. underwoodi are opaque
and some were dissected periodically to exam
-
ine their pigment cells. Upon adult emergence,
live males and females of all species were dis
-
sected in Ringer’s solution and studied with a
Wild M3 dissecting scope and a Carl Zeiss
Jenaval compound microscope. Photomicro
-
graphs were taken with a Nikon CoolPix 995
mounted on the compound microscope, and
drawings of larval and adult structures were made
from these photographs. The chromatophore pat
-
terns in pupae were recorded on a template
drawing of a pupa originally made from pre-
served specimens.
Larvae of E. underwoodi (n = 14) were sam-
pled on 9 June 2006 from shallow pools near
the bottom of Takkakaw Falls (51°29
30
′′
N,
116°28
30
′′
W) 11 km north of Field, British Co-
lumbia (B.C.). They were fed a variety of small
adult insects collected in Salmon Arm, B.C.,
which were dropped on the water surface, gen-
erally once a day. Larvae of M. velutinus (n =
106) and C. trivittatus (n = 43) were collected
on 29 May 2006 from woodland pools 3.3 km
northwest of Enderby, B.C. (50°34
36
′′
N,
119°11
20
′′
W); larvae of C. americanus were
collected in February 1991 (n =21)andon15
May 2006 (n = 18) from an exposed pond lo
-
cated just northeast of the junction of Hwy 97b
and 10 Avenue SE in Salmon Arm (50°41
35
′′
N,
119°13
32
′′
W). Larvae of Chaoborus spp. and
M. velutinus were fed locally collected mos
-
quito larvae and a variety of small crustaceans.
The pigmentation of the gonads of 93
M. velutinus larvae was examined and the lar
-
vae were reared to adulthood to determine sex.
Terms for structures of the male genital tract
follow Sinclair et al. (2007) and Borkent et al.
(2008).
Preserved specimens larvae of Anopheles
gambiae Giles (Culicidae), Corethrella
appendiculata Grabham (Corethrellidae), a nd a
species of Dixella Dyar and Shannon (Dixidae),
and adult males of A. gambiae and A. earlei
Vargas were dissected to make outgroup
comparisons.
Results
In the follo wing we describe the dev elopment
of the chromatophores in Eucorethr a, Mochlonyx,
and Chaoborus, in phyletic sequence.
Eucorethra Underwood
Fourth-instar larva: The larva has no air sacs,
and chromatophores are scattered throughout
the body (Figs. 1A, 1B). Each chromatophore
occurs separately and is not attached to any
structure (i.e., they float freely upon dissection).
The testes are light to medium brown and the
ovaries are pale, lacking any pigmentation.
Pupa: We found no concentration of chromato-
phores in pupae of either sex.
Adult male: Each testis has a layer of reddish
brown fat body on its anterior portion (Borkent
et al. 2008). No chromatophores are present on
the reproductive system.
Adult female: No females were studied.
Remarks: Larv ae of Eucor ethra do not have a
concentration of chromatophores. The testes of
adult males have a layer of lightly pigmented fat
body anteriorly, similar to that in many species
of Culicidae (discussed below). It is uncertain
whether the chromatophores scattered through
-
out the larval body are homologous to those on
the air sacs of Mochlonyx and Chaoborus.
Mochlonyx Loew
Fourth-instar larva: The thoracic air sacs have at
least a fe w scattered very light- to medium-
brown (rarely black) chromatophores dorsally
and at least partially laterally. The abdominal air
sacs have light-bro wn chromatophores concen
-
trated anteriorly (Fig. 2). Some chromatophores
are also scattered along the tracheal trunks. The
testes are medium to dark brown, owing to the
presence of fat-body cells, and the ovaries are
pale, lacking any pigmentation (Figs. 1C, 1D).
© 2008 Entomological Society of Canada
632 Can. Entomol. Vol. 140, 2008
Fig. 1. (A and B) Fourth-instar larva of Eucorethra underwoodi, showing chromatophores scattered in the body.
(A) Head, ventrolateral view (head capsule is 2.28 mm long). (B) Dissected thorax, ventral view (abdominal
segment 1 is 1.75 mm wide). (C and D) Fourth-instar larvae of Mochlonyx velutinus, dorsal view. (C) Male larva
(head capsule is 0.79 mm long). (D) Female larva (head capsule is 0.81 mm long). (E and F) Fourth-instar larva
of Chaoborus trivittatus with chromatophores on the thoracic air sacs (each air sac is 1.61 mm long). (E) Under
bright light. (F) After being in dark conditions for several hours (chrom, chromatophores; tes, testis).
© 2008 Entomological Society of Canada
Borkent and Borkent 633
Male pupa: Upon emergence the anterior
chromatophores are scattered throughout the
thorax, but at least some of the chromatophores
previously located on the posterior air sacs of
the larva have migrated to the testes, adding to
the pigmented fat body. Some chromatophores
remain on the tracheal trunks. After day 2 and
before day 4, dark pigment cells also cover the
vasa deferentia (Fig. 2).
