8o THE BIOLOGY OF INSECTS 



specialised region of the cuticle, thin, tense and delicate, 

 evidently serving as an " ear-drum " which can be thrown 

 into vibration by sound-waves impinging on it. Further, 

 it is to be noted that in most kinds of insects which possess 

 such auditory organs, there are found also special structures 

 for producing sounds. The best known of insect ears are 

 probably those found on the first abdominal segment in 

 ordinary grasshoppers and locusts. On either side of this 

 segment in a locust the sub-circular or ovoid membranous 

 ear-drum is easily seen without the help of a magnifier ; 

 close in front of it is the spiracle of the segment, and over 

 the lower area of its inner surface are spread the fibres of 

 the tensor muscle whose contraction increases its tightness. 

 Attached to the inner face of the drum-membrane, near its 

 centre, is a nerve ganglion in connection with the auditory 

 nerve going to the central system. This ganglion, often 

 called Miiller's organ from its discoverer of a century ago 

 (Johannes Miiller, 1826), contains a number of large nerve- 

 cells, which receive impulses from groups of chordotonal 

 organs, whose distal ends are bound to the drum membrane, 

 and transmit the impulses through the fibres of the auditory 

 nerve to the segmental ganglia of the ventral trunk-cord. 



These remarkable " ears " on the abdomen in grass- 

 hoppers and locusts are among the best known of insect 

 sense-organs. Only recently, however, have we become 

 acquainted v^th ears of a somewhat similar nature on the 

 abdomen in certain moths of the Geometridae, Uraniidae, 

 and allied families, and on the thorax of Noctuidae, Arctiidae 

 and their allies, mainly through the researches of F. Eggers 

 (19 1 9) and H. Eltringham (1923). The tympanal organs of 

 a geometrid or uraniid (Fig. 27) belong to the second 

 abdominal segment, not the first as in a locust. The cuticle 

 of this segment is inpushed on either side to form a hollow 

 vesicle, often much shallower in the male than in the female 

 moth, and at the front aspect of this vesicle the delicate ovate 

 drum-membrane (t) is evident. Connected with the strong 

 ridge surrounding the tympanic membrane are radiating 

 bands of muscle fibres serving apparently to regulate the 



