In female Tetraopes tetraophthalrntis there is significant heterogeneity in allometric growth ratios of prothoracic tubercle size and prothorax width among different areas of the range. This suggests that different relative growth rates in the formation of the adult prothorax may have been selectively favored in different areas of the range. However, males do not show the effect. Furthermore, the relationship between individual and population allomorphosis is also strikingly different in the two sexes; estimates of growth ratios at these two levels are similar in females, very dissimilar in males. The analysis of allometric size relationships, in particular by means of the allometric growth equation of Huxley (1932), continues to be regarded as a powerful tool in the study of animal evolution. Recent restatements of the context within which the Huxley equation is useful have been generally less sweeping than the original claims made on its behalf, but its value for descriptive purposes has never been in question. It has been possible in both modern and fossil populations to estimate some effects of natural selection by means of comparisons of growth ratios among populationis, and in some studies to compare levels of allomorphosis. Present material, consisting of a large number of contemporaneous populations of a single species, provides an opportunity for a detailed examination of ecogenotypic differences in an allometric relationship; it further provides an opportunity for a comparison of individual and population allomorphosis. MATERIALS AND METHODS The ease with which large samples of the cerambycid beetle Tetraopes tetraophthalmus can be collected over a large area of the United States makes this insect a particularly suitable form for population analysis. Moreover, recent work on its geographic variation, involving many metrical characters (Mason, 1964), brought to light a situation of interest in the study of allometry. Among the characters used in the 1964 study were two measures of the width of the prothorax, one of them across the lateral tubercles which jut out prominently on either side of the thorax, and the second at a point just anterior to these tubercles. This second measurement will be referred to as the "prothorax width" (Ps,), an index of the size of the insect, taken as the independent variable for purposes of the analysis. The difference between the two prothorax width measurements, giving the total length of the two tubercles together, will be referred to as "tubercle size" (T), the dependent variable. Tubercle size showed a very strikingly positive allometric relationship to body size (i.e., to P,), in some populations, but the effect did not seem consistent among samples from different localities. For the purpose of examining this question statistically, thirty males and thirty females were sampled at random from collections made in 1962 at each of 35 localities distributed as evenly as practicable across virtually the entire range of the species. (Conclusions drawn here are thus based on data from a total of 2100 individuals.) Simpson (1953) has recognized five levels of allomorphosis, extending the categories of Huxley and of Westoll (1950). Two of these levels can be readily compared in Tetraopes. Heterauxesis, or ontogenetic allometry, cannot be measured in the present instance, since the adult stage of the beetle is the only one in which the char-