Female pupa: Upon emergence the anterior
chromatophores are scattered throughout the tho
-
rax, but the abdominal chromatophores are con
-
centrated in two groups between the lateral
tracheal trunks in the areas where the two poste
-
rior air sacs of the larva were previously located.
The fate of the other larval chromatophores is
uncertain because of the somewhat opaque
body. By day 2 the abdominal chromatophores
© 2008 Entomological Society of Canada
634 Can. Entomol. Vol. 140, 2008
Fig. 2. Schematic diagram showing the presence and (or) fate of chromatophores in three chaoborid genera,
Eucorethra, Mochlonyx,andChaoborus. The shaded area shows the pigmented fat body (fb) and the solid
areas show the chromatophores (chrom).
(including those remaining on the tracheal
trunks) have mostly dispersed in the abdomen,
with some concentrated in segment 7. At day 4
there are clumps of chromatophores in seg
-
ment 7 and some are concentrated on the three
spermathecae.
Adult male: Black chromatophores are present
in patches over the entire testis and all of the
vasa deferentia (Fig. 2; Borkent et al. 2008). A
single sheath of pigmented cells surrounds the
posterior 1/4 of the abutting but not fused vasa
deferentia. Chromatophores are scattered through
-
out most of the body tissues, but it is uncertain
where these originated in the larva.
Adult female: Chromatophores are concentrated
around the spermathecae. Chromatophores are
also scattered throughout most of the body tis
-
sues, but it is uncertain where these originated
in the larva.
Remarks: It was only in males that at least
some abdominal chromatophores moved from
the larval air sacs directly to the gonads. Other
chromatophores dispersed among the body tis-
sues, but it is uncertain whether these were
originally on the larval air sacs. There was
some concentration of chromatophores on the
developing spermathecae in female pupae.
Of 93 larvae reared to adulthood, 50 larvae
with pigmented gonads were males and 43 lar-
vae with pale gonads (not clearly visible) were
females. In 2 of the 50 male larvae, only one
testis was darkly pigmented, with the other ei
-
ther pale or absent.
We followed the dev elopment of the chromato
-
phores in this genus only in M. velutinus.
Chaoborus Lichtenstein
Fourth-instar larva: Each air sac is cov ered dor
-
sally and at least partially laterally with numerous
dark-brown (expanded state) to black (contracted
state) chromatophores (Figs. 1E, 1F). The testes
and ov aries are pale, lacking any pigmentation.
Male pupa: Upon emergence the thoracic and
abdominal chromatophores are concentrated in
four groups between the lateral tracheal trunks,
in the areas where the two anterior air sacs and
two posterior air sacs of the larva were previously
located; some chromatophores may already be
present on the testes; a few abdominal chromato
-
phores are scattered among the body tissues
(Fig. 3). By day 2 of development the chromato
-
phores are concentrated on the testes and trach eal
trunks, with increasing numbers scattered through
-
out the thorax and segment 7 (Fig. 3). By day 3,
the chromatophores are either scattered among
the body tissues or have covered the posterior
4/5 of the testes and the portion of the vasa
deferentia anterior to where they abut. There is
no significant change in the position of the chro
-
matophores from day 3 until adult emergence on
day 5 (Fig. 3).
Female pupa: Upon emergence the thoracic and
abdominal chromatophores are concentrated in
four groups between the lateral tracheal trunks,
in the areas where the two anterior air sacs and
two posterior air sacs of the larva were previously
located (Fig. 3). Some of the thoracic and ab
-
dominal chromatophores are scattered among the
© 2008 Entomological Society of Canada
Borkent and Borkent 635
Fig. 3. Diagram of the distribution of chromatophores
in male and female pupae of Chaoborus americanus of
increasing age (the respiratory organ is 0.92 mm long).
body tissues. By day 2 of development the chro
-
matophores are more widely scattered in the
thorax and segment 7, but there is still a con
-
centration where the thoracic air sacs of the lar
-
vae were located. By day 3 the chromatophores
are scattered among the body tissues, where
they remain until adult emergence.
Adult male: Black chromatophores cover the
posterior 4/5 of each testis and the anterior 1/2
of each vas deferens (anterior to where the two
vasa deferentia abut) (Fig. 2; Borkent et al.
2008). Other chromatophores are scattered
throughout most of the body tissues.
Adult female: Chromatophores are scattered
throughout most of the body tissues, with no
obvious concentration.
Remarks: It was only in males that many of the
chromatophores moved from the larval air sacs
to the tracheal trunks of the pupae, and then to
the testes and vasa deferentia, or, in the case of
those on the posterior larval air sacs, directly to
the testes. Other chromatophores dispersed
among the body tissues. There was no concen-
tration of chromatophores in female pupae dur-
ing development from larva to adult.
We followed the development of the chro-
matophores in C. americanus and C. trivittatus
and found no differences.
Phylogenetic interpretation
The chromatophores on the air sacs of male
Chaoborus larvae move upon pupation to the
tracheal trunks, testes, and vasa deferentia, or
disperse among the body tissues. At least some of
those on the pupal tracheal trunks continue to
move and become part of the pigmented layer
covering the testes and (or) vasa deferentia. The
chromatophores on the air sacs of female larvae
disperse randomly among the body tissues of
the pupae (Fig. 3).
