OCR Text |
Show 356 SEXUAL SELECTION. PART II. teeth, mere rudiments, on the inferior surface of tl~e right wing-cover, which underlies the other and 1.s never used as the bow. I observed the s~me r~dimentary structure on the under side of the r1ght WJ~gcover in Phasgonura viridissima. Hence we may with confidence infer that the Locustidre are desce~ded from a form, in which, as in the existing Achet1dre, both win0'-covers bad serrated nervures on the under surface, and 0 couid be indifferently used as the bow ; but that in the Locustidre the two wing-covers gradually beca~~ differentiated and perfected, on the principle of the diVIsion of labour, the one to act exclusively as the bo~ and the other as the fiddle. By what steps the more simple apparatus in the Achetidre originated, .we do not k~ow, but it is probable that the basal portwns of the wmgcovers overlapped each other formerly as at present, ~nd that the friction of the nervures produced a gratmg sound as I find is now the case with the wing-covers of th~ females.36 A grating sound thus occasionally and accidentally made by the males, if it s~rved th~m ever so little as a love-call to the females, might read1ly have been intensified through sexual selection by fitting variations· in the roughness of the nervures having been continually preserved. . .. In the last and third Family, namely the Acndndro or grasshoppers, the stridulation is. produced. in a very different manner, and is not so shrill, accordmg to Dr. Scudder, as in the preceding Families. The inner surface of the femur (fig. 13, r) is furnished with a longitudinal row of minute, elegant, lancet-shaped, elastic teeth, from 85 to 93 in number ;37 and these are scraped 36 Mr. Walsh also informs me that he has noticed that the femal~ of the Platyphyllum concavum, "when captUl'ed makes a feeble gratmg "noise by shufiling her wing-covers together." 37 Landois, ibid. s. 113. <CaAr. X. ORTHOPTER.A. 357 .across the sharp, projecting nervures on the wing-covers, which are thus made to vibrate and resound. Harris 38 -says that when one of the males begins to play, he first " bends the shank "of the hind-leg beneath ·" the thigh, where it is "lodged in a furrow de ·" signed to receive it, '' and then draws the leg " briskly up and down. "He does not play both "' fiddles together, but al-t 1 fi Fig. 13. Hind-leg of Stenobothrus pratorum: " tern a e y rst upon one 1', the stridulating ridge; lower figure, the ·" and then on the other." teeth, forming the ridge, much magnified (from Landois). In many species, the base .of the abdomen is hollowed out into a great cavity which is believed to act as a resounding board. In Pneumora (fig. 14), a S. A.fi·ican genus belonging to this same family, we meet with a new and remarkable modification : in the males a small notched ridge pro-jects obliquely from each side of the abdomen, against which the hind femora are rubbed.39 As the male is furnished with wings, the female being wingless, it is remarkable that the thighs are not rubbed in the usual manner against the wing-covers ; but this may perhaps be accounted for by the unusually small size of the hindlegs. I have not been able to examine the inner surface of the thighs, which, judging from analogy, ''wuld be finely serrated. The species of Pneumora have been more profoundly modified for the sake of s tridulation than any other orthopterous insect; for 3 ~ 'Insects of New England,' 1842, p. 133. :m ·w estwoocl, ' l\fodern Classification,' vol. i. p. 462. J |