Acoustic and mechanical sensitivity of the distal lobe of the pilifer in choerocampine hawkmoths

Abstract

A descending interneuron response is used to demonstrate that ultrasonic reception in choerocampine sphingids depends upon contact between the medial wall of the second palpal segment and the distal lobe of the pilifer. The medial wall serves as acoustic interface and the distal lobe contains the sensory transducer. When the palp is abducted so as to break this contact acoustic sensitivity drops about 100-fold but may be partially or almost completely restored when the palp contact is replaced by contact of a thin artificial membrane with the distal lobe. The external structure of the distal lobe and its physical relation with the second palpal segment are discussed in this context. If the distal lobe is displaced at frequencies of 5 to 100 kHz by means of a crystal-driven probe it is found to be sensitive to displacements of 0·02 to 0·1 nm at its optimal frequency of 30 to 70 kHz. Other structures on the cranium are 30 to 100 times less sensitive to vibration at these frequencies. Except for registering ultrasound of low intensity the neural response of this acoustic detector contains little or no information about frequency, direction of the source, or intensity differences. Its presence in this subfamily of hawkmoths is discussed in relation to members of other subfamilies.