Arachnida

arachnids

Subclass Guides

11

Arachnida is a class of joint-legged within the subphylum Chelicerata. The group includes approximately 93,000 described across 11 traditionally recognized orders, with Acari (mites and ticks) and Araneae (spiders) comprising the vast majority of diversity. Arachnids originated from marine ancestors during the Silurian or Devonian periods and independently colonized land, developing diverse solutions to terrestrial challenges including respiration, , and locomotion. All extant arachnids are terrestrial except for some secondarily aquatic mites and a few semiaquatic spiders.

Hydrozetoidea by (c) Even Dankowicz, some rights reserved (CC BY), uploaded by Even Dankowicz. Used under a CC-BY license.Sarcoptes by no rights reserved, uploaded by Tero Linjama. Used under a CC0 license.Endeostigmata by (c) Adam J. Searcy, some rights reserved (CC BY), uploaded by Adam J. Searcy. Used under a CC-BY license.

Pronunciation

How to pronounce Arachnida: /əˈræknɪdə/

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Habitat

Arachnids occupy virtually every terrestrial on Earth, from deserts to tropical rainforests and from intertidal zones to high mountain elevations. Different orders exhibit distinct habitat preferences: spiders occur in nearly all terrestrial environments; scorpions are particularly diverse in arid and semi-arid regions; mites and ticks inhabit soil, leaf litter, freshwater, and parasitic environments on plants and animals. Many groups have colonized freshwater habitats, though true marine arachnids are rare.

Distribution

Arachnids have a distribution across all continents except Antarctica, occurring from equatorial to polar regions. Diversity hotspots are concentrated in tropical and subtropical areas. Scorpions show highest diversity in subtropical deserts; spiders peak in tropical forests; harvestmen are abundant in humid tropical and temperate forests; mites and ticks achieve global distribution including remote oceanic islands through small size and abilities. Specific comparative studies have examined arachnofaunas between Mediterranean and Central European regions, Mexico and the USA, Central America and the Caribbean Islands, South Africa and associated Indian Ocean islands, Patagonia, New Guinea, Australia, New Zealand, New Caledonia, and Lord Howe Island.

Diet

The vast majority of arachnids are predatory. Spiders are exclusively predatory, capturing prey through webs, ambush, or active hunting. Scorpions are of insects and other arthropods. Solifugae are fast-running predators. Pseudoscorpions prey on small arthropods. Harvestmen function primarily as scavengers and opportunistic predators. Acari exhibits the greatest dietary diversity, including predatory mites, herbivorous plant-feeding mites, and parasitic ticks feeding on vertebrate blood.

Life Cycle

Arachnids typically undergo direct development without , hatching from as miniature versions of . The generally includes egg, prelarva (in some groups), post-embryonic stages, and adult. Juveniles pass through a series of (instars) before reaching sexual maturity. The number of instars varies: spiders typically have 5-10 instars; scorpions may have 6-7 instars over several years; mites and ticks show highly variable life cycles with some having only three active stages (larva, nymph, adult) and others complex multi-year cycles. Growth is indeterminate in most arachnids, with molting continuing in some groups even after sexual maturity.

Behavior

Silk production occurs in spiders and some other arachnids, used for prey capture, shelter, protection, and . Venom is widespread, particularly in spiders and scorpions, for prey subjugation and defense. Book lungs and tracheal systems represent convergent respiratory adaptations to terrestrial life. (voluntary leg loss) occurs in many groups as an escape mechanism. (death feigning) is common in harvestmen and some spiders. Ballooning (aerial dispersal via silk threads) allows spiders to colonize remote areas. (hitchhiking on larger animals) is practiced by pseudoscorpions and some mites for dispersal.

Ecological Role

Arachnids function as critical in virtually all terrestrial , regulating of insects and other . Spiders are among the most abundant terrestrial predators by in many . Scorpions serve as important predators in arid ecosystems. Mites dominate soil microarthropod , contributing to decomposition and nutrient cycling. Ticks act as for numerous affecting wildlife, livestock, and humans. Some mites are significant agricultural pests, while predatory mites serve as important agents. Harvestmen contribute to decomposition as scavengers.

Human Relevance

Spiders consume an estimated 400-800 million tons of prey annually, mostly insects, providing substantial services in pest control. Spider silk has attracted biomimetic interest due to its strength and elasticity. Ticks and some mites are medically and veterinary significant as and . Some spider venoms contain compounds with potential pharmaceutical applications. Arachnids feature prominently in public education programs addressing misconceptions and arachnophobia. Certain are kept as pets, particularly tarantulas.

Misconceptions

The belief that tarantulas are aggressive is unfounded; North American show little interest in human interaction. Brown recluse spiders do not have established in California despite frequent misidentifications. While nearly all spiders possess venom for prey capture, very few species have venom harmful to humans.

Sources and further reading