14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 94 



of the first abdominal sternum (fig. 9 A). The dilator muscle of the 

 spiracle (14/) takes its origin at the same point. The muscle of the 

 tympanal frame appears to be a tensor of the tympanum. In the an- 

 terior part of the tympanum are two small cuticular thickenings that 

 support the sense organ on the inner surface (fig. 6 B, SO). The prin- 

 cipal support is a wide-angled V-shaped sclerite with a narrow dorsal 

 arm (B, C, b) and a broader ventral arm (c) diverging from an apical 

 knob (a). The last is a pitlike invagination of the external surface 

 of the tympanum, and the major part of the sense organ (B. C) is 

 attached directly to its ventral surface. The other support is a much 

 smaller pyriform sclerite (d) lying posterior to the angle of the \'- 

 shaped sclerite, to which is attached a small fusiform branch of the 

 main sense organ (C). 



The tympanal sense organ (fig. 6 B, SO) is a small oval body com- 

 posed mostly of a mass of sense cells (C, SCls), but in its upper part 

 is a stratum of elongate parallel cells containing sense rods, or scolopes 

 (Sco), beyond which is a layer of large cap cells (CpCls) by which 

 the organ is attached to the knob (a) at the angle of the V-shaped sup- 

 porting sclerite. A slender posterior branch of the main organ contains 

 a second smaller set of scolopes, and is attached by a fascicle of slender 

 cap cells to the pyriform sclerite (d). From the ventral end of the 

 organ the sensory nerve (Nv) proceeds to the large composite ganglion 

 of the ventral nerve cord lying in the metathorax. 



Between the tympana of the opposite sides of the body are two large 

 air sacs given off from the lateral tracheal trunks in the base of the 

 abdomen. The sacs completely occupy the cavity of the first abdominal 

 segment above the alimentary canal, and their outer walls are pressed 

 close against the tympana. The two sacs form such a large air-filled 

 space in the base of the abdomen of Dissostcira that it is possible to 

 look clear through the body of the insect. /. c, into one " ear " and 

 out of the other. 



The tympanal organ of the Acrididae is usually regarded as a sound 

 receptor, though little or no evidence of hearing on the part of the 

 grasshoppers has yet been produced. Few species are capable of mak- 

 ing sounds, and an auditory " sense " would not seem to be one of great 

 importance to a grasshopper, but the elaborate mechanical and sensory 

 structure of the tympanal organs suggest that the latter must subserve 

 some function of importance in the life of the insect. Perhaps we are 

 too prone to conceive of insect " senses " as sensory perception of 

 stimuli. An insect merely reacts through its motor mechanism to 

 certain stimuli. The reaction to stimuli from a tympanal receptor 

 organ, therefore, may be something quite different from a general 

 sensitivitv to sound in the audible sense. 



