Scientists have created the best map of stick-insect evolution to date by combining DNA analysis and knowledge of their varied egg-laying techniques. The first stick insects flicked or dropped their eggs while hiding in the foliage, but they have evolved new egg-laying techniques after colonizing different habitats. Previous evolutionary theories, based on anatomical similarities, are inaccurate, with geographically isolated populations of stick insects more likely to be related than those with similar features.
Known for exceptional mimicry, stick insects have evolved a range of egg-laying techniques to maximize egg survival while maintaining their disguise, including dropping eggs to the ground, skewering them on leaves, and even enlisting ants for egg dispersal. Scientists have now combined knowledge on these varied techniques with DNA analysis to create the best map of stick-insect evolution to date. Contrary to previous evolutionary theories based on anatomical similarities, the new analysis finds the first stick insects flicked or dropped their eggs while hiding in the foliage. It also finds that geographically isolated populations of stick insects are more likely to be related than those with similar features.
Stick insects are increasingly popular in the pet industry on account of their remarkable size, bizarre appearance and gentle nature. They are the only insects where each species has an individual egg form. In the 1950s, scientists based stick-insect evolutionary theories on the traditional method of examining subtle changes in anatomical features. However, this method could not explain why distantly-related species, for example those separated by faraway continents, often shared very similar features.
Using DNA analysis and linking these findings to their variety of egg-laying techniques, the research created their own map of stick-insect evolution. As well as revealing that species geographically isolated with each other were more likely to be related than species that looked similar, the results challenged previous theories on how stick-insect egg-laying strategies evolved.
Stick-insects were thought to evolve from a ground-dwelling adult form that deposited its eggs directly in the soil. We show that ancestral stick-insects actually remained in the foliage and dropped or flicked their eggs to the ground, a technique employed by most of these insects as a strategy to remain in disguise. The hardening of the egg capsule early in the evolution of stick insects represents a key innovation allowing further diversification.
This hardened capsule allows the egg to survive falls from the canopy, to float on water and to pass through the intestines of birds. A further innovation, exclusive to stick insects that flick or drop their eggs, is a food-filled cap on the egg that attracts ants, who then disperse it much further than a female stick insect could achieve on her own.