Ants in service to stick insects and violets.
What does the New Zealand stick insect, our violet, comphrey, corydalis,
dead-nettle, ground-ivy and the celandine have in common?
Stick insects are beautiful, large insects (genus Cheleutoptera), some reaching up to 35cm. They effectively mimic sticks and branches on which they hunt. This is shown on the photograph which depicts a stick insect, that for reasons only known to himself, visits my letter box. They are camouflaged not only by their colour, but also their behavior - when they are threatened they sway on their long legs imitating a branch being blown by the wind.
On the other hand, the plants listed above inhabit our fields and forests.
The answer to the question is somewhat surprising. Stick insects propagate their children in the same way as is done by the mentioned plants. They both use ants as trusty helpers, and their methods for 'employing' the ants to do such work is exactly the same.
The female stick insect, when she feels that her time has come, has the habit of grasping onto a branch, wiggles her abdomen about and drops her eggs on the ground beneath her, usually covered with dead leaves and rich topsoil.
The named plants also drop their seeds onto the ground, and here stops their role in childrearing for both the plants and the stick insects. The ants which are in essence omnipresent, now start their work. They
collect the seeds and the eggs and take them to their nests. They do not do this selflessly - look closely at the photograph. Both the eggs and the seeds have a similar
structure and they also have a special feature - a knobby outgrowth as if to entice the ant to take it in its jaws. This outgrowth is essentially a present for the ant - it contains fat and protein which the ants eat and take back to share with the inhabitants of their nest. Botanists have named this growth elaiosomes, I'm not sure if zoologists have come up with a corresponding name for the similar outgrowths on eggs.
The phenomenon of taking advantage of ants in such a way is called myrmecophilia, and the dispersion of seeds by ants is called myrmecochory. This occurrence was first described over a hundred years ago. I haven't heard stick insects being called myrmechophilic insects. A further twist of fate is that
fates of eggs and seeds are very simikar, as once the ants have consumed their present they are no longer
interested in it. They either remove the seeds and eggs from the nest, or simply forget about them. At the appropriate time the seeds and eggs awaken far away from their origin, usually in a place that is rich in food, hence increasing their chances of survival.
This phenomenon is an excellent example of convergent evolution, which I will also write about one day.