Structure, microbial associations and function of the so-called “mixed segment” of the gut in two soil-feeding termites, Procubitermes aburiensis and Cubitermes severus (Termitidae, Termitinae)

The morphology, histology and ultrastructure of the mesenteric and proctodaeal components of the mixed segment are described in detail, together with the disposition of the associated gut musculature, the patterns of peristalsis and the streaming movements of soil particles in the gut lumen. The mesenteric epithelium is characterized by very extensive basal infoldings which are associated with a large population of mitochondria, but evidence of significant protein synthesis and secretion is lacking. It is proposed that this tissue is a transporting epithelium whose major function is the secretion of a copious, K+ -rich fluid into the intestinal lumen to irrigate the hindgut (proctodaeum) and that alkaline hydrolysis of humic fractions by this fluid is an essential component of digestion. Malpighian tubules are present, but do not participate in fluid excretion. Since the hind-guts of soil-feeding termites are exceptionally voluminous and elongated and the major symbiotic micro-organisms occupy fixed positions by virtue of their attachment to the lining cuticle, it is argued that flushing is necessary to sustain microbial activity and for the recovery of end-products by the host. A survey of the protodaeal epithelium suggests that the anterior colon (P4a) is the most likely site of fluid reabsorption and that the wall of the hindgut anterior to this site is impermeable. The greater degree of ultrastructural differentiation of the mixed segment of Cubitermes severus and its more complete morphological separation from the midgut suggests that this species is a more advanced soil-feeder than Procubitermes aburiensis.