THE PERIODICAL CICADA 



The air chamber is a large, thin-walled sac of the tracheal 

 respiratory system, and receives its air supply directly 

 through the spiracles of the first abdominal segment. 

 From the sac are given off" tracheal tubes to the muscles 

 of the thorax and to the walls of the stomach. 



Many insects have tracheal air sacs of smaller size, 

 and the purpose of the sacs in general appears to be that 

 of holding reserve supplies of air for respiratory pur- 

 poses. The great size of the air sac in the cicada's abdo- 

 men, however, suggests that it has some special function, 

 and it is natural to suppose that it acts as a resonating 

 chamber in connection with the sound-producing drums. 

 Yet the sac is as well developed in the female as in the 

 male. Possibly, therefore, it serves too for giving buoy- 

 ancy to the insects, for it can readily be seen that if the 

 space occupied by the sac were filled with blood or other 

 tissues, as it is in most other insects, the weight of the 

 cicada would be greatly increased; or, on the other 

 hand, if the body were contracted to such a size as to 

 accommodate only its scanty viscera, it would lose 

 buoyancy through lack of sufficient extent of surface — 

 a paper bag crumpled up drops immediately when re- 

 leased, but the same bag inflated almost floats in the air. 



The Sound-Producing Organs and the Song 



The cicadas produce their music by instruments quite 

 different from those of any of the singing Orthoptera 

 —the grasshoppers, katydids, and crickets, described 

 in Chapter II. On the body of the male cicada, just 

 back of the base of each hind wing, as we have already 

 observed, in the position of the "ear" of the grasshopper 

 (Fig. 63, Tm), there is an oval membrane like the head 

 of a drum set into a solid frame of the body wall (Fig. 

 120, Tm). Each drumhead, or tympanum, is a mem- 

 brane closely ribbed with stiff vertical thickenings, the 

 number ot ribs varying in different species of cicadas 

 and perhaps accounting in part for the different qualities 



[ 207] 



