120 INSECT PHYSIOLOGY 



campaniform organs, in the halteres or reduced hind-wings of Dip- 

 tera. These structures are exceedingly sensitive to strains set up in 

 the cuticle and, as we have seen (p. Ill), they enable the insect to 

 respond to the inertia forces set up in the vibrating haltere when the 

 insect changes direction during flight. 



Hearing 



Where these mechanical sense organs are sufficiently sensitive and 

 suitably disposed they may be stimulated by the pressure changes or 

 the air movements associated with air-borne sounds. That applies to 

 the antennae of mosquitoes in which Johnston's organ is highly 

 developed; the entire bushy antenna of the male mosquito is set in 

 vibration by the high-pitched sound emitted by the female in flight. 

 It is this note which provides the stimulus to mating. Even the cerci 

 of crickets, which are primarily tactile organs sensitive to air move- 

 ments, may have their long tactile bristles set in vibration by low- 

 pitched sounds. But the most highly developed auditory organs, 

 which occur in Tettigoniids, Locustids, Lepidoptera, &c, consist of 

 batteries of specially modified scolopidia stimulated by the move- 

 ments of a membranous ear-drum or 'tympanum'. In these insects 

 the auditory organs become much more sensitive to sounds of higher 

 frequencies, and their range of perception extends into frequencies 

 far beyond those which the human ear can detect. They possess re- 

 markable powers of recognizing the sounds produced by their own 

 species. They apparently do this by responding, not to the pitch of 

 the sound, which is the characteristic most noticeable to the human 

 ear, but to the frequency with which the note is 'modulated'. 



Insect sounds are generally produced by a rod (connected with a 

 sound-producing membrane) being moved across a series of teeth ; 

 each tooth strengthens or modulates the note given out by the mem- 

 brane. The tympanal organs of insects have a very small 'time con- 

 stant': they can distinguish sounds separated by intervals as short 

 as 1/100 sec, whereas the human ear cannot perceive intervals of 

 even 1/10 sec. The insect ear is therefore well adapted to perceive the 

 modulation of sound frequency in the songs of their own species, and 

 Noctuid moths are able to perceive and recognize the rapid pulses of 

 high pitched sounds given out by bats hunting by echo-location. 