The situation in Mochlonyx is similar, but the
male testes are already pigmented in the larva,
and have a thin layer of fat body. The more
lightly pigmented, sparse chromatophores on the
air sacs of Mochlonyx larvae a ppear to disperse
in a similar manner to those of Chaoborus,and
in pupae, numbers cover the testes and develop
-
ing vasa deferentia (Fig. 2). The testes of adult
male Mochlonyx are therefore pigmented with a
combination of fat body that was originally on
the testes of the larvae and chromatophores that
were on their air sacs.
The testes of male Eucorethra larvae are pig
-
mented with a thin layer of fat body that ulti
-
mately develops into a thick layer of fat body
on the anterior of the testes of the adult, as in
many Culicidae (see character 3 below). All
adult male Culicidae have a thick layer of fat
body adhering to at least a portion of their tes
-
tes (Hodapp and Jones 1961).
Character analysis
The character states in Mochlonyx form a
nearly perfect intermediate state between those
observed in many Culicidae + Eucorethra and
those found in Chaoborus.
The presence of chromatophores in larvae and
their developmental fate in adult male and adult
female Eucor ethra, Mochlonyx,andChaoborus,
described abov e, are summarized in Figure 2.
These features are interpreted cladistically as
follows; the character numbers are shown on the
cladogram in Figure 4. The relationship between
the monophyletic Cryophila, Australomochlonyx,
Promochlonyx,andMochlonyx has been dis
-
cussed by Sæther (1992).
1. Fourth-instar larva with simple tracheal sys
-
tem with each non-bifurcating portion of
uniform diameter and lacking associated chro
-
matophores (plesiomorphic); with one pair of air
sacs in thorax and another pair in abdominal
segment 7, all bearing at least some chromato
-
phores (apomorphic).
This feature groups five of the six genera of
Chaoboridae and is unique within Diptera.
Colless (1986) indicated that the larva of
Promochlonyx did not have chromatophores on
the air sacs, perhaps because of a postmortem
change. It should be noted that even in well-
preserved Mochlonyx larvae the chromatophores
may be diff icult to discern. Cryophila larvae are
known to hav e chromatophores on the air sacs
(Sæther 1992), as do A ustr alomoc hlonyx larvae,
© 2008 Entomological Society of Canada
636 Can. Entomol. Vol. 140, 2008
Fig. 4. Phylogeny of genera of Chaoboridae. The
numbers refer to character states discussed in the text.
though in the latter case this has not been re
-
ported in the literature (Ogawa 2004).
Although restricted here to fourth-instar lar
-
vae, air sacs are present in all earlier instars of
the group with the derived state. Chromato
-
phores are present at least in third-instar larvae
of Mochlonyx and Chaoborus. Finally, the chro
-
matophores are far more numerous and darker
in Chaoborus than in any other genus of
Chaoboridae and this is a further synapomorphy
of this distinctive genus.
Eucorethra larvae have individual chromato
-
phores scattered throughout their bodies. This
feature appears to be unique and is therefore
potentially an autapomorphy of this monotypic
genus. Alternatively, it is possible that these
chromatophores are precursors of the chromato
-
phores in Chaoborinae and therefore represent an
intermediate derived state. No chromatophores
are associated with the trachea in fourth-instar
larvae of Dixidae (Dixella sp.), Corethrellidae
(C. appendiculata), and Culicidae (Anopheles,
Aedes Meigen), but may be scattered among
their body tissues. Hinton (1958, 1959) noted
sheets of mobile chromatophores one cell thick
just under the epidermis of the thorax and abdo-
men of larvae and pupae of Thaumaleidae and
Simuliidae, but it is not known whether these
chromatophores are homologous to those pres-
ent on the tracheal system of Chaoborinae.
Larval air sacs have been described in the
chaoborids Chironomaptera collessi Jell and
Duncan and both species of the fossil genus
Chachotosha Lukashevich from the Lower Cre
-
taceous (120–144 mya) (Jell and Duncan 1986;
Lukashevich 1996), showing that air sacs origi
-
nated before that time. It is uncertain whether
chromatophores were present on the fossil air
sacs.
2. Fourth-instar larvae without chromatophores
associated with trachea (plesiomorphic); with
chromatophores on air sacs, which disperse to en
-
case testes and vasa deferentia of male pupa
(apomorphic).
The migration of chromatophores from the
air sacs of the larva to the testes and vasa defer
-
entia of the developing male pupa is a unique
developmental pathway and a strong indication
that, of the genera studied here, Mochlonyx
and Chaoborus are sister genera. It will be im
-
portant to study the male genital tract of other
genera closely related to Mochlonyx.Ifour
interpretation here is correct, males of Cryophila,
Austr alomoc hlonyx,andPromochlonyx also have
the derived condition or some further modifica
-
tion of it.
Male larvae of Mochlonyx, Eucorethra,and
most Culicidae (including Anopheles) hav e darkly
pigmented testes (owing to the presence of fat
body) that allow them to be sexed in at least the
fourth instar (female larvae have unpigmented
ovaries; Warren and Breland 1963; Var gas 1968).
