Spiders make up just one of roughly a dozen living orders within the class Arachnida. Scorpions, harvestmen, ticks, mites, pseudoscorpions, vinegaroons, whip spiders, camel spiders, and several far more obscure groups all belong to the same class, and a morphology-based analysis of arachnid relationships has recovered a tree with spiders nested alongside whip scorpions and whip spiders in a group called Tetrapulmonata, while scorpions, harvestmen, mites, and others branch off elsewhere in the arachnid family tree.1Zoological Journal of the Linnean Society. A phylogenetic analysis of the arachnid orders based on morphological characters The variety is enormous. Some of these animals are among the most familiar creatures on Earth; others are so rare and tiny that most entomologists have never seen one alive.
Scorpions
Scorpions are probably the first non-spider arachnid most people think of. They have a fossil record stretching back over 430 million years, making them among the oldest known terrestrial arthropods. All roughly 2,500 described species share the same basic body plan: a pair of pedipalps modified into pincers, a segmented tail curving over the body, and a venom-bearing stinger at the tip called the telson. That stinger is a remarkable piece of engineering. Both experimental and computational work has shown that the curved shape of the stinger makes the venom reservoir structurally robust against forces from almost any direction, an optimization shaped by hundreds of millions of years of natural selection.2Journal of the Mechanics and Physics of Solids. Morphological optimization of scorpion telson
One of the stranger scorpion traits is fluorescence. Under ultraviolet light, scorpions glow a vivid blue-green. Research on the phenomenon has found that the fluorescence spectrum peaks around 475 nanometers and is consistent across multiple developmental stages. When a scorpion molts, the freshly shed exoskeleton pieces recover their fluorescence at different rates depending on the UV wavelength used: under UVA light, shed pincers and stingers regain their glow in about six hours, while under shorter-wavelength UVB and UVC light the recovery can take up to 72 hours.3Spectroscopy Online. Unveiling the Mysteries of Scorpion Fluorescence: Insights from Ultraviolet Excitation Why scorpions fluoresce at all remains debated; hypotheses range from UV detection to a simple by-product of exoskeleton chemistry. What is clear is that the property develops with cuticular hardening, not with a specific life stage, since even juvenile instars glow once their cuticle has cured.
Harvestmen
Harvestmen, order Opiliones, are commonly called “daddy longlegs” in much of the English-speaking world, which leads to no end of confusion because the same nickname is applied to cellar spiders and crane flies. Harvestmen are not spiders. Their body plan gives them away: the cephalothorax and abdomen are broadly fused into a single compact oval, whereas spiders always have a visible waist between their two body sections. Harvestmen also lack venom glands entirely and do not produce silk.
What harvestmen do have is chemistry. When threatened, many species release defensive secretions from specialized scent glands on the front of the body. In the species Rilaena triangularis, for example, the secretion is ejected as a directed jet and smells pungent. Analysis of that spray found it contains mainly 1,4-benzoquinone along with 1,4-naphthoquinone and octanoic acid. That finding was notable because benzoquinone-based defense had previously been documented only in a different harvestman suborder; discovering it in Rilaena extended the known distribution of this chemical strategy across a much broader swath of harvestman diversity.4PubMed Central. Benzoquinones from scent glands of phalangiid harvestmen (Arachnida, Opiliones, Eupnoi): a lesson from Rilaena triangularis
Harvestmen also show surprising parental behavior. In the cave-dwelling species Goniosoma longipes, females lay 60 to 210 eggs on cave walls and guard the egg batch for nearly two months until the hatchlings disperse. Field experiments that removed females showed that egg-guarding has a clear anti-predator function: unprotected clutches suffered far higher losses to predation. Males of the same species actively patrol their egg-guarding mates, and if a female is removed, the male will take over brood care for up to two weeks.5Journal of Zoology. Reproductive biology of the neotropical harvestman (Goniosoma longipes) (Arachnida, Opiliones: Gonyleptidae): mating and oviposition behaviour, brood mortality, and parental care Male parental care is rare among arachnids, making harvestmen an outlier in more ways than one.
