One of the specialized silk-handling devices found in spiders. In general there are four pairs, two pairs on the tenth abdominal segment and two pairs on the eleventh, although few spiders retain the maximum complement in the adult stage. The anterior median pair is usually lost, but sometimes a remnant may remain in the form of a median cone (colulus) whose function is unclear. In some cases the anterior median pair fuse during embryonic development to form an oval plate covered with thousands of tiny spigots (the cribellum). Some spiders retain only two pairs of spinnerets, losing both the anterior median and posterior lateral pairs. Spinnerets are movable, sclerotized tubes composed of several segments; they vary in size, and are mostly conical in web-spinning species. The silk glands produce a protein (fibroin) as a liquid which is emitted through tiny spigots on the ends and ventral sides of the spinnerets. Polymerization of the liquid silk is not an oxidative or evaporative process, but relies on tension.
Subjects: Zoology and Animal Sciences.