Colour change stick insects

What controls the colour of the common mānuka stick insect?

Three stick insects . The same species but different colours living in different environments

When we see how well-camouflaged individual stick insects are, it is hard not to imagine they have chosen either their hiding place or their skin colour, with care.  But stick insects cannot see colour – they don’t know if they are green or brown.   The reason they are so hard to see is that their cousins who were visible have been eaten. The only ones left are the ones that were not seen by the warblers and silver eyes and chaffinches and tui and sparrows.

Some stick insects can change colour – as demonstrated by the Indian stick insect Carausius morosus (Mangelsdorf 1926), whose colour changes in response to light levels. The trick this insect uses is the movement of pigment granules in it’s skin (cuticle) that alter light absorption and light scattering (Umbers et al. 2014).

Other stick insects like the North American Timema walking sticks have their colour determined by their genes and variation among individuals within the same species is maintained by selection, recombination and mutation (Comeault et al. 2015; Villoutreix et al. 2020).   They cannot change colour to suit their environment.

So, what about rō Aotearoa – New Zealand stick insects?  No one knows for sure – but maybe you have an idea? I have made some observations of the common mānuka stick insect Clitarchus hookeri and think that genetics is involved. 

Colour variation in captivity suggests genetic rather than environmental differences explain this phenotypic diversity

Nymphs of the mānuka stick insect Clitarchus hookeri showing the range of colours a single individual can exhibit at different instars

In boxes at home I allowed green Clitarchus hookeri indivudals from Waikato to mate with brown individuals from Whanganui. I collected the eggs that each female dropped and stored them for a few months until the little stick insects hatched. Nymphs were housed in sibling groups and fed mānuka (Leptospermum scoparium) and climbing rātā (Metrosideros perforate). 

All my Clitarchus hookeri offspring hatched as green nymphs. Many remained green as they grew and moulted but some changed colour (see figure). Two males changed to brown when about 30mm long, then speckled grey, before moulting to green as adults, their sisters remained green as they grew. Two females changed to brown when about 25mm long, then pale brown. One matured as a green individual, the other pale brown. Of the twelve stick insects I raised until they were adult eight remained green and four changed colour but only one was still brown when adult. In the next generation, produced by crossing siblings, the similar observations were made; some F2 individuals remained green as they grew, some changed to brown when about 30mm before changing to green as adults.

My observations of the colour of F1 offspring provide preliminary data suggesting that the colour of C. hookeri individuals is genetically determined rather than environmental as nymphs were raised in a common environment. However, I also observed that individuals can be different colours at different stages of their growth. Although changing from green to brown has been recorded in this species (Stringer 1970), changing back to green from brown was unexpected. Individual variation in colour will need to be considered when modelling inheritance and selection of this trait. It is possible that my observations could be explained by relatively few alleles at one or two loci. Further work is needed for us to understand the inheritance of colour traits in the common mānuka stick insect Clitarchus hookeri


  • Comeault AA, Flaxman SM, Riesch R, Curran E, Soria-Carrasco V, Compert Z, Farkas TE, Muschick M, Parchman TL, Schwander T, Slate J, Nosil P. 2015. Selection on a genetic polymorphism counteracts ecological speciation in a stick insect. Current Biology. 25:1975–1981.
  • Mangelsdorf AJ. 1926. Color and sex in the Indian walking stick, Dixippus morosus. Psyche. 33:151–155.
  • Morgan-Richards M, Trewick SA, Stringer IAN. 2010. Geographic parthenogenesis and the common tea-tree stick insect of New Zealand. Molecular Ecology. 19:1227–1238.
  • Morgan-Richards M, Langton-Myers SS, Trewick SA. 2019. Loss and gain of sexual reproduction in the same stick insect. Molecular Ecology. 28:3929–3941. DOI: 10.1111/mec.15203. 
  • Salmon JT. 1991. The stick insects of New Zealand. Auckland, New Zealand: Reed.
  • Stringer IAN. 1970. The nymphal and imaginal stages of the bisexual stick insect Clitarchus hookeri (Phasmidae: Phasminae). New Zealand Entomologist. 4:85–95. 
  • Trewick SA, Morgan-Richards M. 2005. New Zealand Wild: Stick Insects. Reed Publishing: Hong Kong.
  • Umbers KDL, Fabricant SA, Gawryszewski FM, Seago AE, Herberstein ME. 2014. Reversible colour change in Arthropods. Biological Reviews. 89:820–848.
  • Villoutreix R, de Carvalho CF, Soria-Carrasco V, Lindtke D, De-la-Mora M, Muschick M, Feder JL, Parchman TL, Gompert Z, Nosil P. 2020. Large-scale mutation in the evolution of a gene complex for cryptic coloration. Science 369:460–466.

