How Do Stick Insects Lay Eggs? Seed-Like Eggs Carried by Ants

How do stick insects lay their eggs?

After mating, or in some species without mating at all, a female stick insect produces eggs steadily over weeks or months. Depending on the species, she may lay from about 100 to more than 1,000 eggs in her adult life. They usually come one at a time, not in a single clutch.

Stick insects use several methods to place them:

• Dropping. Many species, including the northern walkingstick, let each egg fall from the treetops into the leaf litter.
• Flicking. Some females jerk the abdomen to throw eggs away from the plant. The giant prickly stick insect of Australia can flick its eggs a meter or more.
• Gluing. Others stick eggs to leaves, stems or bark, singly or in small rows.
• Hiding in crevices. Some push their eggs into cracks in bark or between leaves.
• Burying. Ground-laying species, including many heavy, wingless forest species, have a pointed egg-laying tube called an ovipositor that they push into soil.

Unlike many insects, stick insects give no parental care. Once an egg is laid, the mother plays no further part.

What do stick insect eggs look like?

Stick insect eggs are hard capsules, usually about 2–8 mm (0.1–0.3 in) long, and they look so much like seeds that they are often mistaken for them. Each species has its own egg shape and pattern, and specialists can often identify a species from its egg alone.

At one end is a lid called the operculum, which the young nymph pushes open when it hatches. In many species the lid carries a small knob called the capitulum. The northern walkingstick’s egg is about 2.5 mm (0.1 in) long and has been compared to a small legume seed.

During outbreaks of northern walkingsticks in North American oak forests, females drop eggs mostly from late August to October, and so many fall at once that observers have compared the sound to gentle rain.

Why do ants carry stick insect eggs?

The capitulum on many stick insect eggs is rich in fats, much like the elaiosome, an edible appendage found on many plant seeds. Plants use elaiosomes to get ants to carry their seeds away, and stick insect eggs exploit the same habit.

An ant that finds such an egg carries it back to the nest, removes the knob and feeds it to the ant larvae. The rest of the egg, still undamaged, is discarded in the nest’s refuse chambers or waste pile. Researchers think this can shelter eggs from fire, drying out, egg-eating animals and some parasitic wasps. Studies on Australian stick insects in the early 1990s showed that ants treat these eggs in much the same way as seeds with elaiosomes. To learn how ants coordinate this kind of foraging, see how do ants communicate?.

When the nymph hatches, it has to leave the nest safely. In the giant prickly stick insect (Extatosoma tiaratum), the newborn nymphs are black with an orange head and run quickly, looking much like the local Leptomyrmex ants. Once they reach a tree and molt, they slow down and take on the camouflage of their parents.

How long do the eggs take to hatch?

Hatching times vary more than in most insects. Eggs of tropical species often hatch after one to several months, and some take much longer. In the giant prickly stick insect, fertilized eggs usually hatch in about four months, while unfertilized eggs can take up to about nine months.

Species of cooler climates often pass the winter as eggs. This pause in development, called diapause, is broken by winter cold, so the eggs hatch in spring when fresh leaves appear. In the northern walkingstick, eggs dropped in fall usually hatch from late spring to midsummer. In the north of its range many eggs wait through a second winter, which gives populations a two-year cycle.

Moisture matters as well. Northern walkingstick nymphs tend to hatch at night when humidity is high, and in dry conditions some cannot free themselves from the egg.

Eggs that survive being eaten by birds

Stick insect eggs are tough. Their shells contain hard minerals such as calcium oxalate, and in some species they can survive a trip through a bird’s gut. In a 2018 experiment in Japan, researchers fed stick insect eggs to brown-eared bulbuls. Some eggs passed through the birds intact, and some of those later hatched.

This suggests that birds that eat egg-carrying females may occasionally carry eggs far away, which could help flightless stick insects reach new places, possibly even islands. The idea is plausible but still based on few studies. It may matter most for species that do not need males, because a single surviving female egg can found a new population. Read more in can stick insects reproduce without males?.

Eggs as part of the camouflage

Looking like a seed is another layer of disguise. A predator searching the ground for insect eggs may overlook what appears to be a bit of plant debris. Combined with dropping eggs into deep leaf litter, this means a female does not need to guard her eggs at all. It is the same strategy of not being noticed that adult stick insects use, described in how do stick insects hide?.