The exoskeleton of insects is a secreted material in the same way that a human fingernail or toenail is secreted by the underlying cells. Insects have a single layer of epidermal cells that secrete the cuticle. Those secretions contain a polysaccharide, chitin, and a protein matrix. The chitin chains provide structural support and prevent the cuticle from tearing, similar to the way rebar or wire mesh is used to reinforce concrete or the way strong threads are used to reinforce fabrics such as ripstop nylon. A wide variety of cuticular proteins are secreted that have remarkable differences in properties. A part of the insect cuticle that is hard and rigid will contain different proteins than a region that is soft and flexible.
Once secreted, the insect releases chemicals that initiate a polymerization process. Polymerization links the proteins to the chitin and to each other. For the biochemist, polymerization creates a complex material whose component proteins are nearly impossible to characterize. However, new molecular biology techniques allow the analysis of secreted proteins before they are polymerized.
It is now possible to use RNA from cells that secrete the cuticle to determine the nature of proteins that are produced. RNA can be isolated from cells that are producing hard rigid cuticle, such as the front wings of beetles and compared to RNA collected from cells that are producing the hind wings, which are membranous and flexible. The RNA encodes proteins and can be analyzed to predict the structures of the proteins that are produced. Scientists can begin to investigate the proteins responsible for the different properties.
Cuticle of a Cicada Larva