The pigmentation is due to the reddish brown
layer of fat body on the anterior portion of the
testes in adult Culicidae and Eucorethra.In
Mochlonyx, the pigmented layer of the larval
testes remains on the testes but appears in the
adult as a thin covering and lacks the appearance
of fat body. In Mochlonyx some of the air-sac
chromatophores add to the pigmentation of the
testes (as well as covering the vasa deferentia).
Mochlonyx is therefore unique in that pigmen
-
tation of the adult testes appears to have two
different origins; however, though this condi
-
tion is unique, we consider it to be intermedi
-
ate between Eucorethra and Chaoborus (see
character 3).
Some chromatophores on the air sacs of
Chaoborus larvae disperse, upon pupation, to
the tracheal trunks and among various body tis-
sues (producing adventitious spotting in adults).
These unique features may also be considered
either separate synapomorphies of Chaoborinae
or part of a single developmental phenomenon
associated with the dispersal of chromatophores
to the male testes and vasa deferentia.
3. Fourth-instar male larva with pigmented
testes (plesiomorphic); with transparent testes
(apomorphic).
Of the three chaoborid genera studied here,
only Chaoborus has pale larval testes. The char
-
acter states are unrecorded for the chaoborine
genera Cryophila, Australomochlonyx,and
Promochlonyx. The plesiomorphic condition is
present in Mochlonyx, Eucorethra, and many (in
-
cluding Anopheles) but not all Culicidae (Adie
1912; Langeron 1926; Jones 1957; Warren and
Breland 1963; Vargas 1968). Both pigmented
and unpigmented larval testes are present in
Simuliidae (Adler et al. 2004), and larvae of
Chironomidae have pale testes, indicating that
the feature is susceptible to homoplasy. The con
-
dition is unknown in the larvae of Corethrellidae,
Dixidae, and Ceratopogonidae.
The possession of pale testes by Chaoborus
larvae is likely related to their remarkable
an d uniquely transparent body in which virtu
-
ally every structure has become pale and (or)
© 2008 Entomological Society of Canada
Borkent and Borkent 637
transparent, with the exception of the preco
-
cio us adult eye, mandibular teeth, air sacs with
their accompanying chromatophores, and the
gut system when larvae have been feeding. Fur
-
thermore, the pigmentation of the larval testes
in Mochlonyx, Eucorethra, and many Culicidae
is likely to provide protection against ultravio
-
let (UV) radiation. Chaoborus larvae migrate
diurnally into opaque bottom detritus (in shal
-
low lentic habitats) or to the depths of lakes,
thereby obtaining protection from daytime UV
radiation, which would otherwise harm them
(Persaud and Yan 2003, 2005; Persaud et al.
2003). The suggested relationship between
pigmented larval testes and exposure to light is
supported by a parallel situation in Simuliidae,
where pigmented larval testes are more promi
-
nent in species that live in exposed streams
(P.H. Adler, personal communication).
Discussion
Our investigation does not allow us to suggest
the evolutionary origin of the chromatophores
present on the tracheae of Chaoborinae, nor the
nature of the precursor of the mechanism by
which chromatophores move from the larval tra-
cheal system to the pupal male genital tract. It
would be interesting to study the chromatophores
of larvae of Eucorethra and other Culicoidea
further in this regard.
The function of the chromatophores on the
larval air sacs of Chaoborinae has not been ex
-
perimentally investigated. However, behaviour
and habitat suggest that those present on the lar
-
val air sacs of Chaoborus may provide counter
shading or a change in the spectrum of reflected
light (Horppila and Nurminem 2007) in order to
alleviate predation pressures, especially from fish
(Giguère and Northcote 1987). The chroma
-
tophores in Chaoborus larv ae are much darker
and more abundant than those in larv ae of the
other genera of Chaoborinae, which occur only
in ponds and marshes. Giv en the ability of the
chromatophores to respond to light, it is possible
that the innervated chromatophores also function
as secondary photoreceptors, aiding in light/UV
detection for timing daily migrations or reducing
UV-induced mortality (Leech and Williamson
2000; Boeing et al. 2004). Although they may
fulfill these functions in Chaoborus larvae,
the pre se nce of sparse medium-brown chro
-
matophores on the air sacs of other genera of
Chaoborinae with semi-transparent larvae re
-
stricted to small, fishless lentic habitats suggests
that the original selection pressure to have tracheal
chromatophores was not related to protection from
predation or UV radiation. It may be that physio
-
logical restrictions necessitate the production of
larval chromatophores solely as precursors of
protective pigment cells on the testes (and on
the spermathecae in at least Mochlonyx).
The migration of the chromatophores to the
adult testes likely protects the adult gametes from
UV radiation, but it is unclear why Mochlonyx
and Chaoborus (and likely other Chaoborinae) re
-
quire this feature when outgroup members do not
have it. Adult Chaoboridae are not long-lived
(compared with Culicidae) and appear to emerge
with mature spermatozoa. Perhaps the rapid mi
-
gration of the chromatophores to the male genital
tract shortly after pupation affords them protec
-
tion during spermatogenesis at that stage (the tim
-
ing of spermatogenesis is not known). Pupae of
Chaoborinae liv e closer to the water surface than
their larv ae and are therefore likely more suscep
-
tible to UV radiation. Chaoborus pupae remain in
the upper layer of lakes or ponds during the day,
and those of other Chaoborinae breathe directly at
the water surface.
