Does allometric growth explain the diminutive size of the fangs of Scytodes (Araneae: Scytodidae)?

Abstract Spitting spiders eject silk and glue from their fangs when attacking prey. The ejection is complete in less than 35 ms and involves high-frequency fang oscillations that can approach 1700 Hz. Because of Newtonian physical constraints, these oscillations, which cause the spit to be dispersed in a zigzag pattern, could not occur at such high frequencies if the fangs themselves were not very small. We hypothesized that allometric neoteny, in which the developmental rate of a structure is retarded relative to the changing overall size of the growing individual, could explain (in an ontological sense) the small fangs of adult spitting spiders. We measured the fangs, chelicerae, carapaces, and sterna of many sizes of spitting spiders, Scytodes thoracica (Latreille 1802a), brown recluse spiders, Loxosceles reclusa Fertsch & Mulaik 1940, and wolf spiders, Varacosa avara (Keyserling 1877), to discover whether the fangs of spitting spiders grow unusually slowly. Using sternum width as our proxy for spider size, we found that the carapaces of spitting spiders grow disproportionately fast but that the spiders' chelicerae and fangs grow at the same rate as their sterna. The growth patterns in L. reclusa and in V. avara differed both from each other and from S. thoracica. We evaluate these patterns and conclude that the diminutive fangs of adult spitting spiders do not constitute an instance of allometric neoteny.