184 Psyche [December 



tion, — in connection with the first spiracle. The first type of 

 tympanum is that more or less familiar in the Geometridfe, which 

 is used by Prout (Genera Insectorum, fasc. 103) as the most 

 fundamental distinguishing character of that family. This ap- 

 pears superficially as a simple hollow balloon located immediately 

 below the spiracle, opening to the exterior in the fold that nor- 

 mally exists in the sclerite of the abdomen facing the hind coxa. 

 It is usually more or less ellipsoidal, and opens conspicuously on 

 the side, forward and outward. In many cases the two tympana 

 are so large as to be separated from each other only by membrane, 

 and look like a pinhole clear through the moth,^ — as is very con- 

 spicuous in Eudule and many Acidaliinse (Sterrhidse) . In the 

 Ennominse the organ is smaller and less conspicuous externally, 

 but entirely of the same character, and it exists even in the lower 

 Q^nochrominae (Monocteniidw) . In this type the spiracle lies on 

 the upper edge of the organ, and the dorsal part of the segment is 

 normal. 



Internally (Fig. 6) the structure is seen to be somewhat more 

 complex. The tympanum itself may be compared roughly to a 

 kettle-drum with edges curving inward strongly to meet the edge 

 of the membranous area that represents the head, and faces for- 

 ward. (It has been turned back in the figure to show the struc- 

 tures between it and the thorax.) Running almost across this 

 head there is a tapering chitinous tongue, lying in the membrane, 

 to the tip of which are attached a couple of muscles, the soft fleshy 

 connection with the nerve (B), which histological study, is likely 

 to prove the end-organ itself, and a variety of air-sacs and fine 

 tracheae. In fact the whole organ is enveloped in a system of air- 

 sacs, which have mostly been removed in the specimen figured. 

 The external opening of the apparatus is on the side that lies against 

 the body-wall, — as if a large hole had been cut in the side of the 

 kettle. The nerve-supply appears to be from the anterior seg- 

 mental nerve of the first segment of the abdomen, which runs back 

 from the ganglion, which is fused with the metathoracic one. This 

 nerve runs past the tympanum anteriorly, and then over the spir- 

 acle and up to supply the dorsal musculature, etc. The part of it 

 between the tympanum and spiracle is surrounded by large air- 

 sacs, which connect directly with the spiracle. 



The European genus Cimelia is generally placed in the Geomet- 