Mites and Ticks
The order Acari, which includes mites and ticks, is by far the most species-rich arachnid group. Estimates of described species run to roughly 55,000, with the true number thought to be many times that. Their range of lifestyles is staggering: free-living soil predators, plant parasites, aquatic filter-feeders, and obligate parasites of vertebrates all fall under this umbrella. Ticks are the ones most people know (and dread), but it is the microscopic mites living on your own body that really underscore how diverse this group is.
Most humans carry Demodex mites in their hair follicles for their entire lives. Two species are typically found on people: Demodex folliculorum, which lives in hair follicles, and Demodex brevis, which inhabits sebaceous glands. These mites are the only multicellular animals known to live continuously on human skin.6Molecular Biology and Evolution. Human Follicular Mites: Ectoparasites Becoming Symbionts They usually cause no symptoms at all. A skin infestation becomes a problem only when mite density shoots up, often due to immune suppression or oily skin conditions.7PubMed Central. Human demodex mite: the versatile mite of dermatological importance
The evolutionary trajectory of Demodex is fascinating in itself. Genetic work has shown that mite lineages track human ancestry: people of different geographic backgrounds tend to carry genetically distinct mite populations that have been faithfully passed from parent to child for thousands of years.8PubMed Central. Global divergence of the human follicle mite Demodex folliculorum: Persistent associations between host ancestry and mite lineages Some researchers have proposed that D. folliculorum is in the process of transitioning from a parasite to a genuine symbiont, an organism that lives on its host without causing harm and may eventually provide some benefit.6Molecular Biology and Evolution. Human Follicular Mites: Ectoparasites Becoming Symbionts Whether that transition is complete is debatable, but the direction of the relationship seems clear: these mites are becoming less parasitic over evolutionary time, not more.
Pseudoscorpions
Pseudoscorpions look exactly like what the name suggests: tiny scorpion-shaped animals, complete with oversized pincers, but lacking a tail and stinger. Most are two to four millimeters long, and they live in leaf litter, under bark, and inside bird nests. Despite their obscurity, there are over 3,500 described species worldwide, and they are one of only four arachnid orders known to possess venom, alongside scorpions, spiders, and ticks. Venom gland analysis in the species Synsphyronus apimelus identified 131 transcripts coding for proteins similar to known venom components from other arachnids, revealing a surprisingly diverse cocktail dominated by enzymes and protease inhibitors.9PubMed Central. Transcriptomic Analysis of Pseudoscorpion Venom Reveals a Unique Cocktail Dominated by Enzymes and Protease Inhibitors
One of the most charming pseudoscorpion behaviors is phoresy: hitching rides on larger animals to disperse to new habitats. They grip the leg or body of a flying insect with their pincers and travel as stowaways. This strategy has been documented on beetles, flies, and other insects. The Neotropical species Semeiochernes armiger disperses by latching onto the giant timber fly Pantophthalmus tabaninus as the fly emerges from pupal boreholes inside decaying fig trees.10Biological Journal of the Linnean Society. On the threshold of dispersal: hitchhiking on a giant fly favours exaggerated male traits in a male-dimorphic pseudoscorpion In that species, the males that successfully hitch rides tend to have exaggerated body proportions, suggesting that phoresy itself creates selection pressure on body shape. A broader review of pseudoscorpion phoresy on flies confirmed that the behavior is widespread and involves associations with many different fly families, though it had not been comprehensively catalogued for more than two decades.11PubMed Central. Hitchhiking across continents: phoresy of pseudoscorpions (Arachnida, Pseudoscorpiones) on Diptera, with new European records
Vinegaroons and Whip Spiders
Vinegaroons (order Thelyphonida, also called Uropygi) and whip spiders (order Amblypygi) are close relatives of true spiders in the group Tetrapulmonata, but they look wildly different. Vinegaroons are heavy-bodied, dark-colored animals with massive pincers and a long, whip-like tail. They get their common name from the acetic acid spray they deploy in self-defense. Chemical analysis of the spray in Mastigoproctus giganteus, the giant vinegaroon of the southern United States and Mexico, found it is about 83 to 84 percent acetic acid by organic composition, with octanoic acid and water making up most of the remainder.12Journal of Insect Physiology. Chemistry, ontogeny, and role of pygidial gland secretions of the vinegaroon Mastigoproctus giganteus (Arachnida: Uropygi) The spray is ejected forcefully from a pair of glands near the base of the tail and can be aimed in almost any direction.13Journal of Insect Physiology. Defence mechanisms of arthropods—I The composition and function of the spray of the whipscorpion, Mastigoproctus giganteus (Lucas) (Arachnida, Pedipalpida) If you have ever startled a vinegaroon and caught a whiff of vinegar, now you know why.
