The Malpighian tubules and rectum

The main organs of excretion and osmoregulation in insects are the Malpighian tubules acting in concert with the rectum and/or ileum (Fig. 3.17). Malpighian tubules are outgrowths of the alimentary canal and consist of long thin tubules (Fig. 3.1) formed of a single layer of cells surrounding a blind-ending lumen. They range in number from as few as two in most scale insects (coccoids) to over 200 in large locusts. Generally they are free, waving around in the hemolymph, where they filter out solutes. Only aphids lack

Malpighian tubules. The vignette for this chapter shows the gut of Locusta, but with only a few of the many Malpighian tubules depicted. Similar structures are believed to have arisen convergently in different arthropod groups, such as myriapods and arachnids, in response to the physiological stresses of life on dry land. Traditionally, insect Malpighian tubules are considered to belong to the hindgut and be ectodermal in origin. Their position marks the junction of the midgut and the cuticle-lined hindgut.

The anterior hindgut is called the ileum, the generally narrower middle portion is the colon, and the expanded posterior section is the rectum (Fig. 3.13). In many terrestrial insects the rectum is the only site of water and solute resorption from the excreta, but in other insects, for example the desert locust Schistocerca gregaria (Orthoptera: Acrididae), the ileum makes some contribution to osmoregulation. In a few insects, such as the cockroach Periplaneta americana (Blattodea:

Fig. 3.17 Schematic diagram of a generalized excretory system showing the path of elimination of wastes. (After Daly et al. 1978.)
0 0

Post a comment