Comparative morphogenesis of sensilla: A review

The “typical” insect sensillum is formed by a fixed number of cells: one or several bipolar sensory neurons are enveloped by the glia-like thecogen, the trichogen, and the tormogen cells. These cells arise via differential mitoses from an epidermal sensillum mother cell, which is “singled out” in the epidermis by the action of proneural and neurogenic genes, and then inhibits its neighbours from becoming sensillum mother cells themselves (“lateral inhibition”). Morphogenesis begins with the formation of a primary ciliary dendrite (9 × 2 + 0) by the neuron that grows above the epidermal surface. The trichogen cell then develops an apical sprout backed by a microtubular cytoskeleton, which will secrete the cuticle of the sensory hair, whereas the tormogen cell forms the hair socket. After finishing cuticle formation, both cells retract and form the subcuticular sensillum lymph cavity. In hemimetabolous insects preparing for molting, the dendrite leaves the new sensillum via an apical or a lateral pore, remaining connected with the old sensillum. During adult development of holometabolous insects, the primary dendrites also project from the newly forming hairs, being lost when cuticle secretion starts. The definite sensory dendrites grow into the hair shaft, whereas the trichogen cell retracts from the latter in most species.