Previous authors have noted the adv entitious
spotting in adults of various Chaoborus species
(Lane 1953; Cook 1956; Colless 1986, p. 5).
Our study sho ws that this is derived from the
chromatophores originally present on the air sacs
of the larvae. The adults of some Chaoborus
species are more heavily spotted than others
(Lane 1953; A. Borkent, personal observation),
suggesting some adaptive advantage to such
spotting. The movement of chromatophores in
Chaoborinae initially seemed unusual but is known
to occur elsewhere; Hinton (1958, 1959) noted it
in pupae of Thaumaleidae and Simuliidae.
Biologists familiar with old literature know that
there are gems of information hidden there. Late
in this study we realized that Meinert (1886), in a
publication with remarkably detailed and accurate
illustrations, identified a patch of pigment cells in
the abdomen of a Chaoborus pupa as “cellules de
pigment des sacs à air postérieurs rejetés”. He
was, therefore, the first author to recognize that
the larval chromatophores were carried through to
the pupa. It was also previously noted by
Akehurst (1922) that the larval pigment cells of
Chaoborus crystallinus (DeGeer)movedtothe
tracheal trunks upon pupation.
There is one final conclusion that can be made.
This study originated as a Grade 8 science-fair
project when the first author was 13 years old
it was then that the migration of chromatophores
© 2008 Entomological Society of Canada
638 Can. Entomol. Vol. 140, 2008
from the larval air sacs to the pupal testes was
first reported. At a time when science is often
perceived as requiring the use of sophisticated
and expensive methods, a great deal can still be
accomplished using little more than straightfor
-
ward and careful observation and, for entomol
-
ogists, a microscope.
Acknowledgments
We express our thanks to B.J. Sinclair, D.A.
Craig, and two anonymous reviewers for critical
reviews of this paper and to V. Lévesque-
Beaudin for the French translation of the ab
-
stract. The first author thanks the judging com
-
mittee of the 1991 J.L. Jackson Junior Secondary
Science Fair for their recognition of the initial
stages of this work. The second author thanks
his wife, Annette, for continued support for this
and other studies of dipteran systematics.
References
Adie, H.A. 1912. Distinction of sex in the larval and
pupal stages of Anophelines. Paludism, 1912: 41.
Adler, P.H., Currie, D.C., and Wood, D.M. 2004.
The black flies (Simuliidae) of North America.
Cornell University Press, Ithaca, New York.
Akehurst, S.C. 1922. Larva of Chaoborus
crystallinus (De Geer) (Corethra plumicornis F.).
Journal of the Royal Microscopical Society, 15:
341–372.
Allison, E.H., Irvine, K., and Thompson, A.B. 1996.
Lake flies and the deep-water demersal fish com
-
munity of Lake Malawi. Journal of Fish Biology,
48: 1006–1010.
Boeing, W.J., Leech, D.M., Williamson, C.E., Cooke,
S., and Torres, L. 2004. Damaging UV radiation
and invertebrate predation: conflicting selective
pressures for zooplankton vertical distribution in
the water column of low DOC lakes. Oecologia,
138: 603–612.
Borkent, A. 1993. A world catalogue of fossil and
extant Corethrellidae and Chaoboridae (Diptera),
with a listing of references to keys, bionomic in
-
formation and descriptions of each known life
stage. Entomologica Scandinavica, 24: 1–24.
Borkent, A., Borkent, C.J., and Sinclair, B.J. 2008.
The male genital tract of Chaoboridae (Diptera:
Culicomorpha). The Canadian Entomologist, 140:
621–629.
Colless, D.H. 1986. The Australian Chaoboridae
(Diptera). Australian Journal of Zoology, Supple
-
mental Series, 124: 1–66.
Cook, E.F. 1956. The Nearctic Chaoborinae
(Diptera: Culicidae). Technical Bulletin of the
University of Minnesota Agricultural Experiment
Station, 218: 1–102.
Gersch, M. 1956. Untersuchungen zur Frage der
Hormonalen Beeinflussung der Chromatophoren
bei der Corethra-Larve. Zeitschrift für
vergleichende Physiologie, 39: 190–208.
Giguère, L.A., and Northcote, T.G. 1987. Ingested
prey increase risks of visual predation in transpar
-
ent Chaoborus larvae. Oecologia, 73: 48–52.
Hadorn, E., and Frizzi, G. 1949. Esperimentelle
Untersuchungen zur Melanophorne Reaktion von
Corethra. Revue Suisse de Zoologie, 56: 306–315.
Hinton, H.E. 1958. On the nature and metamorpho
-
sis of the colour pattern of Thaumalea (Diptera,
Thaumaleidae). Journal of Insect Physiology, 2:
249–260.
Hinton, H.E. 1959. The function of chromatocytes in
the Simuliidae, with notes on their behaviour at
the pupa–adult moult. Quarterly Journal of Micro
-
scopical Science, 100: 65–71.
