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MISSING STICKMAN FOUND: THE FIRST
MALE OF THE PARTHENOGENETIC
NEW ZEALAND PHASMID GENUS
ACANTHOXYLA UVAROV, 1944
DISCOVERED IN THE UNITED KINGDOM
by Paul D. Brock, Malcolm Lee, Mary Morgan-Richards &
Steven A. Trewick
INTRODUCTION
Three species of New Zealand phasmids have become naturalised in the UK: Prickly Stick-
insect Acanthoxyla geisovii since Edwardian times, Unarmed Stick-insect Acanthoxyla inermis
since the 1920s, and Smooth Stick-insect Clitarchus hookeri since the 1940s. Smooth Stick-
insect is only found on the Isles of Scilly, but the two Acanthoxyla species are found in
localised populations in the south-west of England, principally within Cornwall and Devon.
Around 250–300 reports of stick insects a year are now being received by Malcolm Lee, the
national recorder, almost exclusively through the online reporting system set up by the
Phasmid Study Group. Many of these are now also accompanied by an image enabling
ready confirmation of the species. There are now thought to be just three species of
Acanthoxyla in New Zealand following synonymy of several morphological forms (Brock &
Jewell, 2015), but males have never been recorded in the genus. The whole genus is
considered to be of hybrid origin and contains both diploid (with two copies of every
chromosome) and triploid (with three copies of every chromosome) lineages (Morgan-
Richards & Trewick, 2005; Buckley et al., 2008; Myers et al., 2013). In the light of this, males
have been assumed not to exist and all populations are regarded as parthenogenetic, that is
females lay viable eggs without the need for fertilisation by a male. Males of the similar
looking Smooth Stick-insect are not uncommon in New Zealand. They may account for up
to 50% of insects in some populations, but are absent in other populations which must breed
parthenogenetically (Morgan-Richards et al., 2010). No male Smooth Stick-insect has ever
been recorded in the UK population, which is therefore regarded as wholly parthenogenetic.
THE FIND
On 6 October 2016 a report was received from David Fenwick of a stick insect found that
morning on the side of his partner’s car outside his home in Heamoor, in the north-western
part of Penzance, Cornwall. David had sent an earlier report to ML in 2013 with a clear
image of a tiny stick insect found on his bathroom ceiling, which enabled it to be identified
as a recently hatched Unarmed Stick-insect nymph. He has subsequently come across other
nymphs within his house. The image accompanying this recent report was taken side-on,
and showed a spindly looking 75mm long phasmid, with the end of the abdomen being
quite a different shape to any of the naturalised stick insects previously seen. It was
assumed to have been an escaped or discarded individual from a nearby phasmid
enthusiast and was referred to PDB to see if he could identify the species. His quick
response was to wonder whether this was a eureka moment, and Acanthoxyla had finally
produced that missing male. A second email from David Fenwick included more close-up
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pictures of his phasmid, including dorsal views of the abdomen and head. When he saw
these images, PDB confirmed that, apart from a more spindly appearance and a slightly
more extended black line along the top of the abdomen, this stick insect had everything to
match female Unarmed Stick-insect, including red beneath the forelegs and spines beneath
mid and hind femora. However, this was definitely a male with genitalia broadly similar
to known males of Smooth Stick-insect.
A trip was taken by ML to Heamoor to check out the area around David Fenwick’s
home for more phasmids. Brambles Rubus spp. and other likely foodplants in nearby
hedgerows and gardens were inspected for evidence of the distinctive feeding damage of
phasmids, but no damage was seen and no stick insects were found. David had put the
insect in a cage, so it was taken back to ML’s home at Port Gaverne. An extended check of
hedgerows in adjacent Port Isaac located a female and the two were put together in the
hope of a pairing. There was no evidence seen of any interaction but, sadly, the male died
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Figure 1. The above diagram shows the genetic similarity between individual Acanthoxyla stick-insects sampled
in New Zealand and the UK. Different colours represent different Acanthoxyla morphological ‘forms’ that were
sampled. All the ‘forms’ mentioned above are illustrated in Salmon (1991) and he considered them all to be
subspecies of just one valid species; A. prasina, rather than the current three valid species Black-spined Stick-insect
A. prasina, Prickly Stick-insect A. geisovii and Unarmed Stick-insect A. inermis of Brock and Jewell (2015). The
longer the line (branch) the more different are the DNA sequences represented by the coloured spots. The bigger
the spot the more individual stick-insects are represented. Some forms have the same DNA sequence and so spots
are subdivided. Two of the ‘forms’ (Unarmed Stick-insect and Prickly Stick-insect) are recorded in the UK, and we
found that the male (yellow) had the same sequence as a female Unarmed Stick-insect previously collected at
Mevagissey, Cornwall, in 2013. Genetic similarity was assessed by comparison of partial Cytochrome Oxidase I
mitochondrial DNA sequence.
the following day, as did the female a few days later. In average years, October is towards
the end of the annual life cycle for the naturalised phasmids so this was not unexpected.
