Autotomy in a Stick Insect (Insecta: Phasmida): Predation Versus Molting (Scientific Notes) (Report‪)‬

Autotomy, or appendage loss, is common in many animals, including reptiles, amphibians, mammals, birds, fish, echinoderms, crustaceans, spiders, and insects (see Maginnis 2006a; Fleming et al. 2007 for reviews). In arthropods, there are 2 hypotheses for this phenomenon. First, limbs may be lost through predation attempts; if a predator were to grab a leg instead of the body, the animal can shed the leg and flee to escape predation (McVean 1982; Carlberg 1986; Formanowicz 1990; Robinson et al. 1991). And second, limbs can be shed during complications with molting. As a result of having a skeleton on the outside of the body, arthropods must repeatedly shed their old exoskeleton and replace it with a new one. During this complicated process, jointed appendages can become stuck in the old cuticle and must be shed to survive (Bedford 1978; Foelix 1982; Carlberg 1986; Robinson et al. 1991; Brock 1999). Although many taxa within the phylum Arthropoda experience autotomy, Phasmida is the only order within the class Insecta that regularly sheds and regenerates lost legs (Borrer et al. 1992). As such, it is important to identify the selective pressures for autotomy in this group. In the laboratory, legs are lost to molting complications approximately 30% of the time (Maginnis 2006b). In the field, a sampled population showed approximately 40% of individuals with missing or regenerated legs (Maginnis & Maginnis 2007). However, these rates offer no insight into the selective pressures behind autotomy. The goal of this study was to determine the effects of predation attempts and/or molting complications on rates of leg loss in a population of Didymuria uiolescens, the spurlegged phasmatid, native to south eastern Australia (Campbell & Hadlington 1967).