233 ' THE CAUSES WHICH PROPAGATE 



mirrors, but also the anterior soft membrane separating the ab- 

 dominal cavity from the thorax, tells us " he did not remark any- 

 sensible diminution in the intensity of the sound/^ 



I will therefore attempt a short microscopic description of the 

 structure and nervous connections of these membranes, in hopes 

 that others may more fully follow up the subject. The mirrors 

 {m, Fig. 5a, Plate III.) of the cicadae (membrana tympanicd) 

 generally consist in a membrane surrounded with an outwardly 

 hollow chitineous frame. Among the males (Plate VII., Fig. 5) 

 they approximate in shajDe a harp-shaped triangle, situated at an 

 angle with the median plane, so that the apex is posterior and 

 directed inward, and the inner straight side, which is indurated 

 and blackened, borders inferiorly the tymbal motor muscles, 

 having an inclination of about ^O*^ to the normal vertical. The 

 surface of the enclosed membrane makes some SC^ with the 

 horizontal median line, thus being inclined downwards ; it is 

 also slightly directed outwards. In the females the chitineous 

 frame {trommelring) is a narrower curvilinear triangle transversely 

 situated. The area occupied by the membrane is about one-sixth 

 of a square inch in the males of the largest European cicada, 

 but in its females it barely measures one-half. 



As we shall find in the tympana of the grasshoppers and 

 moths, a portion of the membrane is attenuated ; there this part 

 is posterior, but here in the males it is somewhat central, 

 rhomboidal or diamond-shaped, with the principal axis approaching 

 the vertical ; in the females, on the contrary, it is cordate, and 

 situated more inwardly or nearer the apex of the frame. As in 

 moths also, its superficies is rendered conspicuous from prismatic 

 hues. A yellow disc is surrounded by a band of lake concentric 

 with exterior lines of purple, blue, green, yellow, and lake, which 

 are most perfect in the male. The remainder of the membrane 

 is of a neutral tint that appears in some measure due to a fine 

 mesh of filaments, and to these or to a waviness in the membrane 

 I would ascribe also the bright interferential spot. A concentric 

 striation has also been observed in the corresponding membranes 

 of grasshoppers, and it is probable that this adaptation for light 

 analysis shadows forth an allied capability regarding incident 

 sound waves. Structurally, the mirror is stated to be composed 

 of a layer of elongate, hexagonal, and lenticular cells, analogous 



