Size, peripheral auditory tuning and target strength in noctuid moths

We investigated relationships among body size, the frequency of peak audito ry sensitivity (best frequency) and acoustic conspicuousness (measured as t arget strength) to simulated bat echolocation calls in a range of tympanate moths (Lepidoptera: Noctuidae). Audiograms of Amphipyra pyramidea Linnaeus , Agrotis exclamationis Linnaeus, Omphaloscelis lunosa Haworth and Xestia x anthographa Denis and Schiffermuller are described for the first time. Best frequency was inversely related to forewing length, an index of body size. Models predict that target strength falls off rapidly once wavelength (1/f requency) exceeds some defined feature of target size (e.g. circumference f or spheres). We investigated how target strength varies in relation to targ et size and emitted frequency for simple targets (paper discs) and for moth s. Target strength fell rapidly when target radius/wavelength < 2 for paper discs of similar size to many noctuid moths. Target strength fell rapidly below wing-length/wavelength ratios of 2 in relatively small (O. lunosa, wi ng-length = 15.2 +/- 0.4 mm, best frequency = 45 kHz) and large (N. pronuba , wing-length = 24.6 +/- 0.8 mm, best frequency = 15 kHz) noctuid species, and decreased rapidly at frequencies below 25 kHz in both species. These ta rget strengths were used to predict the detection distance of the moths by bat sonar between 10 and 55 kHz. Predicted detection distances of both spec ies were maximal for fictive call frequencies of 20 kHz, and were reduced a t lower frequencies due to decreased target strength and at higher frequenc ies by excess atmospheric attenuation. Both relatively large and small noct uid moths are therefore strong acoustic targets to bats that echolocate at relatively low frequencies. Bats may emit allotonic calls at low frequency because the costs of reduced detection range are smaller than the benefits of reduced audibility to moths. Because best frequency scales with body siz e and maximum detection distance is not very sensitive to body size, noctui d moths in the size range examined do not necessarily have best frequencies that would match the call frequencies of bats that may detect the moths at greatest distance precisely. Hence, best frequency may be constrained in p art by body size.