We review literature on extant detritivores, in-cluding cockroaches and termites, and conclude that co-prophagy was the key behavior leading to the evolution of hindgut fermentation systems in the stem group of Dictyo-ptera. Coprophagy exploits concentrated microbial con-sortia on cellulose based substrates. These microbes are potential mutualists and food, but they also initiate degrada-tion of cellulose, detoxify allelochemicals and soften the substrate, a phenomenon known as the " external rumen " . We suggest that the evolution of a sophisticated hindgut fauna is a process of internalizing this self-assembled microbial com-munity, accompanied by changes in host-microbe interde-pendence, the source of microbial inoculum for neonates, and host social behavior. Proctodeal trophallaxis evolved from pre-existing intraspecific coprophagous behavior when termite ancestors became subsocial, because the physiology of encystment in oxymonad and hypermastigid flagellates precludes their transfer via cysts in adult feces. The behavior was reinforced by the benefits of using the trophic stages of flagellates as food. The association of these flagellates with the dictyopteran lineage is an ancient one, and may have originated as part of the external rumen in the Carboniferous coal swamps.
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