Arachnid
The Greek word arachne means spider, and it comes from a myth about a human weaver turned into an eight-legged creature. Adult arachnids possess exactly four pairs of legs attached to the cephalothorax. This count distinguishes them sharply from insects which carry only three pairs. Some species modify their frontmost pair for sensing rather than walking. Others grow appendages so large they mimic extra leg pairs. The body divides into two tagmata called the prosoma and opisthosoma. These terms translate to cephalothorax and abdomen respectively. No fossil evidence shows these animals ever had a separate thorax-like division. The cephalothorax usually wears a single unsegmented carapace. The abdomen varies in segmentation across different groups. Scorpions clearly display preabdomen and postabdomen sections. Mites often fuse all abdominal segments together. A telson appears as a stinger in scorpions or a flagellum in whip scorpions.
Internal respiratory surfaces allow most arachnids to survive outside water. Tracheae function as individual tubes similar to insect systems. Ricinuleids and pseudoscorpions utilize sieve tracheae where multiple tubes bundle from a small chamber. Book lungs evolved from modified book gills into vascular lamellae for gas exchange. Arachnid blood composition changes based on respiration mode. Species with efficient tracheal systems may lack oxygen transport in their blood. Scorpions and some spiders carry haemocyanin, a copper-based pigment. This substance functions similarly to vertebrate hemoglobin. The heart sits in the forward part of the abdomen. Some mites possess no heart at all. Excretory glands include up to four pairs of coxal glands along the prosoma side. One or two pairs of Malpighian tubules empty into the gut. Guanine serves as the primary nitrogenous waste product. A waxy layer covers the cuticle to conserve water efficiently.
Most arachnids lack extensor muscles in distal joint appendages. Spiders extend limbs hydraulically using hemolymph pressure. Whip scorpions employ hydraulic extension methods too. Solifuges and some harvestmen use elastic thickenings in joint cuticles to extend knees. Scorpions evolved muscles extending femur-patella and patella-tibia joints simultaneously. Pedipalp equivalent joints in scorpions rely on elastic recoil. Eyes exist as lateral ocelli and median ocelli types. Lateral ocelli evolved from compound eyes and may contain a tapetum. Scorpions can hold five pairs of lateral ocelli. Median ocelli develop from transverse ectoderm folds. Most retinas lack enough light-sensitive cells for proper image formation. Fine sensory hairs cover bodies providing touch capabilities. Trichobothria represent complex structures within these hair systems. Slit sense organs function as pit-like membranes detecting motion. These organs likely handle proprioception and possibly hearing functions.
Arachnids mostly consume pre-digested insect or small animal bodies. Ticks and many mites live as parasites carrying disease. Mite diets include fungi, plant juices, and decomposing matter. Harvestmen eat decaying plants, droppings, animals, and mushrooms. Only harvestmen and house dust mites ingest solid food directly. Spiders sometimes consume their own silk production. Venom glands secrete enzymes to kill prey or defend against threats. Tick saliva contains anticoagulants and anticomplements. Some species produce neurotoxins affecting nervous systems. Stomachs produce digestive enzymes poured over dead prey. Pedipalps and chelicerae deliver fluids onto the target. Juices turn prey into nutrient broth quickly. Arachnids suck this liquid into a pre-buccal cavity. A muscular pharynx acts as a pump moving food forward. The stomach extends through most of the body with diverticula. These extensions absorb nutrients while producing more enzymes.
Males transfer sperm in packages called spermatophores to females. Harvestmen males possess actual penises for direct transfer. Complex courtship rituals ensure safe delivery across many orders. Sexual dimorphism appears frequently among different arachnid groups. Females usually lay yolky eggs hatching into immature forms resembling adults. Scorpions bear live young either ovoviviparously or viviparously. Some mites also give birth to live offspring instead of eggs. Female parental care dominates most arachnid species. Harvestmen stand out as exceptions providing male care too. Gonads sit within the abdomen region. Genital openings locate on the underside of the second abdominal segment. Development proceeds from egg to adult stages directly. No larval stage exists between these phases for most species.
Consensus emerged around 2010 regarding chelicerate relationships. Extant arthropods split into three main clades: chelicerates, pancrustaceans, and myriapods. A 2019 analysis placed horseshoe crabs deeply inside Arachnida. Molecular data showed systematic conflicts affecting Acariformes and Parasitiformes. Slowly evolving genes demonstrated monophyly of Chelicerata and Euchelicerata. Tetrapulmonata includes spiders, whip scorpions, and blunt rump tail-less whip scorpions. Opiliones, Ricinulei, and Solifugae form a supported combination group. Horseshoe crabs nested inside Arachnida suggest complex terrestrialization history. Uraraneida represents an extinct spider-like order from Devonian times. Chimerarachne yingi fossil in amber shows spinnerets and tails. This specimen dates back 100 million years from Myanmar. It resembles primitive living spiders known as mesotheles. Haptopoda remains an extinct group apparently part of Tetrapulmonata.
Over 110,000 named species exist within the class Arachnida. Spiders comprise 51,000 of these described species. Mites account for 32,000 species under Acariformes classification. Ticks make up 12,000 species within Parasitiformes groups. Harvestmen number approximately 6,700 species globally. Scorpions total around 2,700 distinct species worldwide. Pseudoscorpions reach 4,000 species in current records. Whip scorpions include both Amblypygi and Uropygi orders totaling 370 species. Solifuges or camel spiders represent 1,200 species. Palpigradi microwhip scorpions contain only 130 species. Ricinuleids hooded tickspiders number just 100 species. Trigonotarbida became extinct between late Silurian and Early Permian periods. Phalangiotarbida fossils show uncertain affinity with 30 known species. Estimates suggest over one million arachnid species remain undiscovered. Extinct forms like Haptopoda contain single known species.
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Common questions
What is the origin of the word arachnid?
The Greek word arachne means spider and comes from a myth about a human weaver turned into an eight-legged creature. Adult arachnids possess exactly four pairs of legs attached to the cephalothorax.
How do most arachnids breathe outside water?
Internal respiratory surfaces allow most arachnids to survive outside water through tracheae functioning as individual tubes similar to insect systems. Book lungs evolved from modified book gills into vascular lamellae for gas exchange in many species.
When did the Chimerarachne yingi fossil date back to?
This specimen dates back 100 million years from Myanmar and resembles primitive living spiders known as mesotheles. It shows spinnerets and tails within amber from the Devonian times period.
Which arachnid groups have the highest number of described species?
Spiders comprise 51,000 of these described species while mites account for 32,000 species under Acariformes classification. Ticks make up 12,000 species within Parasitiformes groups and harvestmen number approximately 6,700 species globally.