VARIATION IN BODY SIZE AND FLIGHT PERFORMANCE IN MILKWEED BUGS (ONCOPELTUS).

In this paper we report observations on variation in flight performance and body size among tropical, temperate, and island milkweed bugs (Oncopeltus: Hemiptera: Lygaeidae). This is one of a series of papers on life history variation in Oncopeltus; photoperiodism is discussed in Dingle et al. (1980) and size dependent timing of metamorphosis in Blakley and Goodner (1978). Other papers are in preparation. The distribution of these bugs throughout much of the Western Hemisphere (Slater, 1964) allows comparison, both within and among species, of the selective forces acting on life histories. Various species occur in tropical and temperate regions and on islands throughout the Caribbean on milkweeds (Asclepiadaceae) and occasionally on plants of related families. One species, Oncopeltus fasciatus, is a long distance migrant to temperate North America (Dingle, 1968a et seq.), while the remainder are confined to the tropics and subtropics. General discussions of the ecology of these insects may be found in Blakley (1977), Dingle (1968a et seq.), Evans (1979), Ralph (1976, 1977), Root and Chaplin (1976), and Sauer and Feir (1973); a summary is included in Dingle et al. (1980) which also gives collecting sites for the populations discussed here. Migratory physiology is reviewed by Rankin (1978). All species discussed in this paper belong to the subgenus Erythrischius (Slater, 1964). We found that flight performance differed considerably among species and populations of Oncopeltus. Dispersal abilities of insects vary with habitat (Southwood, 1962; Dingle, 1972), and many species occurring in isolated habitats, including islands, are apterous or brachypterous (Carlquist, 1974; Vepsaldinen, 1978). There are, however, only a few studies (e.g., Rose, 1972) which demonstrate that relatively shorter (as opposed to functionless) wings actually lead to reduced flight and none, so far as we know, demonstrating reduced flight in cases where it would be expected, for example, on islands, but where there is no obvious alteration in wing structure. In this report we demonstrate that reduced flight can occur with no apparent changes in wing morphology. Body size has attracted considerable attention from evolutionary biologists. It is easy to measure, and it is associated with a number of other characteristics from life history statistics (Pianka, 1970; Stearns, 1976; Blueweiss et al., 1978) to an array of morphological and physiological traits (summarized by Pyke, 1978). Despite much effort by ecologists and others, however, the adaptive significance of body size is not well understood, and indeed there seems to be no general agreement on the most important selective factors (Pyke, 1978). Body size is also frequently influenced by proximate environmental conditions such as food availability, so that analysis is complex. Regardless of such phenotypic variability, the heritability of body size differences is more often assumed than demonstrated so that their adaptive nature falls short of proof. In this study we demonstrate heritable differences in body size between bugs of island 1 Present address: Institute for Behavioral Genetics, University of Colorado, Boulder, Colorado 80309.