Sticky sex

New Zealand stick insects have invaded the United Kingdom, but in the process they have lost the ability to reproduce sexually. This is odd because the vast majority  (more than 99%) of  multicellular creatures (primarily eukaryotes) engage in sex during reproduction.

Sex involves two individuals with different properties. Typically one sex (the male) produces abundant small and often motile gametes that carry genetic information to the larger egg produced by the other (female). Through this process, genetic information is passed from two parents to their offspring and results in shuffling of genetic variation. The results are readily evident in the variation seen among offspring that is prominent in human families.

Stick insects are (mostly) no exception even though scientist can show that reproduction without two sexes can have a numerical advantage over sexual reproduction. Simply, females that make only self-fertile daughters leave more of their genetics to future generations.  Theoretically it seems that clonal reproduction is advantageous, as long as the environment does not vary too much; producing offspring that are not the same as the parent could make some of them less successful. It is telling then, that despite the numerical advantage of clonal reproduction, that vast majority of large organisms do use sexual reproduction. Natural selection has made its choice.

One group of New Zealand stick insects includes individuals that differ in colour, size, and shape. In particular the number and size of spines they have varies among individuals. This group (genus Acanthoxyla) includes several described species, although in this case defining species is difficult. All are female, which means all come from self-fertile eggs produced by one parent (the mother). Hatchlings grow up to look like their mums, so are effectively clones.

Among the many individuals of common and widespread Acanthoxyla (literally: prickly stick) observed in New Zealand, no male has been encountered. Yet. But recently a male belonging to this genus turned up in England.

Rare males like this emerge among all-female stick insect populations, probably as a result of a random mutation deleting one of the XX sex chromosomes that denotes a female stick insect. XO individuals are male in appearance, but are usually not reproductive.

Research on the New Zealand genus Clitarchus has been revealing about the switching between sexual and asexual (all female) reproduction. As reported in Nature a population of Clitarchus hookeri accidentally introduced to the UK about 100 years ago has lost not only its homeland but also its sex life.

Analysis of genetic variation shows that the origin in New Zealand of the UK stick immigrants was most likely in Taranaki, North Island. This agrees with historical records indicating that native plants collected in this area were shipped to England and then the nearby Isles of Scilly. In particular the Abbey Gardens on Tresco are now home to a range of New Zealand plants, and it is likely that stick insect eggs in the soil around plant specimens were accidentally transported around the world. Hatchlings that grew into adult stick insects able to produce abundant self-fertile females were likely at an advantage. The potential of this species to switch to asexual reproduction has also resulted in a pattern of geographic parthenogenesis in New Zealand.

Genetic variation (mtDNA COI) in Clitarchus hookeri across New Zealand (A, B), highlighting the mainly parthenogenetic lineage in NZ (C), and the lineage associated with the one variant found in the UK population (D).

Closer examination of two New Zealand populations of the same species add to our understanding of the drivers and mechanisms of reproduction strategy switching. The UK population lost sexual reproduction and evolved a barrier to fertilisation, which has been demonstrated by providing captive female stick insects from UK with NZ males. Meanwhile two NZ populations recently gained sexuality and genotypic data indicate this happened via two different pathways.

Original Science:
Morgan-Richards M, Langton-Myers S, Trewick S. 2019. Loss and gain of sexual reproduction in the same stick insect. Molecular Ecology

Trewick SA, Morgan-Richards M. 2018. Missing New Zealand stickman found in UK. Antenna 42: 10–13.


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