Ants are social insects which live in great colonies. Their social structure with workers and a queen makes their society a very interesting and robust way of life. The fact that ants move around so much is gratefully used by plants that need to disperse their seeds. Many plant species make so called “elaiosomes”. This is a fleshy structure which is rich in lipids and proteins and serves as the paycheck for the worker ant that disperses the seed. This elaiosome attracts ants, which take the seed back to their nest. There the elaiosome is eaten and the rest of the seed is disposed in the waste pile of the ants, where the seed can germinate. Surprisingly, stick insects evolved the same trick as the plants!
The Cape stick insect
The Cape stick insect (Phalces brevis, formerly known as Bacillus coccyx) occurs in Africa. Stick insects are slow moving and often flightless insects, which means that they have limited dispersal abilities. Their eggs resemble plant seeds and are laid singly. The eggs are simply dropped or flicked away by the female. They can throw their eggs a couple of centimeters or as far as 5-6 meters away, depending on the species (Carlsberg, 1984). It was already known that many species have an appendage on the egg resembling the elaiosome, called the capitulum (see the figure below). Although it was not necessary for the survival of the egg, it was unknown what it did do. Compton and Ware however figured out what this capitulum was for.
Ants doing the work
Compton and Ware collected eggs from the Cape stick insect and removed the capitulum from some but not from others. Then they looked at whether ants would take the eggs with them. They found that indeed only eggs with a intact capitulum are taken by ants from two species (Pheidole megacephala and Acantholepsis capensis). They left 10 eggs taken by the ants to see if they hatched, 8 of them indeed hatched. Furthermore, the young stick insects were not harassed by the ants at all!
So why would the stick insects use this mode of transportation? Couldn’t they just flick their eggs 6 meters away instead of evolving this elaborate system? There seem to be several benefits for using this system. Forest fires are common in the area where these stick insects live. Ant nests are underground and survive these fires. Having your eggs in those nests makes sure that even though you yourself perish in the flames, your offspring survives safely underground. Furthermore, ant nest provide a neat protection to all kinds of animals that are out to eat the eggs. The ants in return get a nutritious meal out of it. All in all, it’s a very fascinating tale of nature!
Carlsberg, U. (1984). “Oviposition behavior in the Australian stick insect Extatosoma tiaratum.” Experientia 40: 888-889.
Compton, S. G. and A. B. Ware (1998). “Ants disperse the elaiosome-bearing eggs of an african stick insect.” Psyche 98: 207-214.
Goldberg, J., et al. (2015). “Extreme convergence in egg-laying strategy across insect orders.” Sci Rep 5: 7825.