Predation by Spitting Spiders: Elaborate Venom Gland, Intricate Delivery System

Abstract

Spitting spiders (Araneae: Scytodidae) subdue prey by entangling them at a distance with a mixture of silk, glue, and perhaps venom. All of the components of this mixture originate in the venom glands, a pair of relatively elaborate structures consisting of five histologically distinct regions, the anterior three producing venom and the posterior two, comprising the largest lobe of the gland, producing silk and glue. The venom gland and its products anchor a suite of tightly linked adaptations that constitute the predatory system of these remarkable spiders. The spitting requires considerable metabolic expenditure both in the form of maintenance of impressively large venom glands and in the form of the biosynthesis of the proto-silk and the glue that are the primary constituents of the spit. Sensory input allowing identification and localization of the potential prey and neural coordination of the spit ejection itself also must occur. It may be that an important part of expectoration, the oscillations of the fangs (contributing to the zigzag pattern of spit that effectively covers a prey item), once triggered, does not require further neural input, but that possibility remains to be confirmed. The chemical, physical, and microfluidic properties of the proto-silk and glue mixture are also crucial for the proper functioning of the spitting system; at the moment, these properties are poorly understood except insofar as they may be similar to the analogous properties of the proto-silk and glue produced in the opisthosomal silk glands of spiders.