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The flight muscle polymorphism in Cenocorixa bifida Simpson, James Eric

Abstract

Flight muscle polymorphism in Cenocorixa bifida Hung, is considered in several habitats in which it occurs in British Columbia. The differences in the indirect flight musculature and pigmentation of the cuticle between the morphs are described in detail. Observations to date suggest that in the flightless form the indirect flight musculature does not develop beyond the stage noted in the early teneral adult. The flightless adults appeared in the habitats in the latter part of August and became more prevalent in the habitats from that time until late in the fall. Substantial increases in the percentage of the non-flying morph in the habitats appear to be correlated with abrupt decreases in the water temperature in these habitats, but this form is found in some habitats before these decreases occur. Sampling data indicate that the percentage of the flightless morph in the adult population varies considerably from habitat to habitat and from year to year in any one habitat. Evidence is presented which suggests that some of variation results from the duration of oviposition in the habitat the previous spring and the size of the second generation in that season. An ovarian diapause which limits the size of the second generation occurs in females toward the end of July. The onset of this diapause appears to be synchronized with decrease in natural photoperiod during the summer and its' termination the following spring appears to be controlled by water temperature in the habitat. A differential pattern of survival of morphs overwinter appeared. The data suggest that the flightless form may be at an advantage overwintering in habitats of intermediate salinity but is probably selected against in habitats of high and low salinities over winter. Flightless females C. bifida parasitized by the mite Eylais extendens Muller were able to produce mature eggs with chorion whereas no female flying form produced mature eggs while being parasitized by a large nymphal mite. Apparently the flightless female is able to produce mature eggs under conditions of stress where the flying form cannot. The endocrine relationships leading to ovarian diapause and flight muscle polymorphism are discussed and tentative hypotheses for endocrine mechanisms are suggested.

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