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Stick insects – The masters of camouflage

Stick insects are masters of disguise, blending into their surroundings with plant-like camouflage and swaying like branches. From seed-like eggs to defensive sprays and zigzag escapes, these insects are full of fascinating survival adaptations.

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An adult female stick insect of the genus Anatispinosa Cliquennois, 2023 (on a leaf of a Garcinia verrucosa).

An adult female stick insect of the genusΒ AnatispinosaΒ Cliquennois, 2023 (on a leaf of a Garcinia verrucosa). Image: Β© Francesca Ferrari

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  • Male stick insects are typically smaller than females, with a significantly different body form and coloration. As a result, males and females of the same species were often misclassified as belonging to different species or even to different families.
  • The order of Phasmids includes some of the biggest insect species in the world, adult phasmids with a length of 20 to 30 cm being relatively common.Β 
  • On average, adult stick insects measure between 6 to 10 cm and only rarely less than 4 cm.Β 
  • Stick insects are found across all continents but their distribution is mainly tropical and subtropical.

Classification

Kingdom
Animalia
Phylum
Arthropoda
Class
Insecta
Order
Phasmatodea (phasmids)
Family
Genus
Species

Life cycle

Phasmids undergo an incomplete metamorphosis, transitioning through several nymph stages to an adult stage, with nymphs and adults having a similar body form. Stick insects typically molt between 4 and 8 times. Females often undergo one additional molt compared to males due to their larger body size.

Stick insects reproduce mainly sexually, with the male transferring a spermatophore (a package of sperm) to the female during copulation. In addition, parthenogenesis (asexual reproduction) is very common among female phasmids. About 1.2% of all Phasmatodea species are known to reproduce exclusively via parthenogenesis. The eggs produced through sexual reproduction result in both female and male offspring, whereas eggs from asexual reproduction will yield only females. Therefore, for some species females vastly outnumber males, up to the point where in some cases the male insects haven’t been classified at all. If a male approaches a female of the same species copulation is commonly observed: the male quickly climbs onto the female, assuming the mating position, and can remain there for hours or days being carried around by the female.

All you can leaf buffet

Stick insects are strict vegans munching on leaves with their jaws. They closely coevolved with flowering plants using them as food, but also as shelter from birds and bats. Studies show a wide range of feeding preferences. Most stick insects are adapted to a few plant species, but some are very selective and feed on a single species, while others are more flexible and include up to 37 plant species in their diet.

A male stick insect of the genus Antongilia Redtenbacher, 1906 munching on a leaf of Dichaetanthera cordifolia.

A male stick insect of the genus AntongiliaΒ Redtenbacher, 1906 munching on a leaf ofΒ Dichaetanthera cordifolia.Β Image: Β© Francesca Ferrari

Eggs for identification

Eggs may be one of the most taxa-specific characteristics in insects. Studying the form and other characteristics of Phasmatodea eggs is especially important since they display a diversity unseen in other insect groups. Recently, it was discovered that some hard-shelled, seed-like eggs of certain phasmid species can survive for several months inΒ salt water. This characteristic could have facilitated the dispersal of stick insects and for example the initial colonization of Madagascar. Some stick insect species glue their eggs on twigs and leaves.Β 

Most stickΒ insect eggs can be categorized into four main types:

1) Bullet-shaped eggs to facilitate their buryingΒ 

2) Eggs with spine-like extensionsΒ that pierce plant leaves fixing the egg in placeΒ 

3) Adhesion eggs that are glued to a substrate,Β 

4) Capitulum eggs (40% of species) that are usually laid loose. A capitulum is a small fatty attachment on the eggs' surface. It is thought to serve as a snack for ants that carry the eggs into their nests.Β Inside the nest, the eggs are protected and kept at ideal temperature and humidity for hatching.

An egg of a stick insect (length 4 mm) from the genus Leiophasma Uvarov, 1940. The orange attachment on the right side looks like a capitulum although this still needs to be confirmed.

An egg of a stick insect (length 4 mm) from the genus Leiophasma Uvarov, 1940. The orange attachment on the right side looks like a capitulum although this still needs to beΒ confirmed. Image:Β Β© Francesca Ferrari

Tactics to surviveΒ 

MalagasyΒ stick insects are hard to find during the day due to their very effective camouflage and because they are nocturnal. To avoid detection by predators, they move mainly at night in search of food or mates. However, fearsome predators such as birds, reptiles, spiders, rodents and other mammals like bats often hunt stick insects at night.

Stick insects use various effective mimicry* techniques: lying on dead leaves on the ground, staying pressed against a branch or tree trunk (often covered in moos), or lying on or under a plant leaf. Their movements are very peculiarΒ and have evolved to enhance their camouflage. Most of the time stick insects remain motionless, especially during the day. If they need to move, they often walk with a swaying motion resembling a twig moved by the wind.

Despite their camouflage, stick insects can still be discovered by predators, and in such cases, they mainly resort to two defensive strategies depending on the species: thanatosis (playing dead) or β€œscare-and-run”. During thanatosis they stay completely still even when touched, before they usually let themselves fall into the vegetation below to escape unnoticed. The β€œscare and run” strategy employs firstly a β€œscare” such as spraying a non-poisonous but red-staining liquid from their chemical defensive glands, or spreading their wingsΒ accompanied sometimes by the production of a loud sound (defensive stridulation). Following this, the insects try to escape quickly into the surrounding vegetation often walking in a zigzag pattern to confuse the predator.Β 

Click here to learn about Francisca's stick insect expedition to Madagascar.

*Glossary

Mimicry: Mimicry is a widespread phenomenon in nature: certain animals and plants have specialized in mimicking visual, auditory or olfactory signals from other species, thus sending a "false message". The imitator profits by deceiving the receiver of the signal.

References

Brock PD et al. Phasmida Species File Online http://phasmida.archive.speciesfile.org/HomePage/Phasmida/HomePage.aspx Accessed 5.7.2023

Bradler S. 2015. Der Phasmatodea Tree of Life: Überraschendes und UngeklÀrtes in der Stabschrecken-Evolution. Entomologie heute 27: 1-23 https://www.zobodat.at/pdf/Entomologie-heute_27_0001-0023.pdf  Accessed 5.6.2023.

Ragge DR 2009. The wing-venation of the order Phasmida. Transactions of

the Royal Entomological Society of London 106;9:37592. https://doi.org/10.1111/j.1365-2311.1955.tb01272.x

Bradler S et al.2015. Single origin of the mascarene stick insects:

Ancient radiation on sunken islands? BMC Evolutionary Biology 15;196 https://doi.org/10.1186/s12862-015-0478-yΒ 

GonΓ§alo Soutinho J et al. 2018.Β When camouflage fails: Predation of a huntsman spider Damastes sp. (Araneae: Sparassidae) on a stick insect Antongilia sp. (Phasmatodea: Bacillidae: Antongiliinae) from Madagascar.Β Acta Arachnologica 67;1: 31–33

https://doi.org/10.2476/asjaa.67.31

Goodman SM et al. Eds. 2022. The New Natural History of Madagascar, Princeton University Press. isbn: 978-0-691-22940-9 978-0-691-22262-2.Β https://doi.org/10.2307/j.ctv2ks6tbb

The author

Francesca Ferrari 1

Francesca Ferrari

ETH Zurich

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