Hodapp, C.J., and Jones, J.C. 1961. The anatomy of
the adult male reproductiv e system of Aedes ae gypti
(Linnaeus) (Diptera, Culicidae). Annals of the
Entomological Society of America, 54: 832–844.
Horppila, J., and Nurminem, L. 2007. The intensity
and spectral composition of upwelling light in re
-
lation to the density of Chaoborus flavicans
swarms. Fundamental and Applied Limnology
(Archiv für Hydrobiologie), 169: 259–263.
Jell, P.A., and Duncan, P.M. 1986. Invertebrates,
mainly insects, from the freshwater, Lo wer Creta-
ceous, Koonwarra fossil bed (Korumburra Group),
South Gippsland, Victoria. Memoir of the Associa-
tion of Australasian Palaeontologists, 3: 111–205.
Jones, J.C. 1957. A simple method for sexing living
Anopheles larvae (Diptera, Culicidae). Annals of
the Entomological Society of America, 50: 104–
106.
Lane, J. 1953. Neotropical Culicidae. Vol. 1. Univer
-
sity of São Paulo, São Paulo, Brazil.
Langeron, M. 1926. Sexualité des larves de
moustiques. Annales de Parasitologie Humaine et
Comparée, 4: 126–135.
Leech, D.M., and Williamson, C.E. 2000. Is toler
-
ance to UV radiation in zooplankton related to
body size, taxon, or lake transparency? Ecological
Applications, 10: 1530–1540.
Lukashevich, E.D. 1996. New chaoborids from the
Mesozoic of Mongolia (Diptera: Chaoboridae).
Paleontologicheskii Zhurnal, 1996(4): 55–60. [In
Russian with English summary; translation in
Paleontological Journal, 30: 551–558.]
Meinert, F. 1886. De eucephale Myggelarver.
Videnskabernes Selskab Skrifter, 6: 373–493.
Ogawa, J. 2004. Chaoborids [online]. Available from
http://oregonstate.edu/~ogawajo/chaoborid.html
[accessed 14 May 2008].
Persaud, A.D., and Yan, N.D. 2003. UVR sensitivity
of Chaoborus larvae. Ambio, 32: 219–224.
Persaud, A.D., and Yan, N.D. 2005. Developmental
differences and a test for reciprocity in the toler
-
ance of Chaoborus punctipennis larvae to ultraviolet
© 2008 Entomological Society of Canada
Borkent and Borkent 639
radiation. Canadian Journal of Fisheries and Aquatic
Sciences, 62: 483–491.
Persaud, A.D., Arts, M.T., and Yan, N.D. 2003. Photo
-
responses of late instar Chaoborus punctipennis lar
-
vae to UVR. Aquatic Ecology, 37: 257–265.
Sinclair, B.J., Borkent, A., and Wood, D.M. 2007.
The male genital tract and aedeagal components
of the Diptera with a discussion of their phylogen
-
etic significance. Zoological Journal of the Lin
-
nean Society, 150: 711–742.
Sæther, O.A. 1992. Redescription of Cryophila
lapponica Bergroth (Diptera: Chaoboridae) and
the phylogenetic relationship of the chaoborid
genera. Aquatic Insects, 14: 1–21. (Addendum:
Aquatic Insects, 14: 193–194)
Teraguchi, S.E. 1972. Regulation of buoyancy by
Chaoborus americanus (Joh.). Ph.D. thesis, Uni
-
versity of Wisconsin, Madison, Wisconsin.
Teraguchi, S. 1975. Correction of negative buoyancy
in the phantom larva, Chaoborus americanus.
Journal of Insect Physiology, 21: 1659–1670.
Vargas, M. 1968. Sexual dimorphism of the larvae
and pupae of Aedes aegypti (Linn.). Mosquito
News, 28: 374–379.
Warren, M.E., and Breland, O.P. 1963. Studies on the
gonads of some immature mosquitoes. Annals of the
Entomological Society of America, 56: 619–624.
Weber, W., and Grosmann, M. 1988. Ultrastructure
of the chromatophores system on the tracheal
bladders of the phantom larva of Chaoborus
crystallinus (Insecta, Diptera). Zoomorphology,
108: 167–171.
Wemhöner, K., and Weber, W. 1974. Innervation des
Tracheenblasenepithels bei der Büschelmücke
Corethra plumicornis (Chaoborus). Experientia,
30: 1076–1077.
© 2008 Entomological Society of Canada
640 Can. Entomol. Vol. 140, 2008
... In all thirty-seven specimens observed the TO totally restricted the release of hemolymph, tissue or gas from the siphon to the exterior. In several specimens (larvae, 5,22,24,26,27,28,32), gas bubbles are found in the siphon but were stopped from movement beyond the TO (Fig. 4, Supplement Data File). ...
... The consideration that the siphon plays only a vestigial role in respiration is not without precedent. Corethra (also called midge and of the family Chaoboridae) were classified as mosquitoes until the early 1960s 22 ; today, they are considered taxonomically separate but are thought to share a common ancestor 21 . Corethra and mosquito larvae share common physiological traits, and many species look very much alike, including the presence of an apparent larval siphon [21][22][23][24][25][26] . ...
