pH homeostasis «—Digestion of microbiota-«
FIGURE 4.41. Digestive processes in the hindgut of Porcellio scaber (Porcellionidae), including the ingestion of leaf litter, detoxification of ingestion phenolics (Detoxif.) in the foregut, digestion in the anterior hindgut through the activity of endogenous and bacterial enzymes, adsorption of nutrients and copper, microbial proliferation (Microb. Prolif.) in the posterior hindgut, and egestion of feces (from Zimmer, 2002).
expressed in the field due to the difficulty of finding feces beneath the litter layer. However, microbially inoculated feces represent a microbial "hot spot" generating microbial metabolites that might allow the isopod to "home in" on a desirable food source (Zimmer et al, 1996).
The isopod Porcellio has an excretory system that exposes its products to the external environment. Urine from the nephridia is channeled into a water-conducting system on the ventral surface. Ammonia is lost to the atmosphere and oxygen is absorbed during this flow. The ammonia-free water is then reabsorbed in the rectum (Eisenbeis and Wichard, 1987).
Millipedes (Diplopoda) (Fig. 4.42) are a group of widely distributed saprophages. They are major consumers of organic debris in temperate and tropical hardwood forests, where they feed on dead vegetable matter. Millipedes are also inhabitants of arid and semiarid regions, despite their dependence on moisture. Millipedes lack a waxy layer on their epicuticle and are subject to rapid desiccation in environments with low relative humidity. Some are true soil forms, others seem restricted to leaf litter or to cryptozoan habitats. They can be loosely grouped into (1) tubular, round-backed forms such as the familiar Narceus; (2) flat-backed forms (many Polydesmid millipedes); and (3) pillbug types, which roll into a ball. Millipedes range widely in length. Typical North American forms are 5-6 cm in length; tropical ones may reach nearly 20 cm in length. Hoffman (1999) has published a checklist of millipedes of North and Middle America. Keys to North American families of millipedes were published by Hoffman (1990). For an account of millipede biology and ecology, see Hopkin and Read (1992).
Millipedes become abundant in calcium-rich, high rainfall areas in tropical and temperate zones. The southern Appalachian Mountains of the eastern United States support a large millipede population. Millipedes can be important in calcium cycling. They have a calcareous exoskeleton, and because of their high densities they can be a significant sink for calcium. Millipedes are major consumers of fallen leaf litter, and may process some 15-25% of calcium input into hardwood forest floors. In desert areas, millipedes are active following rains, especially in desert shrub communities. They avoid hot, dry conditions by concealment under vegetation or debris (Crawford, 1981). Millipedes are vulnerable to desiccation, because their cuticle generally lacks a waterproof layer, their gas exchange system is not closed, and they lose a considerable amount of water through the mouth, in defecation, and during reproduction (Wolters and Ekschmitt, 1997).
Millipedes appear to be selective feeders, avoiding leaf litter high in polyphenols and favoring litter with high calcium content (Neuhauser and Hartenstein, 1978). Freshly fallen leaves are generally avoided, even though assimilation efficiency is much higher from that source (David and Gillon, 2002). Some millipedes are obligate coprophages. When McBrayer (1973) cultured millipedes in containers, which excluded their feces, the millipedes lost weight. When a small tray containing feces was added to the cultures, the millipedes consumed it and prospered. Such obligate coprophagy indicates a close relationship with bacteria necessary for digestion of vegetable material. It is not known whether millipedes possess a unique gut flora of microbes.
Centipedes (Chilopoda) are common predators in soil, litter, and cryptozoan habitats (Fig. 4.43). They are all elongate, flattened, active forms. Centipedes occur in biomes ranging from forest to desert. The large desert centipedes (Scolopendromorpha) are some 15-cm long; tropical centipedes may exceed 30 cm (Shelley, 2002). Lithobiids are the common brown, flat centipedes of litter in hardwood forests. The elongate, slim geophilomorph centipedes are euedaphic in forest habitats, where they prey on earthworms, enchytraeids, and Diptera larvae (Lock and Dekoninck, 2001). Like the millipedes, centipedes lose water through their cuticles at low relative humidities. They avoid desiccation by seeking moist habitats, and by adjusting their diurnal activities to humid periods in desert and sand dune habitats.
Centipedes are distinguished from superficially similar organisms by the presence of forcipules, the modified first segment upon which the head rests (Mundel, 1990). This segment bears the pincerlike fangs, which have poison ducts opening at their tips. Five orders of centipedes
are recognized, and Mundel (1990) provides keys to orders and families of centipedes of the world.
All centipedes are predators but may ingest some leaf litter on occasion—it can sometimes be seen in their guts. Centipedes are fast runners and actively pursue and capture small prey such as collembolans.
The scorpion, the archetypal generalized arachnid with its long, segmented, stinger-bearing abdomen and chelate palpi, needs no description. It was obvious to the ancients—the only zodiacal sign bearing the name of a soil organism. Scorpions (Fig. 4.44) are inhabitants of warm, dry, tropical, and temperate regions but reach their greatest diversity in deserts. They are highly mobile predators of other arthropods and occasionally even small vertebrates. The selection of prey items utilized is large; for one species of scorpion more than 100 different prey were recorded (Crawford, 1990). Relatively few individual scorpions forage at any one time. Different species demonstrate different patterns of predation, some being "sit and wait" predators and others acting as mobile hunters (Crawford, 1990). Scorpions are also cannibalistic to an unusual extent (Williams, 1987).
Typical cryptozoans, scorpions hide under rocks or logs, or in crevices, during the day and emerge at night to feed. In the southeastern United States, scorpions may be trapped by placing wet cloth on the ground at night; the dampness will attract them. The scorpion cuticle will fluoresce under black light, offering a nighttime survey procedure.
Scorpions' stings are painful, about the same as a honeybee sting but of shorter duration. In the southeastern United States, the tiny scorpion Vejovis carolinus, commonly found under bark of pine stumps, invades houses on occasion. A species with similar habits, Centruroides vittatus, occurs from east Texas to the southwestern United States (Shelley and Sisson, 1995). Only a few species of scorpions are deadly. Of
the 1500 species worldwide, only about 20-25 are dangerous, all in the family Buthidae. Where dangerous species occur, antivenom is usually available (Jackman, 1997).
The impact of scorpions on their ecosystems is unknown. They are not numerous, but in desert ecosystems they may be dominant predators (Polis, 1991).
Spiders (Araneae) (Fig. 4.45) are another familiar group of carnivores. They are solitary hunters, exhibiting a range of strategies from "sit and wait" with silken webs to active pursuit of prey. They are found in all terrestrial environments except truly polar (Arctic/Antarctic) regions. Many species are found in aboveground habitats, but some are cryptozoans in litter and on the soil surface. Some small spiders are euedaphic (Fig. 4.46). Some of the small litter-inhabiting spiders could be considered microarthropods. Spiders may be active hunters or "sit and wait" dwellers in retreats. Wolf spiders (Lycosidae) (Fig. 4.47) are common wandering predators in leaf litter and on soil surfaces, and are often captured in pitfall traps. They are conspicuous ground-dwelling predators in agroecosystems (Draney, 1997).
Spider taxonomy is a dynamic discipline. There are about 100 families in the order, arranged in several suborders (or infraorders). Kaston's (1978) guide, How To Know The Spiders, is an excellent introduction for the novice arachnologist. And Roth's (1993) Spider Genera of North America is invaluable for workers in the United States. Regional works
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