Summary
We used neurophysiological analyses to determine that arctiid moths which emerge, as adults, during the spring possess tympanic organs responsive to ultrasonic stimuli similar to the echolocation signals of bats. These species, however, do not express another form of bat-directed defense: sound production. Both audition and sound production are present in those species which are volant in the summer months. We suggest that the predation pressure exerted by populations of syrnpatric, insectivorous bats (Myoris Iucifugus) increases during the season as a result of the foraging patterns of lactating females and, later, their juveniles. Whereas this pressure is low enough in the spring to allow arctiids to exist with only the long-range defense of audition, by summer it has increased sufficiently for selection to favour the presence of short range defenses (i.e. sound production).
Methodology
Arctiid moths were collected at ultraviolet lights located at the Queen's University Biology Station in Ontario. Canada throughout the seasons of 1977 and 1979 and tested in laboratories at the station (for species lists, emergence times, and acoustic activity see Fullard 1977; Fullard and Fenton 1977). The moths were usually tested on the night of capture or placed in plastic vials for use the next day, Identification of the moths was made using Forbes (1960) and voucher specimens which have been deposited with Carleton University. We used standard neurophysiological techniques (described in Fenton and Fullard 1979) to expose the auditory nerves of moths and to determine the peripheral sensitivity of their tympanic organs. Threshold intensities were derived using 2 kHz frequency increments from 2 to 110 kHz with a Briiel and Kjaer (B and K) bin. (1 in. = 2.54cm) condenser microphone (type 4135) and a type 2606 measuring amplifier. Stimuli were in the form of continual sinusoidal pure tones increased at a rate of 5 dB/s until threshold (first detectable auditory cell response) was attained. Sound production from tactile stimuli using techniques described in Fullard and Fenton (1977) was monitored in those arctiids captured during 1977. Throughout this field season all of the arctiids (30 species; 1099 specimens) which came to the lights from 2000 to 0200 hours were captured, tested. and retained. A colony of Myoris 1rrc;firgrr.s Le Conte (predominantly female) located 25 km from the station was used to assess the population buildup throughout the spring. Weekly counts were made of the bats as they emerged from the colony at dusk. Myoris Iucifugus was selected for this study since it is the most abundant bat in the area and possesses a number of easily accessible roost sites (Fenton 1970). Additional counts were also made at another M. Iurcifugus colony housed in an abandoned silo 5 km from the station.