Class Arachnida

During the middle Paleozoic, chelicerates made the rigorous transition from water to land; only later did over 5000 species of arachnid mites adopt a secondary aquatic existence. Arachnids then rapidly radiated in form and species richness in association with their predaceous and parasitic exploitation of insects. One crucial factor in this success has been the diverse uses of silk by spiders, pseudoscorpions, and some mites. Although some degree of "arachnophobia" afflicts many people, relatively few of the 76,000 described species are directly harmful to humans because of their venom, link with diseases and allergies, or competition for plant resources. More than balancing their negative attributes is the substantial role in biocontrol of insect pests.

scorpions, spiders, and harvestmen

True Scorpions The 1500 to 2000 species of true scorpions (order Scorpiones) are elders of the arachnid clan (Fig. 3). In addition to the true scorpions, several other arachnid orders are called "scorpions": false scorpions (Pseudoscorpiones with 2000 species), wind scorpions (Solifugae with 900 species), whip scorpions (Uropygi with 85 species), and tailless whip scorpions (Amblypygi with 70 species). All are much smaller than true scorpions, but are also typically carnivorous.

FIGURE 3 Desert hair scorpion, Hadrurus arizonensis. (Photograph by Jim Kalisch, courtesy of University of Nebraska Department of Entomology.)

True Spiders (Order Araneae) Most arachnids lack biting mouthparts and must, therefore, partially digest prey tissue before sucking it into their bodies. Prey are subdued with poison injected by fangs present on each chelicera. Arachnids reproduce with indirect fertilization (without a penis), often after elaborate courtship rituals. Their leglike pedipalps are used by males to transfer spermatophores. Eggs are wrapped in a protective silken cocoon, and brood care is common. Silk is produced normally by caudal spinnerets and by a small platelike organ (cribellum) in cribellate spiders only. Uses for this silk include cocoons, egg sacs, linings of retreats, and capture webs. Locomotion is typically by walking or jumping, but aerial dispersal through the process of "ballooning" with long silken threads is common in most spiderlings and adults of some smaller taxa. Most spiders are terrestrial and are found anywhere insects are located. All are carnivorous, and ecological divergence in prey type and capture method has led to the wide evolutionary radiation. In addition to insects, spiders attack other spiders, small arachnids, and a few other prey taxa including small vertebrates. Several spiders are poisonous to humans, such as the black widow and the brown recluse.

Harvestmen The order Opiliones includes arachnids known as "daddy longlegs," a name reflecting its enormously long walking legs. They are also called "harvestmen" because some species undergo a seasonal population explosion each autumn around the farm harvest. They have "repugnatorial glands" that produce an acrid secretion to repel predators. The 5000 species are more closely related to mites than to true spiders. Most are tropical, but taxa are known from colder subarctic and alpine zones. Opilionids frequent humid forest floors, being less arboreal than true spiders. Although carnivory on small arthropods and worms is common, harvestmen are notable as the only arachnids other than mites that consume vegetation.

mites and ticks At least 30,000 species of arachnid mites and ticks have been described in the order (or subclass) Acari. The major habitat of mites is on land, where they are either free-living or parasites of plants and animals, but lakes, streams, and even hot springs support 5000 taxa, with the marine fauna being less diverse.

Many acarines are ectoparasitic in larval and/or adult stages. Animal parasites attack mammals (including humans and domestic animals), birds, reptiles, amphibians, fish, aquatic and terrestrial insects, other arachnids, and some other invertebrates, including echinoderms, mollusks, and crustaceans. They are vectors for several human diseases, and some (e.g., chiggers) have annoying bites. Many people develop allergies to mites living on household dust. On the other hand, microscopic mites commonly consume dead tissue and oily secretions on human faces, and they are used to control harmful insects and mites. Their feeding habits and role in spreading viruses make them severe pests of natural and agricultural plants. A great diversity of mites, however, are free-living, mostly in forest and grassland litter, where they feed directly on litter or on microorganisms decomposing detritus. Many mites prey on other mites, nematodes, and small insects.

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