Whip spiders, by contrast, are flat-bodied, crab-like animals that scuttle sideways across cave walls and tree bark at night. They have no venom, no silk, and no tail. What they do have is an extraordinary pair of sensory legs. The first pair of legs is not used for walking at all; instead, these “antenniform” legs are elongated into thin, flexible whips that can be several times the animal’s body length. Packed with olfactory and tactile sensors concentrated on the tarsal segment, these legs are swept constantly in all directions to detect prey, navigate in total darkness, and communicate with other whip spiders.14Zoodiversity. Sensory Structures on the Antenniform Legs of Whip Spider, Phrynichus phipsoni (Arachnida, Amblypygi), from the Indian State of Goa: Scanning Electron Microscopic Elucidation Whip spiders are entirely harmless to people, though their appearance tends to provoke disproportionate alarm.
Reproductive behavior in whip spiders also has its own character. Males of Cuban phrynid species produce species-specific spermatophores, small sperm packages deposited on the ground during courtship, and in some species the male uses his chelicerae to modify the spermatophore head, a behavior that differs even between closely related species.15Zoologischer Anzeiger. Mating behaviour and spermatophore morphology of four Cuban whip spiders (Arachnida, Amblypygi, Phrynidae) – Taxonomic relevance and evolutionary trends Spermatophore transfer, where the male places a sperm packet on a surface and guides or signals the female to pick it up, is actually common across many non-spider arachnid orders. It is a fundamentally different approach from the direct sperm transfer spiders use with their pedipalps.
Camel Spiders
Solifuges, often called camel spiders, sun spiders, or wind scorpions, are neither spiders nor scorpions. They are stocky, fast-running arachnids found in deserts and scrublands across the world, and they have some of the proportionally largest jaws (chelicerae) of any arachnid. These chelicerae can shear through insect exoskeletons, small lizards, and even bird chicks. Despite internet legends that erupted during the Iraq War, solifuges are not venomous to humans and do not chase people; they run toward shadows for shade.
Their reproductive biology is distinctive. Solifuges are egg-layers, and their method of sperm transfer is unique among arachnids: it is always mediated by the male’s chelicerae, the same oversized jaws used to dismember prey.16Journal of Arachnology. Solifuge (camel spider) reproductive biology: an untapped taxon for exploring sexual selection The male produces a spermatophore and uses his chelicerae to transfer it to the female’s genital opening. The dual use of chelicerae for both feeding and mating is an odd convergence of function, and the details of solifuge mating remain poorly studied compared to other arachnid groups. Researchers have flagged solifuges as an undertapped system for studying sexual selection precisely because so little fieldwork on their courtship and mating has been done.