The male was preserved and has been deposited in the national collection at the Natural
History Museum, London. As stick insects often do, it had shed a leg shortly after capture
and fortunately David had immediately placed it in ethanol to preserve it. This leg was
forwarded to New Zealand, where DNA sequencing was used to confirm with a high level
of confidence that it came from an individual whose mother was an Acanthoxyla (Figure
1). A fragment of the DNA from the maternally inherited mitochondrial genome was
sequenced and compared to DNA sequences from numerous stick-insect species. The male
specimen had a sequence identical to female Unarmed Stick-insect specimens collected
from the UK and very similar to many New Zealand collected individuals.
THE X CHROMOSOME AND MALES
In most stick insects, sex is determined by the number of X-chromosomes. Two X-
chromosomes creates a female, one X-chromosome results in a male. A simple mutation
during cell division can produce an egg with a missing X-chromosome, resulting in the
production of rare males from parthenogenetic females. Because many Acanthoxyla
individuals are triploid the accidental loss of a single X-chromosome is unlikely to result
in a male. Most of the Acanthoxyla individuals identified as diploid have had few or no
spines (Unarmed Stick-insect), similar to their putative paternal species (Smooth Stick-
insect). A mutation resulting in the loss of an X-chromosome in a diploid lineage is much
more likely to produce a son than a similar loss from a triploid individual, therefore it is
of no surprise that the first Acanthoxyla male should have had an Unarmed Stick-insect
mother.
DESCRIPTION OF MALE
The illustrations on pp. 19 & 20 highlight various features discussed in this section, with
the male images on the left and the corresponding female image on the right. These stick
insects come in green or brown colour forms in the female, and it just so happens the male
was brown and the female was the commoner green form.
Measurements: male: body length 75mm, head 3.6mm (width 2.2mm), antennae 28mm,
pronotum 3mm, mesonotum 14mm, metanotum 10.3mm (+ median segment 3.7mm),
femora: fore 22mm, mid 17mm, hind 21mm; tibiae: fore 2mm, mid 16mm, hind 20mm.
Cerci 1.7mm (Natural History Museum, London (NHMUK)).
Brown with bold black longitudinal stripe on pronotum, also black marks on abdominal
segments and at end of mesonotum/start of metanotum, otherwise only a faint indication
of a longitudinal line.
Head: Elongate, eyes brown. Laterally behind eyes and towards hind part of segment with
blackish marks. Antennae with 21 segments, basal segment elongate, segments 3 and 4
short. All segments hairy. Inner base of fore femora reddish.
Thorax: Pronotum shorter than head, with a bold central longitudinal black line. Depressed
towards the top of the segment and in the centre, forming an almost rectangular area,
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Adult Unarmed Stick-insect Acanthoxyla inermis (Photos: D. Fenwick (left) and P. Brock (right)).
Head and thorax of Unarmed Stick-insect Acanthoxyla inermis (Photos: D. Fenwick (left) and P. Brock (right)).
surrounded by a slightly raised area. Mesonotum and metanotum generally smooth, only
very sparsely granulated. Mesonotum almost five times the length of the pronotum and
the same length as the mesonotum and median segment. Upper part of mesonotum with
a hint of a black line, which reappears at base. Underside reddish.
Abdomen: Elongate. Black marks prominent at the end and start of the abdominal
segments 1‒8. Ninth abdominal segment very elongate, the swollen subgenital plate not
reaching half the length of the segment. Tiny hairs ventrally on the ninth segment. Basal
part of anal (tenth) segment hairy; segment rounded at tip. Claspers broad, each clasper
with five unequal stout inner black teeth. Cerci short and hairy, tapering slightly to
rounded tip.
Legs: Elongate. Mid and hind femora with pair of basal spines.
