Detritus

Many cockroaches feed on detritus (Roth and Willis, 1960; Mullins and Cochran, 1987), a broad term applied to nonliving matter that originates from a living organism (Polis, 1991). A unique feature of detritivores is that there is no co-evolutionary relationship between the consumer and the ingested substrate. This is in stark contrast with the relationship between herbivores and higher plants, and in predator-prey systems. A consequence of this lack of co-evolutionary interaction is that detriti-vores are less specialized than predators and herbivores, and they defy classification into straightforward food chains (Anderson, 1983; Price, 2002; Scheu and Setala, 2002). The food of detritivores is nutritionally very different from feeding on living plants or animals because it has been colonized and altered by microbes. Litter is a "resource unit" comprised of recently living material, degraded litter, dissolved organic matter, complex consortia of fungi, bacteria, nematodes, and protozoa, and the metabolic products of these (Nalepa et al., 2001a; Scheu and Setala, 2002). The notion that detritivores may ingest a large amount of living microbial material, and may develop co-evolutionary relationships with these organisms, is not typically considered (Chapter 5).

Dead plant material in varying states of decay is known to be the primary food source for cockroach taxa in a variety of habitats. This is particularly true for species living at or near ground level in tropical forests, which have an unlimited supply of decaying litter within easy reach. Plant detritus is constantly accumulating on the forest floor, either seasonally or constantly. In the rainforest canopy, detritivores have access to suspended litter and the dead material that typically edges herbivore damage on live leaves (Fig. 3.3). Many cockroaches feed on leaf litter (Table 4.4), which in general is of higher resource quality and decomposes more quickly than twigs and other woody materials (Anderson and Swift, 1983); however, decayed wood may serve as a food source more commonly than is generally appreciated (Table 3.2). In rainforests, practically all wood is rotten to some extent, and the division between decayed wood, rotted plant litter, and soil organic matter is difficult to assess (Collins, 1989). Many cockroach detritivores live within their food source—"a situation reminiscent of paradise" (Scheu and Setala, 2002).

Physically tough substrates like leaf litter and wood are macerated by a combination of mandibular action and

Table 4.4. Examples of cockroaches subsisting largely on leaf litter.

Habitat

Cockroach taxon

Reference

Rainforest

Epilampra irmleri

Irmler and Furch (1979)

6 species (Malaysia)

Saito (1976)

20 species of nymphs

WJB (pers. obs.)

(Costa Rica)

Dry forest,

Geoscapheini

Rugg and Rose (1991)

scrub

Thorax porcellana

Reuben (1988)

Desert

Arenivaga investigata

Hawke and Farley (1973)

Edney et al. (1974)

Heterogamisca chopardi

Grandcolas (1995a)

Aquatic

Litopeltis sp.

Seifert and Seifert (1976)

Poeciloderrhis cribrosa

Rocha e Silva Albuquer

verticalis

que et al. (1976)

Opisthoplatia maculata

Takahashi (1926)

Fig. 4.5 Proventriculus of Blattella germanica, transverse section. From Deleporte et al. (1988), courtesy of Daniel Lebrun. Scale bar = 100 ^m. When the "teeth" are closed the inward pointed denticles almost occlude the lumen. Hairs on the pul-villi may help filter the coarse food from the fine (Cornwell, 1968).

Fig. 4.5 Proventriculus of Blattella germanica, transverse section. From Deleporte et al. (1988), courtesy of Daniel Lebrun. Scale bar = 100 ^m. When the "teeth" are closed the inward pointed denticles almost occlude the lumen. Hairs on the pul-villi may help filter the coarse food from the fine (Cornwell, 1968).

passage through the proventriculus, a strongly muscled and often toothed armature that lies just behind the crop (Fig. 4.5). It might be expected that the morphology of this organ is functionally related to diet, but that does not appear to be the case. The various folds, denticles, and pulvilli on the structure are, in fact, useful characters in phylogenetic studies of cockroaches (McKittrick, 1964; Klass, 1998b). The proventriculus of the wood-feeding taxa Cryptocercus (Cryptocercidae) and Panesthia (Bla-beridae), for example, are completely different; that of Cryptocercus resembles that of some termites, and Panesthia has the flaccid, wide proventriculus of a blaberid. Macropanesthia rhinoceros, which feeds on dead, dry leaves, lacks a proventriculus (Day, 1950). This species, as well as Geoscapheus dilatatus, Panesthia cribrata, and Cal. elegans are known to ingest sand, probably to aid in the mechanical fragmentation of their food (Zhang et al., 1993; Harley Rose, pers. comm. to CAN).

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