Just Hang On: Australian Gall Midges Suspend Themselves on Spider Silk Without Becoming Entangled

Spider silk is a versatile material that is exploited by its creators, but potentially also by other animals once deposited into the environment. In this study, we report on gall midges from the family Cecidomyiidae in Australia exploiting spider silk as roosting sites. Across two distant locations in New South Wales, Australia, we observed adult midges suspended motionless on aerial lines by their front pair of legs yet able to liberate themselves when disturbed. The benefit of using silk as a resting surface may include a reduction in predation risk by terrestrial and aerial predators that cannot reach individuals suspended in the air. This does not appear to be associated with an increased risk of predation by the silk-owners, given that the silk exploited for roosting is not part of aerial webs or capture threads that are sticky. Our observations complement those of gall midges from other continents that show an invertebrate actively exploiting the extended phenotype of a spider, which is an unexpected relationship between these animal types where one is typically the prey of the other. The capacity to suspend on silk lines may require certain silk line orientations or web architectures, constraining midges to exploit the extended phenotype of specific types of spiders.