380 EXTERNAL ANATOMY OF INSECTS. 



terior cavity of the alitrunk, and with the meta- 

 phragm it forms the posterior cavity. 



b Postdorsolum (the Postdorsolum). The middle- 

 piece between the mesophragm and the Postscu- 

 tellum. In Coleoptera it consists of a tense elastic 

 membrane, which is quite covered by the Meso- 

 thorax a . 



c Postscutellum (the Postscutellum). A narrow chan- 

 nel running from the Dorsolum to the Abdomen in 

 Coleoptera, forming an isosceles triangle reversed. 

 In other orders it is either a triangular elevation of 

 the middle of the posterior part of the Postdorsolum, 

 or a distinct triangular piece b . 



d Postfrcenum (the Postfrcenum). In Coleoptera the 

 part of the Metathorax in which the Postscutellum 

 lies, at first nearly horizontal, but posteriorly it takes 

 a vertical direction towards the abdomen. In ge- 

 neral it may be defined, the part that intervenes 

 between the Postscutellum and the Abdomen; and 

 which in many cases is connected with the posterior 

 basal margin of the under-wings, and prevents their 

 being pushed too far forwards c . 



e Pleurae (the Pleurce). The space behind the scapu- 

 lars, on which the lower organs of flight are fixed d . 

 They include the Al<£ Inferiores. 



A Al^e Inferiores (the Under-wings). The lower or 

 secondary pair of organs for flight e . They include 

 the Commissura, Tendo, Hamuli, Pterygium, Alula, 

 and Halteres. 



» Plate VIII. Fig. 3, 12, 20. and IX. Fig. 7, 10-12, 15, 20. t. 

 b Ibid. u. « Plates VIII. IX. XXVIII v. 



d Plates VIII. and IX. w. e Plate X. Fig. 4, 10. 



