The major host of Mantispa uhleri Banks in the Midwest is the crab spider, Philodromus vulgaris (Hentz). Larvae are phoretic on spiders and feed on spider hemolymph until the sac is entered and the eggs consumed. The frequency of larvae boarding Philodromus is considerable – as high as 75% in some locales. With such a high rate of spider boarding, one would expect that many of these spiders would have several larvae on them. The opposite is true. Field data from Iowa show that, out of 451 Philodromus collected, 164 had one larva while only 14 spiders had two and only two spiders had three larvae aboard. This suggests that there is some interaction among multiple larvae that increases the likelihood that only one will survive. We examined the rate of larval survivorship through the last spider molt. Spiders in the control group each received one larva; Spiders in the experimental group each received two. Spiders were checked under CO2 anesthetization prior to the molt for larval location. After the spiders molted to the adult, they were again examined to determine how many larvae successfully made it through the molt. Results show a 94% survival rate when only one larva was present on the spider. With two larvae present, their survival rate was only 66%. These differences were significant (P < .001, Binomial Distribution). Aggressive interactions were also directly observed between larvae when they were positioned next to each other on the spider’s pedicel following the molt.
Species 1: Neuroptera Mantispidae Mantispa uhleri
Species 2: Araneae Philodromidae Philodromus vulgaris
Keywords: phoresy, spiders
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