Stick insects

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(HOST) Ruth Page describes one insect mom who drops her eggs in a heap and takes off; she counts on ants to protect them.

(PAGE) Ever spotted a stick insect? You have to be sharp-eyed. They look exactly like slender sticks, a part of the branch under which they stand. And they can stand still for quite a time; it’s movement that attracts the eye.

Unlike most other insects, they appear to make no effort to protect their eggs or nymphs to assure that their genes are preserved for posterity. They just drop their eggs carelessly, one at a time, straight down from the branch where they’re clinging, and go on their way unconcerned.

So there’s this little heap of tiny eggs, enough to be readily visible to predators. One would have expected mom to spread them a- round for protection and put them near a food source, as do most insects. No, she just drops the eggs and walks off.

It takes quite a while for these eggs to hatch, seemingly making it even more difficult for them to survive. When they do hatch, they appear as miniatures of their parents – tiny newborn stick insects rather than larvae. They’re called nymphs. Their instinct is to head for the nearest upright anything and climb it. When they reach edible foliage they stop and have breakfast – and lunch and dinner – as they move leisurely along.

But that’s after they succeed in hatching. How did they escape predation when they lay in one heap or, in some cases, not quite heaped, but pretty close to one another? To start with, each egg carries a tiny larder in a minute sac called a capitulum. It contains a bit of highly nourishing fat. The eggs have firm calcium shells and are about the size and appearance of plant seeds. This helps protect them from predators.

But the capitulum is the prize: when some ant species see an egg with a capitulum, they haul it back to their nest. They pull off the capitulum and feed it to ant larvae. Then they leave the egg, pos- sibly in their dumping area, or anywhere out of the way in the nest. Once underground in an ant nest, the egg is safe from predators and removal of the capitulum has done it no harm.

So all the stick insects found by appropriate ants are spread about, sometimes quite far from the site of their birth, and the future of mom’s genes is much safer.

Scientists are still trying to discover what the scent is that stick- insect eggs provide to attract the ants to that tiny packet of fat. Many plant seeds are dispersed in this way and, in some cases, research has shown what the aroma is that ants like. But so far, the insect eggs’ chemical secret is a mystery.

This is one more example of millions of years of evolution allowing all kinds of associations to develop in the natural world.

This is Ruth Page, happy to find another deeply satisfying exam- ple of the intricacies of the near-invisible life around us.

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