Ecomorphology and Mating Behavior of Two Species of Night-stalking Tiger Beetles, Omus audouini and O. dejeanii

Abstract

Night-stalking tiger beetles (Cicindelinae: Omus) are among the least studied members of the highly diverse Carabid sub-family Cicindelinae, the tiger beetles. Despite populations of Omus being common in the forest floor habitats of the west coast of North America and their conspicuous predatory role within terrestrial arthropod communities, little is known about the biology and ecology of Omus.Field studies showed that two species of Omus existed in the forested areas of Powell Butte Nature Park, Portland, Oregon, USA: Omus audouini and O. dejeanii. The co-occurrence of sympatric, and likely syntopic, species allowed for a comparative approach in examining and analyzing previously unknown or unaddressed aspects of the biology of Omus. Both morphometric and behavioral analysis was used to address specific questions regarding niche partitioning and mating behaviors in the genus.On the basis of the competitive exclusion principle, I predicted that these closely related species with similar ecological requirements would experience selective pressure to minimize niche overlap and competitive pressures through morphological character displacement. In particular, the mandibles of male tiger beetles serve a dual role: one as tools for feeding-including prey capture and prey processing-and another role as secondary sexual organs whereby the males use their mandibles to grasp the female and maintain amplexus. A geometric morphometric approach was used to evaluate and compare shape differences between the two species as well as identify trends of sexual dimorphism and species differences in context of prey base.Tiger beetles obligatorily engage in male-superior mounted mating behavior.Body size was used to first address trends of female-biased sexual size dimorphism ii within the Carabid subfamily Cicindelinae. Female tiger beetles may be expected to experience proportionally greater stress during mating among larger bodied than smaller bodied species and selection would favor increasingly pronounced female-biased sexual size dimorphism among larger-bodied species.The mating duration of Omus was anecdotally reported as an order of magnitude greater than any other tiger beetle but has never been experimentally confirmed. I performed a series of pairings under laboratory setting to (1) establish a baseline of mating duration for the two species and test the effects of (2) time of day mating was initiated, (3) food deprivation and (4) operational sex ratio on mating duration.Morphometric analysis suggested niche partitioning existed between the two species due to an average body size scaling factor of x1.3 and an average mandible length scaling factor of x1.5, i.e. "Hutchinsonian Ratios"-an observed minimum scaling threshold of niche differentiation seen in several natural predator populations. Similar minimum values were not seen between the sexes of either species suggesting an absence of sexual niche dimorphism. Geometric morphometric analysis of the mandibles revealed two distinct regions subject to selective ada...