928 



INSECTA. 



Fig 394. 



A, inferior surface of left wing of Acrida tiiri- 

 dissima. 



B, upper surface of the right, and, C, under sur- 

 face of left wing of Acrida brachelytra, shewing 

 the tympanum a, and bow c, across which the file 

 6 acts. 



l>, the file magnified. 



Until recently it has been supposed that these 

 were the only parts in the male Acrida con- 

 cerned in the production of sound, the me- 

 chanism of which has been explained by Bur- 

 meister* as consisting in a quick attrition of 

 the wings against each other during a forcible 

 expiration of air from the thoracic tracheae and 

 spiracles, which are situated beneath, and are 

 covered by the edges of the wings ; that the 

 air in rushing out of these spiracles is driven 

 against the tympani, which are thus occasioned 

 to vibrate and produce the sound. But this 

 ingenious explanation is not entirely correct; 

 the means employed do not appear sufficient to 

 explain the phenomenon, besides which a part 

 of the structure that is chiefly instrumental in 

 producing the sound has been overlooked. In 

 addition to the tympanum, and parts by which 

 it is inclosed, there is also another part which 

 has not until recently been described. It is a 

 strong, transversely elongated horny ridge, 

 situated immediately behind the tympanum, 

 near the base of the wing, and is most distinct 

 on the upper surface of the wing of the left 

 side. This part was first shown to us by the 

 late Mr. William Lord, in the wing of Acrida 

 viridissima, in February 1838, but it had pre- 



* Manual (translat.), p. 470. 



viously been described by M. Goureau, in an 

 elaborate paper on tlie Stridulation of Insects.* 

 When examined minutely, this ridge, whicli is 

 of the colour and appearance of tortoise-shell, 

 is found to be striated transversely, so as to 

 resemble a rasp or file. Goureau has called 

 it the bow ; a similar ridge or file exists on 

 the under surface of the right wing, but less 

 strongly notched, and is called by Goureau 

 the false bow. When the wings are rubbed 

 briskly together, these rasps or bows produce 

 a loud grating against some projecting or ele- 

 vated nervures on the borders of the wings, by 

 means of which the drum is made to vibrate 

 like any other tensely stretched membrane 

 when agitated, the intensity of the sounds pro- 

 duced depending entirely upon the rapidity 

 and force employed by the insect during the 

 attrition of the parts concerned, and being 

 entirely independent of any forcible expiration 

 of air from the thoracic spiracles, which is 

 thus seen to be unnecessary for the production 

 of the sound. That this is really the case is 

 proved by the fact that in Acrida brachelytra 

 (B, c), the wings are so exceedingly short and 

 narrow that they do not cover, nor are they even 

 near any part of the spiracles, so that the air in 

 passing out of these orifices cannot possibly be 

 driven against the tympanum. On the other hand 

 the tympanum in this species (fig. 394, B, C) 

 is considerably larger than in others of the 

 same genus, and not only has its margins more 

 elevated, but has also a strong bar extended 

 across near its base (r), is itself more tense 

 and vibratory, and has a short, strong bar (d), 

 connected with the ring by which it is inclosed, 

 and also, at a right angle, with the origin of 

 the great marginal nervure of the wing. It is 

 remarkable that both in Acrida viridissima and 

 Acrida brachelytra, the tympanum in one wing 

 differs from that of the other in being less re- 

 gular in its form, much more opaque, and tra- 

 versed by several tracheae, a circumstance 

 which leads us still further to infer that the 

 sounds produced result from the vibrations of 

 one only of these organs, besides which the 

 proper tympani are not in corresponding wings 

 in these two insects. In the former, in which 

 the base of the left wing is covered by that of 

 the right, the proper tympanum is in the left 

 wing, while in the latter insect, in which the 

 right wing is covered by the left, the tympa- 

 num is in the left wing, which is remarkable in 

 being entirely deficient of the file or bow, but 

 which is largely developed on the under sur- 

 face of the left wing. The analogue of the 

 file in the right wing is evidently a strong por- 

 tion of the rim of the tympanum nearest to the 

 base of the wing. It is remarkable also that 

 in this species the whole surface of the right 

 wing, in which the tympanum is situated, is 

 more transparent and free from nervures than 

 the corresponding part of the left wing, so that 

 the whole surface of the wing may perhaps be 

 rendered sonorous. It is remarkable also that 



* Annales dc la Societe Entomologique de France, 



1837, p. 31. Entomological Magazine, January, 



1838, p. 89 ct seq. 