Note: This sex is similar in general appearance to the female, but much more slender
(normal in stick-insects). Unarmed Stick-insect is the only known Acanthoxyla species with
a bold black line on the pronotum. The male is easily distinguished from the often less
plain-looking Smooth Stick-insect male, as the latter has a more conspicuous longitudinal
black line running along the body, which is sometimes broken but always on the head. It
also has expanded eighth abdominal segments, lacking in Acanthoxyla.
PARTHENOGENESIS IN PHASMIDS
The most studied parthenogenetic Phasmid is the Indian Stick-insect Carausius morosus.
Numerous papers have been written about this species, all based on culture stocks first
introduced into Europe in 1898. Crotch (1970) stated, “If the Stick Insect had been a native
around Mount Ararat, Noah would have been hard put to it to find a pair of them for the
Ark! In no breeding culture does the male appear more than once in a thousand adults.
Some authorities give the ratio as nearer 1/10,000.” PDB reared two males in about 200
insects in the 1980s. Eggs were not subjected to the very high temperatures that can result
in males appearing, according to some researchers, but most rearers never see a male. It is
generally believed that the male is incapable of fertilisation, although there have been no
studies in India, where males may occur in some natural populations, as is the case in other
Carausius species. Italian researchers and others have examined parthenogenesis in detail,
in the mainly Mediterranean genus Bacillus, where males occur in some populations of B.
rossius (for example, Scali et al., 2003). Other captive-bred phasmids from various countries
reproduce parthenogenetically in the absence of males, but there have been no detailed,
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End of abdomen, dorsal view of Unarmed Stick-insect Acanthoxyla inermis (Photos: D. Fenwick (left) and P. Brock
(right)).
End of abdomen, lateral view of Unarmed Stick-insect Acanthoxyla inermis (Photos: D. Fenwick (left) and P. Brock
(right)).
End of abdomen, ventral view of Unarmed Stick-insect Acanthoxyla inermis (Photos: D. Fenwick (left) and P. Brock
(right)).
long-term studies. But
the genus Acanthoxyla
has been a mystery up
to now, uniquely with a
lack of male(s) in any
species, either in the
wild or in captivity.
The genus Acanthoxyla
appears to be of hybrid
origin, with the male of
Smooth Stick-insect
the putative parental
species, the female
unknown (Trewick et
al., 2008).
There are some
advantages to partheno -
genesis. A single insect,
or even a single egg,
transported to new
areas could soon lead
to a viable population able to exploit this fresh habitat. This will have been a prime factor
in the establishment of our UK populations. Such exploitation of new habitats is still
happening, as shown by the post-2000 range expansion of Unarmed Stick-insect. A
parthenogenetic population of the same size as one with equal number of males and females
may theoretically produce twice as many eggs. Of course, the eggs of parthenogenetic
insects may not be equally fertile, but a study on Smooth Stick-insect (Morgan-Richards et
al., 2010) showed only marginal differences, with 74% of eggs from parthenogenetic females
hatching out, compared to 85% for females from sexual populations.
In species where males occur only rarely, they may not be able to fertilize females, as
is the case with cultured Carausius morosus. As our male died before any interaction could
be observed, we do not know if it could take any part in fertilisation. The New Zealand
endemic Smooth Stick-insect exhibits a geographic split between sexual and
parthenogenetic populations, with sexual populations apparently restricted to New
Zealand’s North Island. No males have been found on South Island by recent researchers,
with a pinned male specimen in the Museum of New Zealand collected by J T Salmon in
1944 near Christchurch being the sole South Island male known. This could have come
about when cells in the egg of a parthenogenetic female lost an X chromosome during
division, since a single male hatched out of one of the 315 eggs from a parthenogenetic
female during the 2010 study by Morgan-Richards et al. That study also showed mated
females from sexual populations gave rise to eggs that hatched out in roughly equal male
to female ratio, but if such females were not allowed to mate, their unfertilized eggs gave
rise to all females. Where males from sexual populations were allowed to mate with
females from parthenogenetic populations, the vast majority that hatched out were females,
with males representing just 1 in 40 hatchlings.
