Light and electron microscopy of the meconium in the black carpet beetle, Attagenus megatoma, and its effect on the intestinal sporozoan, Pyxinia frenzeli

Abstract

The meconium of Attagenus megatoma (= Attagenus piceus) is the most elaborate of all insect meconia previously described. It is a brown tube, tapered to a sharp point and closed on each end. Three layers form the tube: the outer shell is thin, transparent, and acellular; the middle sheath is brown, thin, and acellular; and the inside tube is thick, opaque, and open at each end. Electron microscopy showed the latter layer to be degenerating midgut epithelium of the larva. Formation of the meconium occurs in the larval midgut when the pupal cuticle has begun to form beneath that of the larva. At this time it is transparent and open on each end. Four hours later the meconium is a closed uniformly dark-brown tube. Defecation of the meconium begins 80 to 90 hr after emergence of the adult. The capsule enters the hindgut, collapses and fragments in the rectal area, and is defecated 86 to 96 hr after emergence of the adult. The meconium forms in larvae free of as well as those containing the midgut sporozoan Pyxinia frenzeli. Pyxinia frenzeli is not an intracellular gregarine but does remain attached to midgut cells of A. megatoma during its growing stage. When the meconium forms in an infected larva, all gregarines attached to the midgut as well as all sporonts and gametocyst stages of the sporozoan in the gut lumen are contained within the meconium. If the adult does not ingest spores, it is completely free of P. frenzeli after meconium defecation. Gregarine-free larvae do not become infected by ingesting meconia defecated by infected A. megatoma. In contrast to other insect-protozoan relationships of a nonparasitic, perhaps commensal nature, in which ecdysone secreted prior to molting induces formation of resistant stages, gametocyst formation in this protozoan is not favored when the meconium appears. The infectivity of this eugregarine is, therefore, greatly decreased by meconium formation. Another dermestid, Trogoderma parabile, was found to have a similar cycle of meconium formation and defecation.