Studies on structural variation in the common European earwig (forficula auricularia Linn)

Abstract

From Scarborough in the south to the moat northerly site, Berwick-on-Tweed, a series of samples of earwigs were collected in the years I966, I967 and 1968. Several parts of the anatomy of the earwigs collected were measured to gain overall information about the variability of this species spatially and from year to year. The results of this investigation may be summarised as follows: there was no evidence from the measurements taken, that the male Earwig is polymorphic in any respect except in the size distribution of the cerci. This polymorphism is restricted to the lengths of the cerci: no evidence was found to show that the lateral axes of the male cerci were polymodally distributed in any sample. Other findings indicated that the occurrence of the high male at the same place, was variable in different years. There was evidence that the earwigs collected in the south were larger than those in the north, and the populations contained proportionately more high males. The female earwigs were shown to be significantly larger than the males in almost all respects except in the cerci which are also dimorphic in shape. Comparisons were made between the Fame Islands' populations of earwigs and those sampled on the mainland. The Fame Islands' earwigs are remarkably different from the mainland populations being much larger on average and containing far more high males. Other work included the development of techniques to permit the objective separation of the populations of males into the separate morphs. Studies of the variation in shape of the cerci indicated the presence of three morphs of the male and two morphs of the female.