10 MB. A. HAMMOND ON THE 



sternum, but with the episternum anterior to each wing, and the 

 trochantin, articulating with the epimeron and coxa of the leg 

 — the paraptera of the prothorax being, according to Audouin, 

 absent." 



These parts constitute the external casing of each thoracic 

 segment exclusive of the appendages, viz. the wings and the legs, 

 and of the internal process known as the entosternum. Of those 

 on the dorsal surface the scutum is the most prominent piece, 

 and to it, in the alary segments, the articulations of the wings are 

 affixed. In front of it is the prsescutum, forming the anterior 

 boundary of the segment, and generally bent downwards to form the 

 horny partitions between the segments known as the phragmata. 

 Following the scutum is the scutellum, a prominent portion of the 

 thoracic skeleton, to which also, in conjunction with the scutum, 

 the membranous portions of the wings (the alulets of the Diptera 

 and Dyticidae) are attached. Lastly, we have the postscutellum, 

 which, like the prsescutuni, is generally bent downwards to form 

 the phragma. These four pieces were regarded by Audouin* as 

 the dorsal portions of four subsegments or annuli, of which the 

 pectoral portions are less easily demonstrable on account of their 

 being frequently confluent and not nearly so greatly developed. 

 The parts forming the pectoral surface have been already suffi- 

 ciently alluded to for my present purpose in the quotation from 

 Newport. Although I cannot indorse the whole series of rela- 

 tions thus indicated by Audouin, and typically exemplified in the 

 structure of the Dyticidse, the general correctness of his views is 

 evidenced to my mind by the fact that on those chief points 

 which separated him from Macleay f, Burmeister J, Westwood §, 

 and Newport ||, to which I shall again have occasion to refer, I find 

 the interpretation which Audouin has put upon these questions 

 the more consonant with my own. 



Where the separation of the three thoracic segments is distinct, 

 as, for instance, in the Coleoptera, the determination of the limits 

 of each is a matter of little difficulty. "Where, however, on the 

 contrary, they are more or less connate, as in the Hymenoptera, 

 and especially in the Diptera, the difficulty is proportionally 

 increased. In the former case this is illustrated by the fact of 

 the dispute which raged over the question as to whether the piece 



* Ann. d. Sci. Nat. torn. i. p. 118 (1824.) t Zoological Journal, vol. v. 



X * Manual of Entomology,' translated by W. E. Shuckard. 

 § Introduction, vol. ii. || Op. tit. 



