The External Rumen

The importance of microbial tissue to an arthropod may reside as much in its metabolic characteristics while on

Table 5.1. Nitrogen levels of various natural materials exploited as food by invertebrates. Compiled by Martin and Kukor (1984).

Nitrogen content

Material

(% dry weight)

Bacteria

11.5-12.5

Algae

7.5-10

Yeast

7.5-8.5

Arthropod tissue

6.2-14.0

Filamentous fungi

2.0-8.0

Pollen

2.0-7.0

Seeds

1.0-7.0

Cambium

0.9-5.0

Live foliage

0.7-5.0

Leaf litter

0.5-2.5

Soil

0.1-1.1

Wood

0.03-0.2

Phloem sap

0.004-0.6

Xylem sap

0.0002-0.1

recalcitrant substrates as in its nutrient content once ingested. The bacteria and fungi responsible for decay predigest plant litter in a phenomenon known as the "external rumen." The microbes remove or detoxify unpalatable chemicals (e.g., tannins, phenols, terpenes), release carbon sources for assimilation, and physically soften the substrate. These changes improve the palatability of plant litter and increase both its water-holding capacity and its nutritional value (Wallwork, 1976; Eaton and Hale, 1993; Scrivener and Slaytor, 1994a; Dix and Webster, 1995). As a result, decay organisms can guide food choice in cockroaches. Both Cryptocercus and Panesthiinae are collected from a wide variety of host log taxa, as long as the logs are permeated with brown rot fungi (Mamaev, 1973; Nalepa, 2003). It is the physical softening of wood that was suggested as the primary fungal-associated benefit for Pane. cribrata by Scrivener and Slaytor (1994a). Ingested fungal enzymes did not contribute to cellulose digestion, and fungal-produced sugars were not a significant source of carbohydrate. Microbial softening of plant litter may be particularly important for juveniles (Nalepa, 1994). Physically hard food is known to affect cockroach development (Cooper and Schal, 1992) and young cockroach nymphs preferentially feed on the softer parts of decaying leaves on the forest floor (WJB, pers. obs.)

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