RECTAL COMPLEX IN LARVAE OF LEPIDOPTERA

In the cryptonephric condition of the excretory system in insects the distal ends of the Malpighian tubules are closely applied to the rectum and enclosed with it in a special chamber, the perinephric space, separated from the rest of the body cavity by the perinephric membrane. The term rectal complex refers to this association of tubules and rectum found in most larval Lepidoptera but not in adults. In the mealworm (Coleoptera) the rectal complex is able to remove water from the feces, but this ability is not conspicuously developed in the larvae of the 2 spp. of Lepidoptera studied: Pieris brassicae and Manduca sexta. These larvae, however, are able to maintain salt balance under heavy dietary loading, and in this the rectal complex plays an important part. A study of salt balance in more detail showed that more Na can be eliminated in the feces than enters the rectal complex from the intestine. Consideration of other possible routes of entry points strongly to the Malpighian tubules. Superimposed upon a net flow of tubule fluid out of the rectal complex is a tidal flow brought about by the rectal musculature and amplified by dilatations of the cryptonephric tubules, which could bring in fluid from the free tubules and enable the uptake of salts. Evidence is presented in support of this view. This tidal flow of tubule fluid and uptake of salts could be the basis of the build-up of high osmolarity in the perinephric fluid and could contribute to the removal of water from the feces. It could also be the basic mechanism in the mealworm, the leptophragmal mechanism being superimposed upon it.