Arachnid

• 1. Introduction

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  Arachnids share the planet with us in extraordinary numbers. Over 110,000 named species have been described so far, and scientists estimate there may be more than a million in total. Spiders alone account for 51,000 of those known species. From the scorpion navigating desert rock to the house dust mite invisible in your bedding, these eight-legged arthropods occupy almost every habitat on earth. Yet for all their familiarity, arachnids remain genuinely strange creatures. Their name traces to a Greek myth about a woman named Arachne, a weaver so skilled and so proud that her hubris cost her human form. What turned Arachne into a spider? And what, exactly, makes an arachnid an arachnid rather than any other creeping thing? The answers involve hydraulic limbs, copper-based blood, a digestive system that works outside the body, and a family tree that scientists are still untangling from the deep Devonian past.

• 2. The Architecture of Eight Legs

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  Eight legs is the most famous number in arachnid biology, but the real count of appendages is higher. Every adult arachnid also carries two additional pairs of structures that have been converted over evolutionary time into tools for feeding, defense, and sensing the world. The first pair, the chelicerae, handle feeding and defense. The second, the pedipalps, serve locomotion, feeding, or reproduction depending on the order. In scorpions and pseudoscorpions, those pedipalps end in pinchers. In the Solifugae, or camel spiders, the palps are so leg-like that these animals appear to have ten legs rather than eight. Larvae of mites and Ricinulei start life with only six legs, gaining a fourth pair after moulting into nymphs. Adult mites in the family Eriophyoidea carry only four legs throughout life. The adult females in some members of Podapolipidae have a single pair. The range is wide enough to make the eight-leg rule feel more like a guideline. What all arachnids lack, and what sharply separates them from insects, are antennae and wings. Their body divides into two sections: the prosoma at the front, and the opisthosoma behind. The abdomen in many groups contains a heart and respiratory organs, organs that traditionally belong elsewhere in animal anatomy.

• 3. Moving and Breathing on Land

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  Spiders and whip scorpions extend their limbs not by muscle but by hydraulic pressure, pushing hemolymph through the appendages to straighten them. Solifuges and some harvestmen use a different method: highly elastic thickenings in the joint cuticle act as springs. Scorpions, pseudoscorpions, and some harvestmen evolved muscles that can extend two leg joints at once, the femur-patella and patella-tibia joints, while the pedipalp joints of scorpions rely on elastic recoil. These varied solutions all serve the same terrestrial demand: moving efficiently on land. The respiratory adaptations are equally varied. Some arachnids breathe through internal tube systems called tracheae. Others use book lungs, an internal structure of vascular lamellae derived from ancestral book gills. Ricinuleids, pseudoscorpions, and certain spiders possess sieve tracheae, in which multiple tubes emerge in a bundle from a single small chamber. Scientists believe this type evolved directly from book lungs, which means arachnid tracheae and insect tracheae are not the same structure at all, despite looking similar. Blood composition shifts with the method of respiration. Arachnids with efficient tracheal systems do not need to carry oxygen in the blood and may have reduced circulatory systems. Scorpions and some spiders, though, use haemocyanin, a copper-based pigment that functions like haemoglobin in vertebrates. Some mites have no heart at all.

• 4. Feeding From the Outside In

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  Arachnids are mostly carnivorous, but they do not chew. Instead, they produce digestive enzymes in their stomachs and pour those enzymes over dead prey using the chelicerae and pedipalps. The tissues dissolve into a nutrient broth, which the animal then sucks into a pre-buccal cavity in front of the mouth. A muscular, hardened pharynx behind the mouth acts as a pump, drawing the liquid through into the oesophagus and beyond. In some arachnids the oesophagus doubles as a second pump. The stomach is tubular, with branching diverticula that extend through most of the body, both producing enzymes and absorbing what the digestion yields. Scorpions, spiders, and pseudoscorpions reinforce this system with venom glands, whose secretions include not only toxins but pre-digestive enzymes. Ticks have a different strategy: their saliva contains anticoagulants and anticomplements, and several species produce a neurotoxin. Harvestmen and some mites, including the house dust mite, are the only arachnids capable of ingesting solid food, which opens them to internal parasites in turn. Harvestmen eat an unusually wide range of material: other animals, decomposing plant and animal matter, droppings, and mushrooms. One species of spider has been recorded as mostly herbivorous. And while it might seem counterintuitive, spiders occasionally eat their own silk, a habit that is not unusual within the group.

• 5. Senses and Signals

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  Two kinds of eyes appear across the arachnid orders: median ocelli and lateral ocelli. Lateral ocelli evolved from compound eyes and in some species carry a tapetum, a reflective layer that amplifies incoming light. Scorpions can carry up to five pairs of lateral ocelli, the maximum found in the class. Median ocelli develop from a fold of the ectoderm and represent an older lineage of light-detection. In most arachnids, the retina does not contain enough light-sensitive cells to form a true image. The cornea of the eye doubles as a lens and is continuous with the outer cuticle of the body itself. Beyond vision, fine sensory hairs covering the body provide touch. Many arachnids also carry trichobothria, more complex hair-like structures that detect air movement. Finally, slit sense organs, narrow pits covered with a thin membrane, contain a small hair that registers the membrane's motion. These organs are believed to contribute to proprioception, the animal's sense of its own body position, and possibly to hearing. Together, these three sensory systems operate without any antennae at all.

• 6. Reproduction and Parental Care

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  Male arachnids in most orders deliver sperm to females packaged in a spermatophore rather than through direct transfer. Harvestmen and some mites depart from this pattern: males in those groups have a penis. Complex courtship rituals have evolved widely across the class, ensuring that sperm reaches the female safely before any defensive instincts can interfere. Sexual dimorphism is common across many orders. Most arachnids lay yolky eggs that hatch into immatures resembling small adults. Scorpions are an exception: depending on species, they are either ovoviviparous or viviparous, giving birth to live young. Some mites are also ovoviviparous or viviparous. Parental care is rare. In most species, only females provide it at all. Harvestmen stand out as one of the few groups in which males also participate in looking after young.

• 7. A Family Tree Still Being Written

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  A broad consensus on arthropod relationships emerged around 2010, drawing on both fossil evidence and molecular data. Living arthropods divide into three main clades: chelicerates, which include arachnids; pancrustaceans, which group crustaceans with insects; and myriapods, the centipedes and millipedes. Within chelicerates, a 2019 molecular analysis placed horseshoe crabs not as an outgroup to arachnids but nested deeply within Arachnida itself. That finding implies the story of how arachnids moved from sea to land is more complicated than earlier models suggested. Relationships within the arachnids have proven harder still. A study in 2014, based on the largest molecular dataset compiled at that time, found systematic conflicts in the phylogenetic information, particularly for Acariformes, Parasitiformes, and Pseudoscorpiones, groups with unusually fast evolutionary rates. More recent genetic analyses have placed pseudoscorpions as the sister group of scorpions, combining them into a clade called Panscorpiones. Six orders within the broader grouping Arachnopulmonata share an ancient whole-genome duplication. Horseshoe crabs have gone through two whole-genome duplications and now carry five Hox clusters with 34 Hox genes, the highest count found in any invertebrate. One fossil, Chimerarachne yingi, preserved in amber estimated at around 100 million years old from Myanmar, carries spinnerets like a modern spider but also a tail like the much older Uraraneida, a combination that places it squarely in contested evolutionary territory.