ARACHNID

any member of the class Arachnida, phylum Arthropoda, which is composed of primarily carnivorous invertebrates, including the spiders, scorpions, mites, and ticks. Arachnid bodies are segmented, with well-developed heads and hard external skeletons. They range in size from the tiny mite 0.1 mm (0.004 inch) long to the 18-centimetre (7-inch) black scorpion of Africa. The body is divided into two parts: the cephalothorax (joint head and thorax), with six pairs of appendages, and the abdomen, or opisthosoma. The first two pairs of appendages, the chelicerae and the pedipalps, are used to hold or grasp prey. The remaining pairs are walking legs. Arachnids have no antennae but rely on tactile hair, simple eyes, and slit sense organs for sensory information. Courtship among arachnids is necessary, with the male giving off a chemical or visual key or exhibiting a characteristic behaviour. The male arachnid usually deposits the sperm on the ground as a spermatophore or constructs a special web called a sperm web and then transfers it into the female. Most arachnids lay eggs, but some species (e.g., scorpions) bear live young. The degree of maternal care varies among arachnids but is an important part of survival for newborn scorpions, which are carried on the female's back. As arachnids grow they molt, or shed their cuticles, several times before reaching maturity. All arachnids, except harvestmen, are unable to digest their food internally; they inject digestive fluids into their prey and suck the liquefied remains into their mouths. Many arachnid forms are distributed worldwide, inhabiting nearly every region from deserts to rain forests. Most arachnid groups are free-living, but many species of mites and ticks are parasitic. The latter also are carriers of serious human and animal diseases. Certain venomous spiders and scorpions pose a danger to humans, but most are harmless and prey on insect pests. any member of the class Arachnida, part of the invertebrate phylum Arthropoda. Arachnida includes forms such as spiders (Araneida), harvestmen (daddy longlegs; Opiliones), scorpions (Scorpionida), and mites and ticks (Acari), as well as lesser-known groups. Only a few species are of economic importance, including the acarids that transmit diseases to humans, other animals, and plants. Additional reading The arachnids are discussed in Vicki Pearse et al., Living Vertebrates (1987), ch. 22, Chelicerates, pp. 529564, an introduction; Robert D. Barnes, Invertebrate Zoology, 5th ed. (1987), with a particularly good discussion of arachnids in ch. l3, The Chelicerates, pp. 492553; Alfred Kaestner, Invertebrate Zoology, vol. 2, trans. and adapted by Herbert W. and Lorna R. Levi (1968; originally published in German, 1965), an expanded textbook of arachnology and myriapodology; J.L. Cloudsley-Thompson, Spiders, Scorpions, Centipedes, and Mites, new ed. (1968), a general account of natural history and ecology; and P. Weygoldt and H.F. Paulus, Untersuchungen zur Morphologie, Taxonomie und Phylogenie der Chelicerata, Zeitschrift fr zoologische Systematik und Evolutionsforschung, 17 (2):85116 (June 1979), a discussion, in German, of the phylogeny of the arachnids. Detailed higher classifications, with brief summaries of each group, are provided in Sybil P. Parker (ed.), Synopsis and Classification of Living Organisms, vol. 2 (1982). Marie Louise Goodnight