How the Ecdysozoan Changed Its Coat

1696 M ost people are aware that insects are crunchy on the outside. What they don't always realize is that the external crunchy parts form the animal's skeleton, and that this body design makes the apparently simple process of growing extremely complex. In addition to providing a barrier against desiccation and protection from mechanical injury, an insect's external skeleton (exoskeleton) does what skeletons do for all animals: it gives the insect its shape and also provides the frame to which muscles are attached, and, therefore, is critical for its behaviors. External skeletons are found in a variety of animals, including arthropods (" jointed-foot invertebrates, " including insects, crustaceans, spiders, millipedes, and centipedes) and nematodes. Arthropods include some of the most successful organisms on earth. Ants alone are believed to represent up to one-third of the terrestrial animal biomass [1], and as many as half of earth's species may be insects. The undeniable success of insects is most certainly due, in part, to the group's rugged exoskeleton. Nevertheless, living within an external skeleton immediately raises a logistical problem: how to grow. Although many insects, especially at immature stages, have elastic skeletons, continuous growth eventually requires that the skeleton be replaced with a larger one. The process that effects this replacement is called molting, and although the same term is used to describe the replacement of the outer skin layer of some vertebrates such as snakes, the process of replacing an exoskeleton is infi nitely more diffi cult. The ability to replace an exoskeleton is currently believed to have evolved only once during animal evolution, giving rise to a clade of animals called Ecdysozoa, which includes arthropods and nematodes [2]. The exoskeleton, or cuticle, is a well-defi ned inert structure that is secreted by, and strongly attached to, the underlying epidermal cells. Although its composition varies signifi cantly among ecdysozoans (e.g., consider the skeleton of a beetle versus that of a crab), the process of molting itself is similar within the clade: the epidermis may undergo a round of cell division (thereby producing a larger surface) and separates from the exoskeleton. A new exoskeleton is then secreted by the epidermis, but is soft until the remains of the overlying old cuticle are shed at ecdysis. The new cuticle then expands and hardens. In nature, this process appears simple only because ecdysozoans are experts at molting. But underlying this seamless process is a …