Ants Disperse the Elaiosome‐Bearing Eggs of an African Stick Insect

The seeds of many plants possess elaiosomes which facilitate their dispersal by ants (Beattie, 1985). Here we describe an apparent example of convergence in dispersal mechanisms between kingdoms, where an African stick insect also uses ants to disperse its progeny. We found that the capitulum on the eggs of the stick insect, Bacillus ? coccyx Westwood (Phasmida, Phasmatidae), functions in a similar way as an elaiosome. The eggs are attractive to ants and the capitulum provides them with a handle to carry the eggs. A further parallel with elaiosomes on seeds is that the ants were able to remove and eat the capitulum without reducing egg viability. Female stick insects are slow moving, often flightless insects and consequently have limited dispersal abilities. This is particularly the case with gravid females, which may be unable to fly even if they possess wings (Bedford, 1978). Stick insect eggs are hard-shelled and often closely resemble seeds, a mimicry which may help reduce predation by insectivorous birds (Carlberg, 1986; Hinton, 1981). They are laid singly, and are often simply dropped or flicked away by the ovipositing female (Bedford, 1978). In many species the eggs have an appendage, the capitulum, which can be detached without reducing their viability (Clark, 1976a, 1976b). The function of the capitulum was discussed by Hinton (1981), who showed that previous ideas about capitula being involved with respiration were incorrect, but concluded that their size and complexity indicated an important, but unknown, function.