Monday, September 6, 2010

Cuticular appendages

Cuticular appendages:
Cuticular structural components, waxes, cements, pheromones (Chapter 4), and defensive and other compounds are products of the epidermis, which is a near continuous, single-celled layer beneath the cuticle. Many of these compounds are secreted to the outside of the insect epicuticle. Numerous fine pore canals traverse the procuticle and then branch into numerous finer wax canals (containing wax filaments) within the epicuticle (enlargement in Fig. 1); this system transports lipids (waxes) from the epidermis to the epicuticular surface. The wax canals may also have a structural role within the epicuticle. Dermal glands, part of the epidermis, produce cement and/or wax, which is transported via larger ducts to the cuticular surface. Wax-secreting glands are particularly well developed in mealybugs and other scale insects. The epidermis is closely associated with molting – the events and processes leading up to and including ecdysis (eclosion), i.e. the shedding of the old cuticle. Insects are well endowed with cuticular extensions, varying from fine and hair-like to robust and spine-like. Four basic types of protuberance (Fig. 2), all with sclerotized cuticle, can be recognized on morphological, functional, and developmental grounds:
1. Spines are multicellular with undifferentiated epidermal cells;
2. Setae, also called hairs, macrotrichia, or trichoid sensilla, are multicellular with specialized cells;
3. acanthae are unicellular in origin;
4. microtrichia are subcellular, with several to many extensions per cell.
Setae sense much of the insect’s tactile environment. Large setae may be called bristles or chaetae, with the most modified being scales, the flattened setae found on butterflies and moths (Lepidoptera) and sporadically elsewhere. Three separate cells form each seta, one for hair formation (trichogen cell), one for socket formation (tormogen cell), and one sensory cell (Fig. 4.1). There is no such cellular differentiation in multicellular spines, unicellular acanthae, and subcellular microtrichia. The functions of these types of protuberances are diverse and sometimes debatable, but their sensory function appears limited. The production of pattern, including color, may be significant for some of the microscopic projections. Spines are immovable, but if they are articulated, then they are called spurs. Both spines and spurs may bear unicellular or subcellular processes.

Fig. 1 The general structure of insect cuticle; the enlargement above shows details of the epicuticle.

Fig. 2 The four basic types of cuticular protuberances: (a) a multicellular spine; (b) a seta, or trichoid sensillum; (c) acanthae; and (d) microtrichia.

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