Effect of spider webs on the natural selection of insects.

Abstract

Although signatures of evolution are all around us, it is actually quite difficult to observe the process taking place. The webs of spiders have allowed us to take a snapshot of what may be natural selection in action. Spiders, as an important natural predator, have the ability to place selective pressure on insect populations. Five different species of insects were studied. Four of them were midges (Chironomidae family) and the remaining species was a mosquito (Culicidae family). Insects were collected from webs and a free (control) group was established through the use of a black light bug trap. Both groups were measured for wing length and thorax length. The caught insects were compared to their respective control cohorts within each species. Variances and means were established for each species and compared between each group. The findings showed three of the species had significantly shorter wings in the caught population. A significantly shorter thorax was also observed in two of the species in the caught population. The final portion of the study compared all four midges of the Chironomidae family to each other. As the mean wing to thorax ratio of the species increased, so did their mortality rate. This finding was consistent with our initial prediction that longer wings would make insects more likely to get caught in the web, while a smaller thorax would not provide the power necessary to escape from the web. However, these results were only observed at the interspecific level and not at the intraspecific level. In the test area, spider webs appear to be acting as a selective force on wing and thorax length of the insects studied.