The Rarest Orders
Beyond the better-known groups lie arachnid orders so obscure that encountering one in the wild would be a genuine event. Ricinulei, commonly called hooded tick-spiders, number fewer than 100 described species. They are small, slow-moving, heavily armored animals found in tropical leaf litter and caves. Their most striking feature is the cucullus, a hinged hood at the front of the body that can be raised or lowered to cover the mouthparts. Their functional feeding apparatus includes the cucullus, chelicerae, pedipalps, and a labrum, all of which are thought to be protruded using internal fluid pressure and retracted by muscles.17PubMed. Gross morphology, histology, and ultrastructure of the alimentary system of Ricinulei (Arachnida) with emphasis on functional and phylogenetic implications A recently described species from a Venezuelan cave, Cryptocellus armasi, showed the elongated limbs and reduced pigmentation typical of cave-adapted animals, representing the first troglobitic ricinuleid known from South America.18PubMed. Cryptocellus armasi, a new troglomorphic hooded tick-spider from Venezuela (Arachnida, Ricinulei)
Palpigrades, or microwhip scorpions, are even tinier and more elusive. Most are under two millimeters long and live in soil, caves, or the gaps between grains of sand deep underground. Almost nothing was known about their diet until researchers examined gut contents of palpigrades from Slovak caves and found them packed with single-celled cyanobacteria. This was remarkable: nearly all other arachnid orders are predators, making palpigrades one of the only arachnid groups known to feed on photosynthetic microorganisms.19PubMed. Microwhip scorpions (Palpigradi) feed on heterotrophic cyanobacteria in Slovak caves–a curiosity among Arachnida The cyanobacteria in question were heterotrophic, meaning they derive energy from organic matter rather than sunlight, which fits with the deep-cave habitat where no light reaches. Digestibility was confirmed by the presence of metabolic waste products inside the palpigrade bodies.
Schizomids, sometimes called short-tailed whip scorpions, round out the obscure end of the arachnid spectrum. They are small, pale, and eyeless, living under rocks and in caves throughout the tropics. With around 350 described species, they are better known than ricinulei or palpigrades but still rarely encountered. Like their larger relatives the vinegaroons, schizomids belong to the broader whip scorpion lineage, but they lack the acetic acid spray and impressive body size that make vinegaroons memorable.
Extinct Arachnids
The arachnid family tree includes entire orders that no longer exist. Trigonotarbids, for instance, were among the earliest land-dwelling arachnids, known from fossils as old as the Silurian period, over 420 million years ago. They resembled heavily armored spiders but lacked spinnerets and could not produce silk. Fossils continue to be described from Carboniferous coal deposits; one species, Archaeomartus roessleri, was identified from the Upper Carboniferous of Hagen-Vorhalle, Germany, adding to the known diversity of this long-vanished group.20Fossil Record. A new trigonotarbid arachnid from the Coal Measures of Hagen-Vorhalle, Germany Other extinct orders include the Haptopoda and the Phalangiotarbida, both known only from Paleozoic fossils and both deeply mysterious. The fossil record makes clear that arachnid diversity was once even broader than it is today, with body plans and ecological roles that have no modern equivalent.
Why People Confuse Them With Spiders
Part of the confusion is anatomical. All arachnids share the eight-leg body plan, and many have pedipalps that can look like a fifth pair of legs or like small arms. At a glance, a whip spider flattened against a wall or a pseudoscorpion perched on a piece of bark could be mistaken for an unusual spider. The confusion runs deeper in common names: “camel spider,” “whip spider,” “book scorpion” (a name for pseudoscorpions found in old libraries), and “daddy longlegs” all blur the lines between orders.
A useful quick check is the waist. Spiders always have a narrow pedicel connecting the cephalothorax to the abdomen. Harvestmen, mites, and most other non-spider arachnids have a body that appears as a single unit or has a much less pronounced division. Scorpions, vinegaroons, and whip spiders do have a visible two-part body, but their conspicuous tails or whip-like appendages immediately set them apart. If it has eight legs and no antennae, it is almost certainly an arachnid. If it also has a narrow waist and spinnerets, it is a spider. Everything else in that eight-legged club belongs to one of the orders described above, or to the handful too rare to have common names at all.