In this report we analyze patterns of size-assortative mating in an insect genus by (1) describing the consistency and strength of assortative mating among populations of a species and among species of a genus, and (2) determining the role of sampling artifacts in generating this nonrandom pattern of mating. Our data consist of 18 samples of soldier beetles (8 species of the genus Chauliognathus) from 17 sites within the southwestern United States. Of the 18 samples, just 6 exhibited significant correlations between elytron lengths of mating pairs. We show how these and other significant correlations are generated by 3 kinds of sampling artifacts: mixed-species samples, mixed-population samples, and mixed-microsite samples.

Hypotheses about the functions of the male genitalia and the male scape in insects were tested by measuring the slopes of allometric relations in six populations of Chauliognathus scutellaris. All allometric relations used elytron length as the indicator of overall body size. Male genitalia have lower slopes than male pronota (a structure not involved in reproduction), male scapes (secondary sexual characters) have higher slopes than male pronota, and female scapes have slopes that are not different from the slopes of female pronota. These results support Eberhard's one-size-fits-all hypothesis regarding the size of male genitalia in insects, and they raise questions about the role of the male scape in reproductive activities.

Soldier beetles of 2 species, Chauliognathus basalis and C. deceptus, were examined to test the Crespi hypothesis that positive assortative mating by size is caused by mate choice. Specifically, we tested the prediction that if mate choice involves choosing the largest mate available, then mating individuals will be larger than nonmating individuals. Four samples were taken, at different times during the mating season, from each of 2 sites. Each sample consisted of mating pairs, nonmating males, and nonmating females. Some of the samples contained beetles of both species; others contained beetles of a single species. For each gender elytron lengths of mating individuals were compared with elytron lengths of nonmating individuals. We found no effect of mating status (mating vs, nonmating) on elytron lengths in samples that exhibited assortative mating (which occurs where 2 species coexist). Surprisingly we found a consistent effect of mating status on elytron lengths in samples that did not exhibit assortative mating (which occurs where only 1 species exists). Our results do not support the mate-choice hypothesis. Instead, mate choice and assortative mating appear to be alternative mating patterns in which mate choice occurs where a single species exists and assortative mating occurs where 2 species coexist.

Soldier beetles of two species, Chauliognathus basalis and C. deceptus, were examined to test two hypotheses of assortative mating: (1) male and female availability associated with temporal covariation in body sizes and (2) female availability associated with a large-male mating advantage. Four samples were taken, at different times during the mating season, from each of two sites. Males were identified to species by their copulatory organs. Females were placed tentatively in the same species as their mates, and then this identification was confirmed by species-specific allometric relationships. In two of the eight samples, body sizes (as measured by elytron length) of mates were positively correlated. At both sites, body size did not change with progression of the mating season, thereby disproving the temporal covariation hypothesis. Where assortative mating was found, correlations between the sizes of mates consisted of points symmetrically distributed around the regression line, which indicates no mating advantage to larger males. The two samples that exhibited assortative mating differed from the other six samples in that they contained conspecific pairs of both species. Each species exhibited assortative mating only when the other species was present Thus, surprisingly, each species altered its mating behavior in the presence of the other species.

Individuals within a population of Chauliognathus basalis (LeConte) in Colorado were found to exhibit 11 discrete patterns of pigmentation on their pronota. The patterns range from no dark pigment at all to a pronotom almost completely covered with dark pigment. Pigmentation class is related to body size: smaller beetles are darker than larger beetles. As a group, males are smaller and darker than females. The adaptive significance of color polymorphism, particularly with regard to thermoregulation, is discussed.