The chelicerae () are the mouthparts of the subphylum Chelicerata, an arthropod group that includes , , and . Commonly referred to as "", chelicerae may be shaped as either articulated fangs, or as a type of pincers. Some chelicerae, such as those found on nearly all , are hollow and contain (or are connected to) venom glands, used to inject venom into prey or a perceived threat. Both pseudoscorpions and Opiliones — neither of which have that venom channel — have additional structures on their chelicerae that are used for grooming (papillae in pseudoscorpions, cheliceral teeth in Opiliones).
Types
Chelicerae can be divided into three kinds: jackknife chelicerae, scissor chelicerae, and three-segmented chelicerae.
Jackknife chelicerae
The jackknife chelicera is subchelate (with fixed finger much reduced or absent) and is composed of two segments. This type of chelicera occurs exclusively in the
Tetrapulmonata.
Jackknife chelicera are described in two different forms: orthognathous and labidognathous. Orthognathous chelicerae are articulated in a manner that enables movements of the appendages parallel to the body axis. This kind of chelicera occurs in the Mesothelae and Mygalomorphae spiders and in the related orders Amblypygi, Schizomida and Uropygi. Labidognathous chelicerae move at right angles to the body axis. This kind of chelicera is rotated and occurs exclusively in the Araneomorphae spiders.
Spider chelicerae
The chelicerae consist of a base segment, sometimes called the "paturon", that articulates with the
cephalothorax (or
prosoma) and a
fang portion that articulates with the base segment.
Almost all spiders have venom glands and can inject the venom through openings near the tips of their fangs when
spider bite prey. The glands that produce this venom are located in the two segments of the chelicerae, and, in most spiders, extend beyond the chelicerae and into the cephalothorax.
The fang, the organic functional equivalent to a hypodermic needle is what penetrates the skin, fur, or exoskeleton of the spider's target—spider mouthparts are primarily intended for envenoming a spider's prey in most species, typically
and other small
.
The basal portion includes all or part of the spider's venom glands, which can be squeezed to control the amount of venom forced out of the glands.
Such control permits a spider to administer either a
dry bite, a dose appropriate to the nature of the prey or enemy, or a maximal dose.
The control is also necessary for actions such as the spitting of venomous silk by members of the family
Spitting spider; they depend on that mechanism both in hunting and defence.
When a spider bites, the two parts of the chelicerae come together like a folding knife, and when making a threat display or actually preparing to bite, the spider will open the angle of the fangs with the basal portion of the chelicerae and also open the angle of the basal portion with the cephalothorax.
Uncate chelicerae
850 8879 Solifugae.jpg|Uncate chelicerae of a solifuge
Anterior view of Loxosceles reclusa (Brown Recluse Spider) (22830337766).jpg| Front view of a brown recluse spider, showing its chelicerae
Pseudoscorpion (7586033338).jpg|Uncate chelicerae of a pseudoscorpion
The uncate chelicera is chelate and composed of two segments and occurs in the orders
Pseudoscorpiones,
Solifugae,
Ricinulei, and
Spider (e.g., brown recluse,
Pholcidae, and
Crevice weaver).
Three-segmented chelate chelicerae
Limulus polyphemus 201780311 (cropped).jpg|Three-segmented chelicerae of an Atlantic horseshoe crab
Pantopsalis albipalpis 1276804.jpg| Pantopsalis albipalpis, a species of harvestman with exceptionally long three-segmented chelicerae
Acutiramus cummingsi (YPM IP 018609).jpg|Isolated fossil chelicera of a pterygotid eurypterid, Acutiramus
Having three-segmented chelate chelicerae is the primitive condition and occurs in
such as the
and the
Opiliones, as well as in non-arachnid Chelicerata such as the
Xiphosura and
Eurypterida.
The chelifores of the
Pycnogonida may be homologous to chelicerae.
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