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Smooth Stick-insect Clitarchus hookeri (male). Waiorongomai Valley, New
Zealand, 28 February 2016 (Photo: P. Brock)
THE HEAMOOR, PENZANCE COLONY
In the last 10 to 15 years, the Unarmed Stick-insect has undergone a massive surge in its
distribution, especially in Cornwall (see Figure 2). These stick-insects have little capacity
to distribute themselves, and such a spread is undoubtedly associated with this species
turning up within the grounds of several garden centres in Cornwall and Devon. As we
buy their plants and take them home, stick-insect eggs, or small nymphs may tag along
too. In Penzance there is an unsubstantiated report of stick-insect sightings in the town in
the late 1960s, but this could relate to Carausius morosus, so often kept, and then discarded,
by children. Such discards can occur anywhere in the UK. Apart from that anomalous one,
there are 39 verified stick-insect sightings in Penzance, including Heamoor, with the first
in 2003. The first for the Heamoor area itself was in 2010, so both dates are consistent with
the colonies being of recent introduction.
WHY THE UK?
It seems a puzzle why the first male Acanthoxyla should turn up in a small offshoot
population on the other side of the world from their native land. It may be that males are
to be found in New Zealand, but their rarity means they have yet to come to the notice of
researchers in this field. An indication of the likely rarity of this male comes from the
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Figure 2. Unarmed Stick-insect Acanthoxyla inermis tetrad distribution in Cornwall, Devon & Dorset (inset)
at 30 June 2015, showing original pre-2000 tetrads in red.
database kept by the national recorder. At the time of receiving David Fenwick’s report,
there were 2,646 separate phasmid records. Excluding records which came from the few
UK recorders who regularly search for them in the field, this leaves 2,343 reports that
represent truly random encounters by individuals who come across, typically, a single
insect in their homes or gardens. Of those reports, 1,019 were accompanied by an image of
what they had found. If we assume those images are of a single insect (around 98% are),
this suggests a rarity for this male of at least one in a thousand, a similar ratio to that
estimated for male Carausius morosus (Crotch, 1970). These masters of camouflage are so
difficult to spot, even to experts, that it is unlikely researchers in New Zealand have seen
anything like this number of Unarmed Stick-insect. Were a male Unarmed Stick-insect to
be within a colony of the similar Smooth Stick-insect it could easily be overlooked as a
male of the latter species.
No doubt this find will spur the search for Acanthoxyla males in New Zealand.
ACKNOWLEDGEMENTS
Our thanks go to David Fenwick for bringing this, so far unique, phasmid male to our
attention, and for taking the excellent close-up images whilst the insect was still alive.
REFERENCES
Brock, P.D. & Jewell, T., 2015. An updated checklist of New Zealand phasmids. Phasmid Study Group
Newsletter 134: 13–14.
Buckley, T.R., Attanayake, D., Park, D., Ravindran, S., Jewell, T.R. & Normark, B.B., 2008.
Investigating hybridization in the parthenogenetic New Zealand stick insect Acanthoxyla
(Phasmatodea) using single-copy nuclear loci. Molecular Phylogenetics and Evolution 48: 335–349.
Crotch, W., 1970. Rearing Stick Insects. London: AES Leaflet 30: 1–20.
Morgan-Richards, M., Trewick, SA., 2005. Hybrid origin of a parthenogenetic genus? Molecular
Ecology, 14: 2133–2142.
Morgan-Richards, M., Trewick, S.A. & Stringer, I., 2010. Geographic parthenogenesis and the
common tea-tree stick-insect of New Zealand. Molecular Ecology 19: 1227–1238.
Myers S.S., Morgan-Richards, M. Trewick, S.A., 2013. Multiple lines of evidence suggest mosaic
polyploidy in the hybrid parthenogenetic stick insect lineage Acanthoxyla. Insect Conservation and
Diversity 6(4): 537–548.
Salmon, J.T., 1991. The Stick Insects of New Zealand. Reed Books, Auckland, New Zealand.
Scali, V., Passamonti, M., Marescalchi, O., & Mantovani, B., 2003. Linkage between sexual and
asexual lineages: genome evolution in Bacillus stick insects. Biological Journal of the Linnean Society
79: 137–150.
Trewick, S.A, Morgan-Richards, M. & Collins, L.J., 2008. Are you my mother? Phylogenetic analysis
reveals orphan hybrid stick insect genus is part of a monophyletic New Zealand clade. Molecular
Phylogenetics and Evolution 48: 799–808.
Paul D. Brock¹, Malcolm Lee², Mary Morgan-Richards³ & Steven A. Trewick³
¹ Scientific Associate, Department of Life Sciences, Natural History Museum,
Cromwell Road, London, SW7 5BD
² National Phasmid Recorder, Gullrock, Port Gaverne, Port Isaac, Cornwall
³ Ecology Group, Institute of Agriculture & Environment, Massey University,
Private Bag 11-222, Palmerston North, New Zealand
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