The Evolution of Insect Locomotion
Figure 6. Sotsuka, 2002.
(See appendix, click to enlarge)
Hexapoda (or insects) are the most successful class within the phylum arthropoda (Ritzmann et al., 2004), which is the most successful phylum in the phylogenetic tree of life (see Figure 6) (Ritzmann et al., 2004). Their ability to navigate so quickly with such agility is undoubtedly the largest contributor to their success in almost any environment around the globe, and thus studying the mechanisms and evolution involved in their locomotion is beneficial to roboticists attempting to recreate it (Ritzmann et al., 2004). Legged locomotion, though used by both vertebrates and arthropods, evolved independently in these phyla due to the selection pressure for a mode of transport that minimized friction and gravitational forces (Ritzmann et al., 2004). The remarkable similarities in the locomotion mechanisms of vertebrates and arthropods suggest that they evolved due to a lack of other viable solutions to terrestrial locomotion (Ritzmann et al., 2004). Both phyla developed the same solution to their physical constraints and therefore roboticists examine these mechanisms very closely when designing robots (Ritzmann et al., 2004).
There are many theories as to how insects evolved legs, but the most prominent view in scientific communities today is that arthropods evolved from a segmented ancestor of the annelida (Damus, 1996). This primitive organism, possibly resembling a modern-day earthworm (See Figure 7) may have developed pairs of leg-like growth structures on each segment to aid its locomotion and eventually some of these segments fused to create the head, thorax and abdomen of what we now call insects (Damus, 1996). The extra pairs of appendages were then modified to become antennae, mouth parts, pincers, or just disappeared completely (Damus, 1996). There are various pieces of evidence to support this theory including the three pairs of legs found on an insect thorax (derived from the three merged segments) and the number of nerve bundles found in the thorax (Damus, 1996). Annelids have one nerve bundle, called ganglia, per segment and the thorax of an insect has three ganglia present, again suggesting that three segments merged to form the thorax (Damus, 1996).
Once insects developed the structures necessary for legged terrestrial locomotion, natural selection began to act on the use of these limbs to eventually develop into the highly adapted insects of the present (Damus, 1996). The speed, agility and efficiency with which they locomote was most likely achieved by small modifications through competition and a predator-prey arms race (Gullan & Cranston, 2005). Insects with the limbs most specifically adapted to their environment were more successful at catching prey and evading predators, and so their predators and prey needed to develop mechanisms of predation or defense which were still effective against these insects (Gullan & Cranston, 2005). These small changes eventually resulted in complicated leg structures and stable mechanical gait patterns that contribute to the success of arthropods, such as in cockroaches, which have front, middle and rear legs of different lengths and morphology (Delcomyn, 2008).