... Corethra (also called midge and of the family Chaoboridae) were classified as mosquitoes until the early 1960s 22 ; today, they are considered taxonomically separate but are thought to share a common ancestor 21 . Corethra and mosquito larvae share common physiological traits, and many species look very much alike, including the presence of an apparent larval siphon [21][22][23][24][25][26] . Krogh found that larvae of the genus Corethra appeared to respire through only the skin and concluded that this organism fills its air sacks with gas from a non-atmospheric source (i.e., tissues) 27 . ...
Article
Full-text available
Acoustic larviciding (AL) occurs by exposing mosquito larvae to acoustic energy that ruptures their dorsal tracheal trunks (DTTs) by the expulsion of gas bubbles into the body. In studying this technique, we serendipitously identified undescribed anatomical and physiological respiratory features. The classical theory of respiration is that the siphon and DTTs play obligate roles in respiration. Our results contradict the accepted theory that culicine larvae respire via atmospheric gas exchange. We identified an undescribed tracheal occlusion (TO) at the posterior extremities the DTTs. The TOs appear necessary for the acoustic rupture of DTTs; this constriction prevents the escape of energized gas from the siphon and allows the tracheal system to be pressurized. With a pressurized isolated tracheal system, metabolic gas exchange directly with the atmosphere is unlikely and could mostly occur through the chitin and setae. Future studies are needed to explore respiration and elucidate the mechanisms of oxygen absorption and carbon dioxide elimination.
... However, Corethrellidae do not appear to have a layer of fat body associated with the testes (McKeever 1985). The testis of M. velutinus is covered by a combination of a thin layer of fat body and chromatophores, whereas that of C. trivittatus is covered with chromatophores only (Borkent and Borkent 2008), and these extend at least partially along each vas deferens. Although some other nematocerous Diptera also have dark testes (e.g., some Anopheles species), the chromatophores encasing the testes of species of Mochlonyx and Chaoborus have a unique developmental pathway and this is a synapomorphy of the two genera (Borkent and Borkent 2008). ...
... The testis of M. velutinus is covered by a combination of a thin layer of fat body and chromatophores, whereas that of C. trivittatus is covered with chromatophores only (Borkent and Borkent 2008), and these extend at least partially along each vas deferens. Although some other nematocerous Diptera also have dark testes (e.g., some Anopheles species), the chromatophores encasing the testes of species of Mochlonyx and Chaoborus have a unique developmental pathway and this is a synapomorphy of the two genera (Borkent and Borkent 2008). The attachment of the anterior end of the accessory gland to the posterior portion of the testis (rather than to the vas deferens) in C. trivittatus is unique in Culicoidea and is therefore derived. ...
Article
Full-text available
La région génitale mâle de Chaoboridae, représentée par Eucorethra underwoodi Underwood, Mochlonyx velutinus (Ruthe), et Chaoborus trivittatus (Loew), est décrite pour la première fois. Tous les genres ont des glandes accessoires appariées qui sont attachées antérieurement au canal déférent ou à la base des testicules, un trait qui est proposé en tant que synapomorphie de Chaoboridae + Culicidae. Mochlonyx Loew et Chaoborus Lichenstein ont des cellules pigmentées distinctives recouvrant leurs testicules et une partie du canal déférent. La région génitale mâle simplifiée de Corethrellidae + Chaoboridae + Culicidae est corrélée à l'unique rotation de 180°, abrupte et permanente, des organes génitaux masculins entre les segments 7 et 8. Dans les taxa avec un complexe de glandes accessoires, les organes génitaux mâles sont tournés d'une manière plus graduelle, souvent pendant la reproduction.
... Chaoborus albipes was redescribed by Felt (1904) and Richardson (1912) soon after its initial description; the tergal pattern in both publications is consistent with the current concept of C. albipes. Lateral specks may also be partly due to the dispersion of chromatophores from the larval air sacs to the tissues of adult flies (Borkent & Borkent 2008), but it is evident that the abdominal patterns in C. albipes (especially eastern Nearctic populations) and C. flavicans are specific (Fig. 5). Sayomyia rotundifolia is treated as a new junior synonym of C. albipes. ...
Article
Full-text available
Chaoborus flavicans (Meigen) is a widespread and much studied lacustrine phantom midge. As larvae, these insects are important aquatic predators. Based on the available type material, morphology of immature stages and adults, their aquatic habitat, and DNA barcodes, C. flavicans is shown to be a composite of at least four species, with three of these named here. Chaoborus flavicans is primarily a lake-dwelling species with a Holarctic range. Chaoborus albipes (Johannsen, 1903 stat. rev.) and C. posio Salmela sp. n. are pond-dwelling Holarctic and north European species, respectively. The position of the larval subordinate mandibular tooth at the vertex of the second and fourth teeth is a synapomorphy of the Chaoborus flavicans species complex. We present an identification key to fourth instar larvae, pupae, and adult males. We also designate the lectotype and paralectotypes of Sayomyia rotundifolia Felt, 1904 (syn. nov. of C. albipes). We hypothesize that a fourth species of the species complex is present in Japan. Our revision indicates that Holarctic shallow ponds contain a hidden diversity of predators (C. albipes and C. posio sp. n.).
