wings with the left overlapping the right, and in this position the file of the former lies above the ridge (s) of the latter. If now the wings are moved sidewise, the file grating on the ridge or scraper causes a rasping sound, and this is the way the katydid makes the notes of its music. The tone and w)lume of the sound, however, are probably in large part produced by the vibration of the thin basal membranes of the wings, which are called the tympana (Tm).
The instruments of different players differ somewhat in the details of their structure. There are variations in the form and size of the file and the scraper on the wings of different species, and differences in the veins supporting the tympanal areas, as shown in the drawings of these parts from a conehead (Fig. 27) given at A, B, and C, of Figure 19. In the true katydid, the greatest singer of the family, the file, the scraper, the tympana, and the wings themselves (Fig. 26) are all very highly developed to form an instrument of great efficiencv. But, in general, the instruments of different species do not differ nearly so much as do the notes produced from them by their owners. An endless number of tunes may be played upon the same fiddle. With the insects each musician knows only one tune, or a few simple variations of it, and this he has inherited from his ancestors along with a knowledge of how to play it on his inherited instrument. The stridulating organs are not functionally developed until maturity, and then the insect forthwith plays his native air. He never disturbs the neighbors with doleful notes while learning.
Very curiously, none of the katydids nor any member of their family has the earlike organs on the sides of the body possessed by the locusts. What are commonly supposed to be their organs of hearing are located in their front legs, as are the similar organs of the crickets. Two vertical slits on the upper parts of the shins, or tibiae (Fig. 19 D, e), open each into a small pocket (Fig. 20 A, E) with a tympanumlike membrane (Tm) stretched across its inner wall. Between the membranes are air cavities (Tra) and a com-
