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akg ha-1 year-1. From Whitford, 2000.

akg ha-1 year-1. From Whitford, 2000.

ogy, and social structure, the work by Holldobler and Wilson (1990) is unsurpassed.

Most of the solitary wasps in the superfamily Vespoidea construct nests in the soil. (Fig. 4.56) The adult female wasp first constructs a small nest cavity. Then a suitable prey item (another insect or a spider) is located, which the wasp then stings to paralyze it and hauls it to the nest. An egg is laid on the paralyzed victim, and it is then entombed. Some of the social wasps, especially Vespula spp., nest in the ground. Often natural cavities such as abandoned rodent burrows are used as nesting sites. Vespids are carnivorous, feeding their larvae on captured prey (insects or spiders), although adult wasps generally feed on nectar, sap, or similar juices (Michener and Michener, 1951).

Diptera

Many of the true flies can be considered soil insects, at least in some stage of their life histories. At least 75 of the 108 dipteran families in North America have some contact with soil ecosystems (McAlpine, 1990). This listing excludes strictly aquatic families, aboveground herbivores, and some parasitic species. Many species that live in above-ground habitats pupate in the soil, thus participating, involuntarily,

FIGURE 4.56. A digger-wasp (family Sphecidae). These solitary wasps usually prepare nests in the soil, which they provision with arthropod prey before depositing eggs (Pratt and Stojanovich, 1967).

in soil food webs. McAlpine (1990) provides a well-illustrated key to families of Diptera that have relations with soil systems.

Many species of fly larvae are important saprovores in soils. They are restricted to moist situations rich in organic matter. Some larvae are predatory and these have adaptations to reduce moisture loss; they occur in drier situations (Teskey, 1990). Fly larvae have a major impact on decomposition rates of carrion. Together with some beetle species, maggots of various types hasten the decomposition rate significantly. When Payne (1965) used window screen to exclude insects from decaying corpses of baby pigs, the bodies became mummified and decomposed slowly compared with corpses exposed to insect attack. Fly larvae are also important in forensic entomology, where their identification has been helpful in determining time of death of human corpses (Catts and Haskell, 1990).

Isoptera

The Isoptera (Fig. 4.57), the termites, are among the most important of soil fauna, in terms of their impact on soil structure and on decomposition processes. Termites are social insects with a well-developed caste system. Through their ability to digest wood they have become economic pests of major importance in some regions of the world (Lee and Wood, 1971; Bignell and Eggleton, 2000). Termites are highly successful,

FIGURE 4.57. Isoptera (Castes of termites: (a) worker, (b) winged reproductive, (c) soldier, (d) queen (courtesy of Banks and Snyder and the U.S. National Museum) (from Borror et al, 1981).

constituting up to 75% of the insect biomass and 10% of all terrestrial animal biomass in the tropics (Wilson, 1993; Bignell, 2000).

Termites in the primitive families, such as Kalotermitidae, possess a gut flora of protozoans, which enables them to digest cellulose. Their normal food is wood that has come into contact with soil. Most species of termites construct runways of soil and some are builders of spectacular mounds (Fig. 4.58). Members of the phylogenetically advanced family Termitidae do not have protozoan symbionts, but possess a formidable array of microbial symbionts (bacteria and fungi) that enable them to process and digest the humified organic matter in tropical soils, and to grow and thrive on such a diet (Breznak, 1984; Bignell, 1984; Pearce, 1997). Interestingly no one adaptive feature or mechanism appears to distinguish the guts of soil-feeding termites. As a result, approximately 67% of the genera in the family Termitidae now consist of these forms. A

FIGURE 4.58. Termite Mounds: Diagrammatic representation of different types of concentrated nest systems. (a) Hodotermes mossambicu. (b) Macrotermes subhyalinus. (c) Nasutitermes exitiosus (from Lee and Wood, 1971).

speculative and generalized sequence of events in a typical Termitinae soil-feeder gut is given in Fig. 4.59 (Brauman et al., 2000).

A number of inquilines (organisms existing in, and sharing, common space) occur in termite nests—ants, collembolans, mites, centipedes, and beetles that have become morphologically specialized for that habitat.

Although termites are mainly tropical in distribution, they occur in temperate zones and deserts as well. Termites have been called the tropical analogs of earthworms, because they reach a large abundance in the tropics and process large amounts of litter. Three nutritional categories include wood-feeding species, plant- and humus-feeding species, and fungus growers. This latter group lacks intestinal symbionts and depends upon cultured fungus for nutrition. Termites have an abundance of unique microbes living in their guts. Using the criterion of 97% sequence identity, one recent study of bacterial microbiota in the gut of the wood-feeding termite Reticulitermes speratus found 268 phylotypes of bacteria (16S rRNA genes, amplified by polymerase chain reaction [PCR]), including 100 clostridial, 61 spirochaetal, and 31 Bacteroides-related phylotypes (Hongoh et al., 2003). More than 90% of the phylo-

The EXTERNAL RUMEN: Incorporation of feces into internal mound materials and fresh constructions, with stimulation of microbial activity and further processing of soil organic matter.

P3, with aerobic periphery and anaerobic core

Mandibles with crushing molar plates

FOOD = organic rich soil + internal mound material

Anterior P1 with peristaltic musculature

Enteric valve and armature

P4b with peristaltic and antiperistaltic musculature

P3, with aerobic periphery and anaerobic core

Mandibles with crushing molar plates

Anterior P1 with peristaltic musculature

Enteric valve and armature

P4b with peristaltic and antiperistaltic musculature

FOOD = organic rich soil + internal mound material

Labile end-products Labile end-products

Selection of soil fractions rich in silt and clay

Non-

enzymatic secretion or microbial lysis?

K+ secretion; filament propagation and mixing with ingested soil

Alkaline hydrolysis at pH up to >12.0

Fermentation in core, and mineralization at periphery of lumen

Neutralization of alkaline pH and aerobic processing of soil organic matter?

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