Static allometry in two species of neotropical stalk-eyed fly

The eye stalk is a sexually selected trait that has evolved several times within true flies (Diptera). It has been suggested that selection on secondary sexual traits such as the eye stalk favors positive allometry (disproportionate increase in a trait in relation to body size), but it remains unclear how common positive allometry is in such traits compared with nonsexual traits. Although the morphological evolution of eye stalks has been extensively investigated in diopsids, little is known about the evolution of eye stalks in other dipteran families. Our study aimed to assess the allometric relationships of sexual and nonsexual traits in two species of the Neotropical ulidiid genus Plagiocephalus. We estimated the covariance between body size and eye stalks, and between body size and four other nonsexual morphological traits, in males and females of P. latifrons and P. lobularis. We found a significant positive scaling relationship between eye stalk length and body size in both species, representing the steepest allometric exponents ever observed in stalk-eyed flies. However, our results also show evidence that positive allometry is not exclusive for exaggerated traits. Our findings also lend support to the hypothesis that the allometric intercept is subject to more variation during evolution than the allometric slope.