Audition in spring species of arctiid moths as a possible response to differential levels of insectivorous bat predation

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 sympatric, insectivorous bats (Myotis lucifiigits) 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).