The class Arachnida is a group of mostly terrestrial arthropods which includes spiders, scorpions, mites ticks, harvestmen, false scorpions (pseudoscorpions), and sun spiders. Many arachnids have highly developed chelicerae in the head region (also found in pycnogonids and horseshoe crabs), which usually take the form of a pair of pincer-like organs.

Arachnid Anatomy

Like all arthropods, arachnids have a hard, chitinous exoskeleton and jointed appendages, but unlike most of their relatives, true arachnids have no antennae. Their bodies are divided into a cephalotorax and an abdomen. The cephalothorax (or prosoma) is usually segmented and its upper surface is covered with a carapace. The lower region is usually protected by sternal plates. The abdominal segments of most arachnids, apart from scorpions, are fused. In ticks and mites both the cephalothorax and abdomen have fused to form a single body. The respiratory system of arachnids comprises specialized breathing organs, called lung-books and a network of tracheae (tubes that carry air from the exterior to the internal organs). The circulatory system is usually open, that is, arteries carry blood from the heart into a series of blood spaces (hemocoele).

The brain consists of two ganglia (bundles of nerve cells) above the esophagus, joined to more ganglia below it. Most arachnids have simple eyes. spiders usually have 8, scorpions 12; even so, their eyesight is, in most cases, very poor.

Reproductive system vary in arachnids, but sexes are separate. Fertilization is by copulation, usually preceded by an elaborate courtship ritual.


There are nearly 40,000 species of spiders, the largest of which are the tarantula of South America (suborder Orthognatha), some species of which have a body 3 inches across with a leg spread of 8 inches. Tarantulas have poison glands in their chelicerae, but most spiders have them in the cephalothorax, although the poison is administered via the chelicerae. The poison is used to paralyze or kill the prey, or in self-defense.

Unique Features of Spiders

[Photo of wolf spider]
Wolf spider

Spiders have abdominal glands which exude silk through organs called spinnerets. The silk threads are used to make webs, to catch prey and make cocoons. Not all spiders spin webs, however; trap-door spiders (family ctenizidae) dig a tunnel often more than 2 inches deep, which they line with silk. The tunnel is closed at ground level by a hinged door. When a small animal passes by, the spider jumps out and grabs it. Many wolf spiders (Lycosidae) and jumping spiders (Salticidae) do not trap their food but stalk prey and then leap on it.

Black widow spider
Black widow spider (female) (Latrodectus mactans)

Note the characteristic red hourglass located on her inferior abdominal surface, which can vary in coloration from yellowish, to shades of orange and red, and at times, can even be white. The female's body is an overall shiny jet-black in color.

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Brown recluse spider (Loxosceles reclusa) is able to inflict a highly venomous bite. Sometimes its bite can go unnoticed, or may feel as slight as a pinprick. However, usually, after 2–8 hours, there is a severe pain, redness, and localized tissue necrosis caused by the venom's proteolytic enzymes.


Based on their habitats, the three species of black widow include the southern (L. mactans), northern (L. variolus), and western ( L. hesperus). Human beings are bitten not as prey, but when a female spider feels threatened. The venom is 15 times more potent than that of the rattlesnake, when it bites its victim, while the male L. mactans is harmless. Though venomous, the quantity of poison is so minute that death from a black widow bite is rarely fatal, though usually very painful.


[Photo ]
Source: PHIL

Scorpions (order Scorpiones) have remained virtually unchanged for about 450 million years. The largest of the true arachnids, reaching 5 to 8 inches in length, they are nocturnal and live in tropical and subtropical regions. The segmented tail has a sharp sting at the tip with a poison gland, which causes paralysis and death to insects and small animals. One of the results following Tityus trinitatis scorpion envenomation is pancreatitis due to its ability to hyperstimulate an intra-pancreatic reaction that converts trypsinogen to trypsin.

Arachnids Anatomy

Other Arachnids

Mites and ticks (order Acarina) are small arachnids, usually less than 1 millimeter in length. Many are parasites, living on blood and tissue fluids, causing skin irritation and occasionally transmitting diseases to their hosts. Their chelicerae can pierce skin, making ticks difficult to dislodge. Harvestmen (order Opiliones) are spider-like animals with long legs and only 2 eyes. False scorpions (order Pseudoscorpiones), like true scorpions, have pincers (large chelate pedipalps), but no tail or sting. Sun spiders (order Solufugae), which can be up to 2 inches long, have simple elongated pedipalps that make them look like ten-legged spiders.

Whip Spiders

Whip spiders (Amblypygi) is a small arachnid order with a worldwide distribution and the largest number of described species in the Neotropics. Currently, about 170 species are known and several new ones have been described in the last few years from all over the world. Eight new species of Charinus are described for the Brazilian Amazon. The majority of them are threatened by the destruction or alteration of the physic conditions of the caves they inhabit and have been included in the red list of the Brazilian Environmental Ministry.9

whip spider Source: PubMed Central

Arachnid Anatomical Structures

  • chelicerae - one of a pair of grasping and piercing organs present in arachnids and Horseshoe crabs
  • carapace - the part of the exoskeleton of a crustacean that spreads over the head and thorax; in other animals, for example, turtle, the carpace is the dorsal covering of bony plates
  • thorax (in arthropods) that part of the body directly behind the head and in front of the abdomen which, in insects, consists of 3 segments bearing the legs and wings.
  • pedipalps - either member of the second pair of head appendages of arachnids, specialized for different functions in different forms. For example, seizing prey in scorpions, locomotion in king crabs, fertilization in male spiders (where the tip becomes a specialized container for sperm transfer), squeezing and chewing food, or for sensory purposes.


  1. Mike Janson and Joyce Pope. The Animal World
  2. Public Health Image Library (PHIL) Photographs, Illustrations, Multimedia Files
  3. U.S. Fish and Wildlife Service Multimedia database
  4. Mites of greenhouses: identification, biology and control. Zhi-Qiang Zhang
  5. Medical and veterinary entomology. D. S. Kettle
  6. Multicellular animals. Peter Ax
  7. Insects and Spiders of the World, Volume 10. Marshall Cavendish Corporation
  8. The encyclopedia of land invertebrate behaviour. Rod Preston-Mafham, Ken Preston-Mafham
  9. Eight New Species of Charinus Simon, 1892 (Arachnida: Amblypygi: Charinidae) Endemic for the Brazilian Amazon, with Notes on Their Conservational Status. Alessandro Ponce de Leão Giupponi1, and Gustavo Silva de Miranda, Claude Wicker-Thomas, Editor. 2016

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