Distortion coordinates (Cartesian Transformations) are used to compare the cranial morphology of first, second, and third instars of Agabus disintegratus (Crotch) and third instars of A. disintegratus and Matus bicarinatus (Say). These analyses revealed significant ontogenetic changes in the cranial architecture of A. disintegratus and differences between crania of third instars of A. disintegratus and M. bicarinatus. Differences between A. disintegratus and M. bicarinatus are interpreted as evidence that larvae of the two species exploit prey regimes with different characteristics. It is hypothesized that the ability of prey to resist capture may be an important characteristic responsible for the ontogenetic changes in cranial morphology observed in A. disintegratus. Anisometric increases in the adductor musculature responsible for closing the mandibles would explain the posterior expansion of the cranium of A. disintegratus during ontogeny. These changes are interpreted as evidence that A. disintegratus larvae may exploit prey regimes with different characteristics during each stadium.
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