The spider species Caerostris darwini is known for utilizing the largest orb webs, built from the biomaterial which by tensile strenght exceeds all other known silk spiders and even most of the sythetic fibers. Recent publications for this species also report sexual size dimorphism, which is linked with unusual sexual behaviors. However, sexual behavior in this species is virtually unexplored. Therefore, we conducted field research in Madagascar and controlled experiments in the laboratory. Our study reveals that C. darwini exhibits sexual strategies that are similar to those in other sexually size-dimorphic species. These include male preference for freshly molted (i.e. teneral) females, male palp breakage within female genitals during copulation and sexual cannibalism. Male courtship is intense and includs mate binding. During courtship, we also observed a previously unknown spider behavior that we term “oral sexual contact”. Males salivated onto female genitalia pre-, during, and post-copulation, irrespective of female’s age or mating status. The adaptive significance of “sexual oral contact” remains elusive. All insertions resulted in male palp damage. The males self-emasculated their damaged palps to become eunuchs. Females were often aggressive towards males after mating and sometimes cannibalized them. While males could not reuse their damaged palps, previously mated females were able to reuse their plugged genitals, implying a polyandrous mating system in C. darwini. Comparing adult and teneral females showed that adult females typically respond aggressively to male courtship, have shorter copulations and higher occurrences of sexual cannibalism.
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