THOBACIC APPENDAGES. 65 



gular membrane, in the free edge of which is a leaf -like thickening. Three 

 curved ridges extend from the base of these processes upon the inner 

 side of the prse-scutum of the meso-thorax, and give origin to muscles, 



The meso- thoracic tergum is marked externally by four transverse 

 lines, which divide it into four nearly equal portions. These are separated 

 internally by ridges, which give origin to muscles as well as strengthen 

 the whole structure ; that between the scutum and scutellum sends 

 inwards a small pointed process near the origin of the wings, from which 

 muscles arise for their movement, The lateral edges of the scutellum 

 form a pair of thickened rods for the support of the articulation of the 

 wing, and a loose fold of integument unites the scutellum to the lateral 

 and dorsal plates of the meta-thorax and gives the chief freedom of 

 motion to the whole dorsal region, as the scutum, praescutum, episternum 

 and sternum of the meso-thorax are firmly soldered to each other, but are 

 ufficiently elastic to allow of some movement in the folds between them. 

 The cavil y of the post-scutellum is entirely filled by a pair of air vessels 

 ( Plate V, Fig. 6). 



The tergum of the meta-thorax is almost vertical in its direction. It 

 forms the posterior wall of the thorax, and it is distinctly divided into four 

 parts by ridges corresponding to the same parts of the meso-thoracic 

 tergum ; two of these, however, the scutellum and post-scutellum, only 

 appear internally, the first abdominal segment arising apparently from 

 the junction of the scutum and scutellum. 



Section VJLL The Thoracic Appendages. 

 PLATE I. ; PLATE V., Fias 1214 ; PLATE VL 



The wings consist, as has been stated above, of a double 

 membrane.* This is kept extended by a system of nervnres, 

 which chiefly arise from two primary nervnres. An anterior 

 or costal, which runs along the anterior margin of the wing to 

 its extremity, and a posterior or sub-costal, which arises in 

 common with the former, and runs for a short distance 

 parallel to it, and then splits up into four trunks, which all 

 terminate by joining the costal nervure. Another set of 



* Page 18. 



