The most anterior (forward) pair of head
appendages of arachnids, lying just in
front of the mouth. Each one consists of basal segment and a hollow claw;
the claw has an opening near its point and is connected by a tube or duct
to a poison gland inside the cephalothorax.
When a spider, for example, "bites," poison is forced through the opening
in the claw into the wound. Since it always bites with both chelicerae,
it makes two punctures, close together; a wasp or hornet, when it stings,
makes only one. In countries where dangerous spiders are found their bite
can be recognized by this feature, and appropriate treatment can be given.
The marks of a spider bite are closer together than those made by the fangs
of snake. Soifugae, which make up a distinct
order of arachnids, have unusually large chelicerae.