Convergent Evolution in Perching and Patrolling Site Preferences of Some Hilltopping Insects of the Sonoran Desert

Abstract

Males of the pompilid wasp, Hemipepsis ustulata, defend palo verde trees on mountain ridges in the Sonoran Desert. The preferences of the wasps for different trees on one ascending ridge remained essentially the same over four spring flight seasons. The wasps preferred large trees high on the ridge. Other insects unrelated to the wasp behaved similarly and focused their territorial or non-territorial mate-searching on or near the same sites most preferred by the wasps. Convergent preferences of hilltopping male insects may reflect the capacity of the compound eye of insects to detect some distant landmarks more easily than others. The prominent ridges that rise from the upland Sonoran Desert of central Arizona attract a diversity of insects, including tarantula hawk wasps, various butterflies, and bot flies. In the spring months males of these hilltopping species patrol or perch in or near conspicuous palo verdes (Cercidium microphyllum) growing on the backbone of the ridges. There they await the arrival of receptive females. Males of some species defend their patrolling or perching locations; in other species males exhibit little or no aggression toward conspecifics. But whether territorial or not, hilltopping males are not distributed evenly along ascending mountain ridges. Previous studies (Alcock, 1981, 1983a,b; Alcock and Schaefer, 1983) of three territorial members of the hilltopping complex (a pompilid wasp, Hemipepsis ustulata; a hairstreak butterfly, Atlides halesus; and a rodent bot fly, Cuterebra sp.) have documented that: (1) certain locations are much more likely to be occupied than others by territorial males, (2) territorial preferences appear to be stable from year to year, and (3) different species have similar preferences for available territory sites. The purpose of this report is (1) to provide new data on the year to year stability of territorial preferences of the wasp Hemipepsis ustulata, (2) to identify the key parameters of perch sites that determine their relative attractiveness to the wasps, and (3) to present new evidence showing that other species, both territorial and non-territorial, rank available mate- locating sites along a ridge in much the same order as the wasp.