Structure and spatial pattern of the sensilla of the body segments of insect larvae.

We describe the types and patterns of sensilla present on the thorax and abdomen of newly hatched larvae of representative species of several insect orders, among them Saltatoria, Mantodea, Blattaria, Heteroptera, Lepidoptera, and Diptera. Sensilla of non-Dipteran species almost exclusively comprise mechanoreceptive hairs or bristles (trichoid sensilla) of various sizes and numbers. In higher Dipterans, peg sensilla (sensilla basiconica, sensilla coeloconica) and so-called papilla sensilla predominate. The pattern of early larval sensilla falls into three main classes, which can be described as 1) fixed pattern, 2) variable pattern, and 3) variable pattern with fixed elements. In larvae exhibiting a fixed sensillum pattern (found in all Dipteran species investigated), sensilla are invariant in number; they are precisely placed in relationship to each other and typically form a single row behind the middle of each segment. A variable pattern (common in most insect groups) typically consists of several rows of relatively evenly spaced sensilla encircling the middle of each segment. In animals with a variable pattern including fixed elements, some sensilla, recognizable by their size or shape, are precisely placed, whereas other sensilla surrounding them are variable.