Figure captionOn top of the basal lamina lies the epidermis itself, a single celled layer that lies just under the cuticle and is responsible for the building of the cuticle. The cuticle is secreted in a distinct order during the moults of the insect. The moult is the change of cuticle that allows an insect to change from an exoskeleton that is too small to one that will allow it to grow bigger. For a description of the growth of insects see the section on moulting.
Within the epidermis there are different cells, the basic epidermal cells, oenocytes, gland cells, tormogen and trichogen cells. The epidermal cells are together responsible for secreting different parts of the cuticle and the communication across the insect cuticle.
The epidermal cells comprise the bulk of the epidermis. The epidermal cells are gland cells in the sense that they produce and secrete substances that form the cuticle. Secretion by the epidermal can be through the very folded cell wall that is in contact with the cuticle or through pore canals that can extend all the way through the cuticle opening in the upper layers of the epicuticle. Secretion directly through the wall is usually associated with the building of the endo and exocuticle or with the secretion of agents that can alter the cuticle making it more elastic, harden or causing the cuticle to degrade allowing most of it to be consumed but the epidermal cells during moults. Secretion through the epidermal pore canals is mostly related to the building of the epicuticle by secretion of epicuticular waxes, cements and quinones that all make up the epicuticle and or give the cuticle its mechanical properties.
The oenocytes can be seen as the chemical factories of the epidermis, that produces many of the lipids and polyphenols that are required to produce the outer and inner epicuticle (Wigglesworth 1988). The oenocytes are derived from epidermal cells and are in most insects found in the epidermis where they attach to the basal lamina and lie surrounded by the epidermal cells. In other insects oenocytes can be observed singularly or in clusters outside the epidermis just under the basal lamina. The pruducts synthesized in the oenocytes are transferred to the epidermal cells that then secrete them through pore canals straight to the epicuticle (Chino 1985, Gu et al. 1995).
Like the oenocytes, the epidermal gland cells are also derived from dermal cells. There are a great variety of cuticular gland cells and many of them are specific to specific genera or orders of insects. One of the functions of the epidermal gland cells is the production and secretion of phermones directly to the cuticle surface through pore canals (Chapman 1988).
The tormogen and trichogen cells perform mechanical and chemical sensory functions and are in that sense involved in the communication through the cuticle. Especially tactile sensory function of the environment is achieved by setae that extend from the cuticle forming hair like structures that when touched or disturbed activate nerve cells sending sensory input that in turn effects the behaviour of the insect. Setae can be both tactile or olifactory, meaning they can be sensitive to mechanic or chemical stimulus (see setae). The insect seta is formed by the tormogen and trichogen epidermal cells, the latter forming the exterior extension of the seta and the tormogen cells forming the setal socket that is so characteristic of an insect seta. Setae come in a variety forms and perform a large array of functions some of which are described in the section about setae where the setal sensory cell is also depicted.