Article
Full-text available
Solar ultraviolet radiation (UVR) has been demonstrated to have damaging effects on zooplankton, but little is known about what factors influence UVR tolerance in nature. Here we examined the relationship between UVR tolerance (the sum of photopro- tection and photorepair processes) and zooplankton taxon, body size, and source lake UVR transparency. Zooplankton of various sizes and taxa from lakes of different UV transparency were exposed to different intensities of a constant artificial UVR source. UVR tolerance was expressed as the UVR dose at which 50% mortality was observed for a given species. Smaller zooplankton species showed a uniformly high UVR tolerance, while larger zoo- plankton varied in their UVR tolerance both among and within species. The smaller rotifers, Keratella in particular, showed a high UVR tolerance while the larger, more transparent rotifer (Asplanchna) showed an intermediate UVR tolerance. Both cyclopoid and calanoid copepod adults were more highly tolerant of UVR than nauplii. Late-instar larvae of the predatory insect Chaoborus were more UVR tolerant than earlier instars. UVR tolerance showed no relationship to the UVR transparency of the source lake. Differential UVR tolerances among zooplankton taxa may alter community and ecosystem structure and function during anticipated changes in underwater UVR environments.
Article
In small temperate lakes, predation by fish generally regulates the species structure and abundance of larval Chaoborus. Yet, Chaoborus abundance may also vary appreciably among lakes with no fish. Many fishless lakes in Sudbury, Ontario, have transparent waters. This raises the possibility that low abundance of Chaoborus in such lakes may be attributable to UVR-induced mortality. To determine whether UVR affects Chaoborus survival, we performed 6 in situ experiments over 2 to 4 day periods at 4 depths in Ruth-Roy Lake (a clear fishless lake with few Chaoborus). Third and fourth instar C. punctipennis were randomly allocated to 3 treatments: quartz (UVR+PAR), OP3 acrylite (PAR only) and dark controls. Survival under UVR+PAR was significantly reduced in comparison with the other treatments. Survival under PAR only was high, and did not differ from the dark controls. Time to death increased with incubation depth and larval stage. These results suggest that the small Chaoborus population in Ruth-Roy Lake, and perhaps in other fishless, clear lakes may be attributed to UVR-induced mortality.
Article
Two new chaoborid species of the extinct genus Chachotosha gen. nov. are described based on features of all the life stages. In numerous (especially larval) characters the new genus is close to the recent Chaobonis Lichtenstein, 1800. Preimaginal stages of fossil chaoborids were for the first time studied using a scanning electron microscope.
Article
The Lower Cretaceous Koonwarra Fossil Bed, encountered in a road cutting S of Leongatha on the S Gippsland Highway, yielded >80 species of invertebrates. >70 are insects, associated with an ostracode, a syncarid (Koonaspides indistinctus gen. et sp.nov.), an anostracan and a cladoceran among the Crustacea, 2 spiders and a harvestman among the arachnids, possible earthworms, bryozoan statoblasts and a bivalved mollusc. The insects represent 12 orders dominated by the Hemiptera, Coleoptera and Diptera in terms of diversity but dominated numerically by aquatic immature Ephemeroptera and Diptera; Odonata, Blattodea, Plecoptera, Orthoptera, Psocoptera, Mecoptera, Trichoptera, Hymenoptera and probably Siphonaptera are lesser components. Insects are identified specifically and described taxonomically. However, the majority of the fauna is left in open nomenclature because features of generic or specific importance among living relatives are not clearly preserved on the fossils. The emphasis is on immature stages of the insect. The palaeoenvironment indicated by the invertebrate fauna is a shallow, semi-isolated body of water marginal to a shallow freshwater lake with periodic, probably seasonal, replenishing of the fauna from the lake and periodic mass-mortality of the isolated fauna; the mechanism for this mortality remains uncertain. A present-day environment in Lake Muirhead just E of the Grampians in W Victoria is suggested as possibly comparable to the fossil setting. -from Authors
Article
With three plates (figs. 2-4) SUMMARY In the larvae and pupae of the Simuliidae the cuticle and epidermis of the thorax and abdomen are more or less transparent, and the colour pattern is formed by cells (chromatocytes) that contain pigment granules..These cells always lie below the base-ment membrane. It is possible to photograph the chromatocytes without damage to the animal and thus to make photographic records of the behaviour of particular chromatocytes over a period of many days when the animal is moving about and feed-ing in the normal way. The chromatocytes accumulate lipids. The accumulation of lipids during larval growth and their depletion during adult development has been photographed in normal undamaged animals. Conspicuous changes in the colour pattern, especially of the thorax, occur at meta-morphosis. Such changes are due to mass migrations and the formation of new aggregation patterns by the chromatocytes. While such movements of the chromato-cytes are taking place in some parts of the body, the aggregation patterns of chroma-tocytes in other parts of the body remain unchanged and appear to be unaffected by the events that initiate and accompany moults and ecdyses.