An introduction on the biology of phasmids

On this page, we’ll dive in the wondrous world of phasmids – better known as stick insects, walking sticks of leaf insects. This bug is a favourite to keep as an exotic pet. Their strange shapes and interesting behaviour makes them very intriguing to keep. Generally speaking, they are relatively easy to take care of.

There are over 3000 species of phasmids described, and there are many different species kept in captivity. The Indian stick insect, also known as the ‘common’ stick insect, is as its name suggest one of the most known and popular stick insects. But there is more to discover than its extraordinary shapes only. We will give you more information about their morphology, their life cycle and natural behaviour.

Phasmid, Phasmida, Phasmatoptera or Spectra
Phasmatodea
1.5cm (0.6″) – 56.7cm (12″)
1gr – 65gr
4 months – 3 years

Morphology

Like many other insects, phasmids have six legs, an exoskeleton and its body exist of 3 bodyparts- the head, thorax and the abdomen. Some have wings and can do some controlled of uncontrolled flights, but many species are wingless or have underdeveloped wings. So they mostly rely on the often very long and adapted legs. Although many phasmid species have long or strange shaped legs, they can move quite quickly, with every part of legs have its own function. Their hind legs are most functional as power. Their middle legs are responsible for steering. Their front legs probe the surface and the environment.

Phasmids, better known as walking sticks, do give credits to their name. They are among the largest of all insects with often very slim bodies. Where some just as tiny as 1.5 cm (0.6″) there are species tot grow to over 30 cm (12″). The longest stick insect even measures up to 56.7 cm (22.3″). Despite being relatively long, their weight is almost nothing but a couple of grams.

Phasmids have exaggerated sexual dimorphism, which means that the males and females look very different and can be determined by how they look. Females are almost always larger than males. Sometimes even two or three times bigger. Also, males are often thinner than females.

The giant prickly stick insect (Extatosoma tiaratum) have a body covered with thorn-like spikes primarily as camouflage. It is a beautiful species to keep.

Camouflage, colouration and predator avoidance

Almost everyone knows phasmids of their camouflage. Their body, often including legs, are designed to mimic the form of a stick or leaf. They rely on this camouflage to evade predators and can remain absolutely stationary so that predators do not see them. Phasmids are true masters of mimicry, and some species will take it even a step further. Some make rocking motions to mimic the movement of leaves and twigs in the wind. Some species use catalepsy (feigning death) as a strategy to avoid predation. The nymphs of the spiny leaf insects have been known to mimic ants by moving their abdomen upwards.

Besides, some species use active defence beside their camouflage. For example, some species have large spines on their leg and when at danger from a predator will move their spiny legs rapidly to inflict painful wounds. Some species are even venomous and use poison to attack predators. These phasmids often don’t use camouflage anymore and have bright colours.

Many phasmids have another, more extreme, defence mechanism that is known as autotomy. It means that a stick insect can break off a leg at a weak point using a special muscle. Phasmids can regenerate missing limbs every time they moult. It can take several moults to regenerate a limb fully. When at the adult stage a stick insect or leaf insect won’t moult anymore, and so can’t regrow a limb anymore (however, there are some cases an adult can force themselves to moult in order to regrow a lost leg).

Life cycle

Phasmids belong to the group of hemimetabolous insects – which means they develop through a series of several nymphal stages called instars – and not undergo a complete metamorphosis as butterflies and beetles do. Newborn phasmids moult around 4 to 9 times. With every mould, the nymph will grow until it reaches the adult size. They cannot grow between moults because of their exoskeleton.

Eggs

Most phasmid females randomly drop their eggs where they fall on the ground. Some females have other strategies, like bury their eggs in the soil, glue their eggs to a suitable surface, or pierce them into a leaf or shooting eggs far away. A single female will “lay” 100 to 1000 eggs in her life.

Hatching times can diverse immensely, like a few weeks to a couple of months. It also depends on the condition of the environment of the eggs.

Even eggs have camouflage. Eggs look like seeds in shape, size, and even feel like seeds. Just like seeds have, these eggs have a lid-like structure from where the nymph will emerge. Some eggs attract ants, and they will take them to the ant nest, where it will have the defence of the ant colony.

A fabulous video about the hatching of a Lord Howe Island stick insect egg (Video by Melbourne Zoo)

Nymphs

The nymphal stages are numbered. L1 represents nymphs that are newly hatched. L2 is after the first moult of the nymph. They generally moult until stage L6 to L9, depending on the sex and species how much moult it takes to reach this adult stage. However, the last and second to last stage is often not specified with a number. Females usually moult one additional time.

After every moult, the phasmid will (frequently) eat its own moulted skin. These skins have precious nutrients, and like in nature, nothing is left to waste.

Newly hatched nymph of the giant prickly stick insect – Extatosoma tiaratum (Photo by Jonas)

Adults

After a couple of months and several moulds, phasmids reach adulthood. It will no longer mould again. The lifespan of phasmids varies by different species. The average is about 12 months, but they can live longer in captivity. Some species can live up to three years.

In this stage, they also breed again and lay new eggs. Many phasmids species are parthenogenic – an asexual reproduction: without mating females lay fertile eggs – and these are complete copies of themselves. Nymphs born out of these eggs are therefore always female.

Habitat distribution and behaviour

With more than 3000 different species of phasmids, they can be found on every continent on the planet (except for Antarctica, there it is too cold). They can almost be found in any environment, tropical or temperature, low or high altitudes, dry and or wet climate. However, there is a higher density and diversity in tropical areas. The island Borneo, for example, is a hot spot with over 300 phasmid species.

Stick insects and leaf insects have a surviving strategy to mimic twigs or leaves and are mainly found in bushes and trees. However, some species do live on grassland entirely. Phasmids are nocturnal, concealing from predators during the day trusting their camouflage. They primarily feed during sundown. Also, the eggs hatch during the night when predation is low.

Both stick insects and leaf insects are tame. You don’t have to worry they attack people. However, they are very delicate and fragile. You’ll need to cautious when handling them. It is best when you’ll let them walk on your finger or hand. They prefer to walk vertical and up, so take advantage of that by holding up your hand higher than the animal. Never grab their leg or pull them off a twig; they can break and lose a limb.

Food habits

All phasmid species that we know are herbivorous. Some species have a wide variety of leaves they accept to eat, but others have a very special preference and only feed on one specific plant. What the animals prefer varies by species, but generally accepted plants are bramble, oaks and beech.

Further reading

Check out these pages as well:

Similar bugs to stick insects and leaf insects

If you like stick insects and leaf insects you’ll probably also like the insects below. Otherwise, check out our page ‘Choose your bug species‘ to find more different bugs.

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