Chelicerae in the context of "Spiders"

Spiders (order Araneae) are air-breathing arthropods that have eight limbs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species diversity among all orders of organisms. Spiders are found worldwide on every continent except Antarctica, and have become established in nearly every land habitat. As of June 2025, 53,034 spider species in 136 families have been recorded by taxonomists. However, there has been debate among scientists about how families should be classified, with over 20 different classifications proposed since 1900.

Anatomically, spiders (as with all arachnids) differ from other arthropods in that the usual body segments are fused into two tagmata, the cephalothorax or prosoma, and the opisthosoma, or abdomen, and joined by a small, cylindrical pedicel. However, as there is currently neither paleontological nor embryological evidence that spiders ever had a separate thorax-like division, there exists an argument against the validity of the term cephalothorax, which means fused cephalon (head) and the thorax. Similarly, arguments can be formed against the use of the term "abdomen", as the opisthosoma of all spiders contains a heart and respiratory organs, organs atypical of an abdomen.

A chela (/ˈkiːlə/) – also called a claw, nipper, or pincer – is a pincer-shaped organ at the end of certain limbs of some arthropods. The name comes from Ancient Greek χηλή, through Neo-Latin chela. The plural form is chelae. Legs bearing a chela are called chelipeds. Another name is claw because most chelae are curved and have a sharp point like a claw.

Chelae can be present at the tips of arthropod legs as well as their pedipalps. In some pseudoscorpions, the chelate pedipalps can be venomous. Chelae can be functionally overlapping with chelicerae, which specifically refers to appendages used as mouthparts in Chelicerata. In spiders those usually contain venom glands, but in other chelicerate allies the chelicerae can be chelate without venom glands.

Habeliida is an order of extinct arthropods that existed during the middle Cambrian. It is divided into two families, Habeliidae (monotypic, containing only Habelia), and Sanctacarididae (containing Sanctacaris, Utahcaris and Wisangocaris). They are thought to have been durophagous, with robust gnathobases (spined basal sections of limbs) used to shred hard-shelled organisms. Remains of trilobites have been found as stomach contents in Wisangocaris. Messorocaris has been suggested to be part of the order in some studies, but this is uncertain. They are suggested to be stem-group chelicerates, though they lack the chelicerae present in true chelicerates.

Cladogram after O’Flynn et al, 2023:

A fang is a long, pointed tooth. In mammals, a fang is a modified maxillary tooth, used for biting and tearing flesh. In snakes, it is a specialized tooth that is associated with a venom gland (see snake venom). Spiders also have external fangs, which are part of the chelicerae.

Fangs are most common in carnivores or omnivores, but some herbivores, such as fruit bats, have them as well. They are generally used to hold or swiftly kill prey, such as in large cats. Omnivorous animals, such as bears, use their fangs when hunting fish or other prey, but they are not needed for consuming fruit. Some apes also have fangs, which they use for threats and fighting. However, the relatively short canines of humans are not considered to be fangs.

Pedipalps (commonly shortened to palps or palpi) are the secondary pair of forward appendages among chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to the chelicerae ("jaws") and anterior to the first pair of walking legs.

A chela (/ˈkiːlə/) – also called a claw, nipper, or pincer – is a pincer-shaped organ at the end of certain limbs of some arthropods. The name comes from Ancient Greek χηλή (khēlḗ), through Neo-Latin chela. The plural form is chelae. Legs bearing a chela are called chelipeds. Another name is claw because most chelae are curved and have a sharp point like a claw.

Chelae can be present at the tips of arthropod legs as well as their pedipalps. In some pseudoscorpions, the chelate pedipalps can be venomous. Chelae can be functionally overlapping with chelicerae, which specifically refers to appendages used as mouthparts in Chelicerata. In spiders those usually contain venom glands, but in other chelicerate allies the chelicerae can be chelate without venom glands.