Biological studies on certain forms of the harvestman Mitopus morio (Fabr.) (Opiliones, Arachnida)
The harvestman Mitopus morio has been studied over a large geographical area in northern Britain, and specimens from other parts of Europe have been examined, to investigate the relationships between the different forms of this species. Previous workers have suggested that M. morio has three forms in Britain, two from upland areas (W. morio cinerascens and M. morio alpinus), and one from lowland regions (M. morio morio). The results of this study show that M. morio cinerascens and M. morio morio are not isolated forms, but that they lie at opposite extremes of a dine of decreasing size with increasing altitude. The form called M. morio alpinus is not a variety of m. morio but is a distinct British species; it is described in this thesis as Mitopus ericaeus sp.n. The mean lengths of the second femurs of all instars of M. morio decrease progressively with increasing altitude. Analyses of specimens of M. morio from other parts of Europe show that site altitude and site latitude account for half of the variation in femur length between sites. Body dry weights and lengths of the corpora penes of males also decrease clinically with increasing altitude. The phenology of M. morio is similar at all sites, but the date on which a particular mean instar is reached is delayed at the higher altitudes; temperatures are, on average, lower at these upland sites throughout the season. m. ericaeus sp.n. is taken at sites above 250m where it co-exists with M. Morio but is less abundant. M. ericaeus sp.n. is larger and more darkly pigmented than m, morio at all instars, and adult males differ in their genitalia morphology, m. ericaeus sp.n. develops and matures earlier than M. morio at the same site, preventing congeneric competition. The two species do not interbreed in the laboratory. The effect of chilling on the egg diapause of both species has been investigated.