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 purposes. The great size of the air sac in the cicada's abdomen, 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 buoyancy 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 released, 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 membrane closely ribbed with stiff vertical thickenings, the number of ribs varying in different species of cicadas and perhaps accounting in part for the different qualities
[ 207 ]
