EXTERNAL ANATOMY OF INSECTS. 561 



or parallel with it, on each side a flat plate ; and from the 

 angle of that part in the first case, and from one of its 

 processes in the last, you will further perceive a ridge or 

 nervure which runs along this plate, in one forming an 

 angle, and in the other being nearly straight, to the base 

 of the tegme?i, where it becomes a marginal nervure to a 

 membrane that is attached to the posterior part of the 

 base of the Anal and Costal Areas; and that this marginal 

 nervure, like a trachea, consists of a spiral thread, or 

 rather of a number of cartilaginous rings connected by 

 elastic membrane a , and consequently is capable of con- 

 siderable tension and relaxation, as the tegmen rises and 

 falls in flight. In the Lepidoptera it appears to be a 

 short piece overhung by the scutellum, which as it ap- 

 proaches the base of the wing is dilated. In the Libel- 

 lulina, to go to the Neuroptera, it has the same kind of 

 elastic nervure connected with the Anal Area of the wing 

 which I have just described in the Homopterous Hemi- 

 ptera ; another nervure, in JEshna at least, appears to 

 diverge upwards from the scutellar angle to the Interme- 

 diate Area b : a structure little different distinguishes the 

 rest of the Neuroptera, and even the Trichoptera. In the 

 Hymenoptera this part varies somewhat ; in the majority 

 perhaps of the Order, as well as in the T>iptera, it ap- 

 pears to be merely the lateral termination of the scutel- 

 lum where it joins the wing; but in some tribes, as in 

 Tenthredo L. (especially Perga Leach), Sirex L., and 

 the Iclineumonidce, a ridge, and sometimes two, runs 

 from the scutellum to the wing ; the upper one, where 



» Plate XXVIII. Fig. 11./'. 



b Chabrier Sur le Vol des Ins. c. iii. t. viii— v. B. i. 



VOL. III. 2 O 



