EXTERNAL ANATOMY OF INSECTS. 569 
As to the Lepidoptera Order, in Pontia Brassice at least, 
the postdorsolum is represented by a pair of nearly equi- 
lateral triangles whose vertexes meet in the centre of the 
metathorax, and between which and the scwtellum is a 
deep cavity; but in Macroglossa Stellatarum and La- 
siocampa Quercus, there appears to be also a central 
transverse piece between them. In the Newroptera there 
is no material or striking difference between the parts of 
the mesothorax and metathorax*. In the Hymenoptera 
more variety occurs in this part. In the saw-flies, &c. 
the postdorsolum is a transverse piece covered by the 
scutellum ; in the Ichneumonideé it is smaller, but not 
covered; in the Vespide it is apparent, transverse, and 
with the postscutellum obtusangular; in Apis it is over- 
hung by the scutellum. The Diptera exhibit some varia- 
tions in this part. In Z%pula it consists of three pieces 
placed transversely, the central one quadrangular, and 
the lateral ones roundish; in the Aside and most 
others of this Order, with the postscutellum, it forms a 
segment of a circle*, sometimes armed with a pair of 
spines, as in Stratyomis, and is what has been usually 
regarded as the real scutellum, though, as I have endea- 
voured to show, not correctly*. 
10. Postscutellum®. The postscutellum bears the same 
relation to the postdorsolum that the scutellum does to the 
dorsolum, but it is seldom, if ever, a distinct piece. In the 
Coleoptera it is represented by the longitudinal narrow 
channel that terminates the postdorsolum towards the 
anus‘: this usually figures an isosceles triangle with the 
AGATE ONG: Lc. (feF. » Tbid. Fic. 11. Z'. 
* Ibid. Fie. 19, 20. #’. 4 See above, p. 5d6—. 
Puates VIII. UX. wu. f Prate VIII. Fie. 3. a’. 
ae ° f 
