Omnivores and detritivores contact microbes at much higher rates than do herbivores or carnivores (Draser and Barrow, 1985). In cockroaches, a high frequency of encounter is obvious from the habitats they frequent and from the abundant literature on their role as vectors. A large number and variety of bacteria, parasites, and fungi are carried passively on the cuticle of pest cockroaches (Roth and Willis, 1957; Fotedar et al., 1991; Rivault et al., 1993). Despite being nonfastidious feeders with regard to bacteria, however, cockroaches are scrupulous in keeping their external surfaces clean (Fig. 5.5). More than 50% of their time may be spent grooming (Bell, 1990) and in many species the legs are morphologically modified with comb-like tubercles, spines, or hairs to aid the process (Mackerras, 1967b;Arnold, 1974).Mackerras (1965a) described the concentration of hairs on the ventral surfaces of the fore and hind tibiae of Polyzosteria spp. as "long handled clothes brushes" used to sweep both dorsal and ventral surfaces of the abdomen. The final stage of the grooming process is to bring the leg forward to be cleansed by the mouthparts (Fig. 1.18). It seems reasonable to assume that microbes and other particulate matter concentrated on the legs during grooming activities are ingested at this point and may be used as food. This suggestion is strengthened by studies of the wood-feed ing cockroach Cryptocercus. An average of 234 microbial colony-forming units/cm2 cuticle have been detected on C. punctulatus (Rosengaus et al., 2003), and the insects are known to allogroom, using their mouthparts to directly graze the cuticular surface of conspecifics.Young nymphs spend 8% of their time in mutual grooming (Fig. 5.5B) and 15-20% of their time grooming adults. Grooming decreases with increasing age, and allogrooming was never observed in adults (Seelinger and Seelinger, 1983). Grooming has a number of important functions, and high levels of autogrooming may be related primarily to the prevention of cuticular pathogenesis in their microbe-saturated habitats. Digestion of some of the gleaned bacteria may be an auxiliary benefit, particularly if resident gut bacteria play a role in neutralizing ingested pathogens. Intense allogrooming in developmental stages with high nutrient requirements is suggestive that there may be a nutritional reward for the groomer, in the form of microbes, cuticular waxes, or other secretions. Starvation is known to increase grooming interactions in termites (Dhanarajan, 1978), and the observation that young Cryptocercus nymphs spend up to a fifth of their time grooming the heavily sclerotized adults, presumably the most pathogen-resistant stage, further supports this hypothesis. However, young nymphs also may be acquir-
Fig 5.5 Grooming behavior. (A) Periplaneta americana passing an antenna through its mouth during autogrooming. Modified from Jander (1966), courtesy of Ursula Jander. (B) Fourth-in-star Cryptocercus punctulatus allogrooming a sibling. Photo by C.A. Nalepa.
ing antimicrobials or other non-nutritive beneficial substances from adults during grooming, and keeping nest mates free of infection is in the best interest of the groomer as well as the groomee. Radiotracer studies are necessary to confirm the assimilation of ingested microbes.
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