280 MR. J. 0. WESTWOOD ON NYCTERIBIA. 
the almost rudimental state of the prothoraw in the true Dipterous Insects, it seems clear 
(especially when the remaining portions of the thorax are examined) that the space in- 
closed by this fine line constitutes all that remains of the prothoraa, giving insertion, 
however, to its ordinary attachments, viz. the head and the pair of anterior legs. 
The central portion of the dorsum of the thoraz is inclosed by a narrow crustaceous 
line, and is occupied by an oval plate, rather dilated towards the abdomen, and com- 
posed of a brownish coriaceous membrane, slightly depressed in the middle in a dried 
specimen, and offering a slight transverse elevation in the centre. Latreille describes 
this portion as forming a dorsal channel, and having its posterior extremity terminated 
in the common French species “‘ par une partie élevée, formant le capuchon,” in which 
the head, when thrown back, is received. I have seen nothing of this capuchon, and 
but little of the channel, in any of the specimens which I have examined, and quite 
agree with M. Dufour in regarding them merely as being occasioned by the desiccation 
of the insects after death: the ‘‘ groove down the middle {of the thorax] to receive 
the head,” described by Mr. Curtis, is doubtless attributable to the same cause. 
On each side of this central portion, about midway on each side, is to be observed 
another slender crustaceous bar, directed obliquely towards the head, and extending to 
the sides of the dorsum of the thorax, whence it is prolonged nearly in a line to the place 
of insertion of the fore legs, thus inclosing on each side an elongate and somewhat tri- 
angular plate of a whitish colour, the anterior margins of which do not extend to the 
margin of the thorax, permitting the pectus to be seen from above. To the narrow 
posterior extremity of this lateral portion is attached the base of the intermediate pair 
of legs; and behind these, on each side, a pair of short and narrow portions, similarly 
separated, are to be observed, to the exterior of which the basal portion of the posterior 
legs is attached. Hence it appears to me that the central and anterior lateral triangular 
plates represent the dorsum of the mesothoraz, and the small posterior lateral ones that 
of the metathoraz. 
The disposition of these portions of the thorax is, however, very different on the ven- 
tral surface. This is quite flat, and of a uniform crustaceous texture, of a somewhat oval 
form, without the least indication of the insertion of the legs, and having a central lon- 
gitudinal line running from the anterior to the posterior extremity. M. Dufour de- 
scribes it as ‘‘un plastron d’une seule piéce ;” but Ihave uniformly found an impressed 
line of division extending from the posterior base of the intermediate legs, and running 
parallel with the anterior margin, thus exhibiting the pectus of the mesothorax in the 
form of a lunate plate, and that of the metathorax as much more extensive. 
But the most remarkable organ connected with the thorax is a pair of pectinated 
processes placed between the base of the anterior and intermediate legs, and received 
in a cavity (formed by the lateral productions of the dorsum and pectus of the anterior 
parts of the thorax), one on each side of the thorax. On detaching one of the inter- 
