Chelicerata

Chelicerates are distinguished from all other arthropods by the presence of chelicerae—appendages located before the mouth that are deutocerebral in origin. This contrasts with Mandibulata (crustaceans, myriapods, hexapods), which possess antennae on the first segment and mandibles on the third. The absence of antennae is a definitive chelicerate trait. Within Chelicerata, major groups are recognized by respiratory structures (book gills in Xiphosura, book lungs and/or tracheae in most arachnids), body tagmosis, and appendage specialization. Recent phylogenomic analyses suggest mites (traditionally united as Acari) may not form a monophyletic group, with Parasitiformes more closely related to other chelicerates than to Acariformes.

Chelicerates occupy virtually every terrestrial habitat from deserts to tropical forests, as well as marine environments including coastal zones and deep ocean. Marine species include four living horseshoe crab species and approximately 1,300 sea spider species. Terrestrial species dominate in diversity, with arachnids particularly abundant in soil, leaf litter, vegetation, and as symbionts of other organisms. Specific microhabitats vary enormously: spiders occupy webs, burrows, or hunting territories; scorpions favor arid rocky areas; mites inhabit soil, water, plants, and animal hosts. Some spiders construct underwater diving bells using silk to trap air bubbles.

Cosmopolitan. Marine chelicerates occur in coastal waters worldwide, with horseshoe crabs concentrated in the Atlantic and Indo-Pacific. Sea spiders are found from intertidal zones to deep ocean. Terrestrial chelicerates occur on all continental landmasses and most islands, from polar regions to equatorial zones. Diversity centers for major groups include the tropics for many spider families, arid regions for scorpions, and global distribution for mites.

Activity patterns vary by group and climate. Temperate species often show seasonal cycles with winter dormancy or reduced activity. Many spiders and scorpions are nocturnal, while some spiders (notably jumping spiders) are diurnal. Horseshoe crabs exhibit mass spawning events timed to spring tides and lunar cycles. In tropical regions, activity may continue year-round with peaks during wet seasons.

Chelicerates employ all major feeding strategies: predation, parasitism, herbivory, scavenging, and consumption of decaying organic matter. Most spiders and scorpions are predators that subdue prey with venom or physical capture. Many mites are parasitic (ticks, Varroa) or herbivorous (spider mites). Harvestmen include predators, herbivores, and scavengers. Most modern chelicerates have narrow guts and pre-process food by grinding with chelicerae and pedipalps, flooding with digestive enzymes to liquidize it before ingestion. Harvestmen and horseshoe crabs are exceptions capable of processing solid food.

Development is typically direct, with eggs hatching as miniature adults or nymphs that gradually acquire adult features through molting. Horseshoe crabs use external fertilization with trilobite-like larvae bearing full appendage sets. Most air-breathing chelicerates use internal but usually indirect fertilization, with males often employing elaborate courtship rituals. Many species lay eggs, but scorpions and some mites are viviparous or ovoviviparous, retaining eggs internally until young emerge. Post-embryonic development involves multiple molts; in some species (e.g., amblypygids), mature individuals continue molting and growing throughout life. Parental care ranges from absent to prolonged, with scorpions carrying young on their backs and some spiders protecting and feeding offspring.

Chelicerates rely heavily on modified bristles (setae) for sensory information about touch, vibrations, air currents, and chemical changes. Most have limited vision; jumping spiders are exceptional with acute color and UV vision. Many spiders produce silk from abdominal spinnerets for webs, egg sacs, or dispersal. Venom has evolved independently multiple times: in spiders, scorpions, and pseudoscorpions (and possibly ticks if hematophagous secretions are considered venom). Chemical defense occurs in whip scorpions, harvestmen, and some mites. Social behavior is rare but occurs in some spider species; most are solitary. Tailless whip scorpions (Amblypygi) exhibit kin recognition and prolonged subsocial grouping in at least one species (Damon diadema).

Chelicerates occupy critical positions in food webs as predators, parasites, and prey. Spiders are among the most important terrestrial predators of insects. Mites play major roles in decomposition, nutrient cycling, and as plant pests or pollinators. Ticks and some mites are significant disease vectors and parasites of vertebrates. Horseshoe crab eggs provide seasonal food resources for migratory shorebirds. As prey, chelicerates support numerous vertebrate and invertebrate predators. Some mites are used as biological control agents against agricultural pests.

Medical and economic importance is substantial. Spider and scorpion venoms are sources of bioactive compounds under investigation for treatments including cancer, cardiac arrhythmia, Alzheimer's disease, and erectile dysfunction. Horseshoe crab blood contains Limulus amebocyte lysate, used to detect bacterial contamination in pharmaceuticals and medical devices. Spider silk proteins have been produced through genetic engineering in goats, silkworms, and bacteria for commercial applications. Negative impacts include: venomous bites from some spider and scorpion species causing human fatalities (approximately 100 spider deaths in the 20th century, ~1,000 scorpion deaths annually in Mexico); tick-borne diseases including Lyme disease and Rocky Mountain spotted fever; mite allergies and asthma; and agricultural damage from spider mites and other pest species. Some spiders and scorpions are consumed as food in Cambodia and Venezuela.

• MandibulataThe other major arthropod subphylum, distinguished by presence of antennae and mandibles rather than chelicerae. Includes crustaceans, myriapods, and hexapods (insects).
• OnychophoraVelvet worms, sometimes confused with terrestrial chelicerates due to soft bodies and similar habitats, but possess antennae, slime glands, and different body plan.

The historical classification of mites as a unified taxon (Acari) based on the gnathosoma is now considered artificial. Recent phylogenomic analyses indicate mites are not monophyletic: Parasitiformes appear more closely related to other chelicerates than to Acariformes. The gnathosoma itself is not a reliable unifying character, as palpal coxae fusion—the key structural basis—does not occur in many mite species. Additionally, tailless whip scorpions (Amblypygi) are frequently misidentified as spiders or scorpions due to their appearance, but represent a distinct arachnid order without venom or silk production.

• arthropod
• arachnid
• spider
• scorpion
• mite
• tick
• horseshoe crab
• sea spider
• venom
• silk
• predator
• parasite

• BugGuide
• Wikipedia
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Damon diadema—Tanzanian giant tailless whip scorpion | Beetles In The Bush
• Arachnida | Beetles In The Bush | Page 5
• entomologists | Beetles In The Bush | Page 4
• chelicerates tree - Entomology Today
• Mites Are a Made-Up Taxon: New Analysis Further Debunks Long-Held Classification
• Lebenszyklus und postembryonale entwicklung der geibelspinne Tarantula marginemaculata C. L. Koch (Chelicerata, Amblypygi) im laboratorium Life cycle and postembryonic development of the tailless whip scorpion, Tarantula marginemaculata (Chelicerata, Amblypygi) in the laboratory