Chelae

For other uses, see Chelae (disambiguation).

Fiddler crabs like Minuca pugnax, and other members of the family Ocypodidae, have chelae of different sizes: a large left chela and a small right chela.

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. Another common name is claw because most chelae are curved and have a sharp point like a claw.

Arthropod chela mechanics

A chela is form by the modification of at least 2 distal segments (podomeres). The terminal segment is a mobile unit to open or close the chela, refer to as movable finger or free finger. The basal segment, which housing muscles to move the movable finger, bore a protrusion called fix finger that facing towards the retracting (closing) direction of movable finger, where both fingers contact and working together as a pincer-like structure. Segments with these structures are referred to as chelate or cheliform.

On the other hand, intermediate form between an unmodified and chelate limb, such as raptorial front legs of mantis or grasping claws of lice, are called "subchela" (subchelate).

In crustaceans

Lobster such as Homarus americanus have prominent chelipeds.

Chelae are common amongst Decapoda, a group which include familiar crustaceans like crabs, lobsters, shrimps and prawns. Legs (thoracopods) with chela are called chelipeds. Astacidea (crayfish and true lobster) and Dendrobranchiata (true prawn) have 3 pairs, Caridea (true shrimp) have 2 pairs, while Brachyura (true crabs) and Anomura (hermit crabs and relatives) usually have only 1 pair on their front legs. Polychelida have 4 or all 5 pairs of their legs ended with chelae. Achelata is the only group of decapods that doesn't have chelae, although females may have tiny chelae on their hind legs. Shrimps of the genus Psalidopus bearing an exceptional pair of scissor-like chelae that both fingers are mobile, with one of them possibly modified from an originally fixed finger.

Outside of decapods, other crustaceans known to have chelae are tanaids, some amphiopods, pennellid copepods (on antennae) and Pleomothra remipedes (on mouthparts).

In chelicerates

Front view of a pseudoscorpion, showing pincer-like chelicerae (middle) and pedipalps (sides).

Chelae are also common amongst chelicerates, especially at their name-giving first appendages known as chelicerae. While these feeding appendages are usually small, some taxa such as pterygotids and solifugids have exceptionally large chelicerae. In tetrapulmonate arachnids however, the chelicerae modified into a pair of subchelate, jackknife-like structure, which function as venomous fangs in most spiders.

Scorpions and pseudoscorpions have their pedipalps (second appendages of chelicerates) modified into a pair of prominent chelae. Ricinuleids also have tiny chelae at the end of their hidden pedipalps. In horseshoe crabs and offacolids, almost all of the legs are also terminated with chelae.

In other arthropods

Bocchus thorpei, a dryinid wasp.

Restoration of the Cambrian arthropod Tokummia, one of the oldest arthropods to bear chelae.

Chelae are rare outside of crustaceans and chelicerates. Sphaerotheriid millipede is the only myriapods that bore chelae, which located at their posterior telopods (mating appendages). While subchelate limbs are common within the highly diverse insects, there are only 4 insect lineages evolved true chelae: Carcinocorini (assasin bug), female Dryinidae (parasitic wasp), female Carcinothrips (thrips), and Carcinonepa (water bug), all occurred at their front legs.

In terms of fossil-exclusive taxa, The Cambrian hymenocarine genus Tokummia is one of the earliest arthropods that known to have true chelae. While similar structure occurred in megacheiran great appendages, these are formed by 3 or 4 segments/fingers, similar to vinegaroon grasping pedipalps and referred to as "multi-chelate".

Uses

Chelae have a wide variety of uses, but most commonly they are used for handling prey and for defense. These uses are often reflected in the morphology of the chelae. For instance, some species, such as the members of the families Ocypodidae and Alpheidae show asymmetry between their paired claws. Possessing one enlarged chela used for defensive and courtship purposes and a smaller chela for shearing and feeding. For some species, this asymmetry between chelae may be a sexually dimorphic trait. whereas in others, like many species of scorpions, it is not. An example of specialization of these asymmetrical chelae can be seen in the Alpheus heterochaelis, the bigclaw snapping shrimp. The enlarged snapping claws of these shrimp are capable of snapping shut with such force to shoot a jet of water and create a loud popping noise, which they use to deter predators and other members of their species.

In scorpion species, the chelae are often used to grab hold of prey and then further subdue them by injecting them with the venom from their stingers, although some species rely solely on the chelae to subdue their prey. Scorpions also use their chelae for defense by using them to shield and protect their bodies. For scorpions, the chelae are formed at the end of the pedipalps and covered in sensory hairs that they use in a similar manner to insect antennae. In some pseudoscorpions, the chelate pedipalps can be venomous.

Further uses of chelae include digging, burrowing, and climbing. Chelae also play an important role in many species mating rituals, such as to communicate and attract prospective mates, wherein species with asymmetrical chelae use their enlarged chela as a display to attract mates. Chelae are also used in the act of mating, where the male species will often use them to hold onto the female during the act.

See also

• Pincer (tool)