EXTERNAL ANATOMY OF INSECTS. 535 
forms a right angle with the rest of the ora, and becomes 
the lower part of the cavity that receives the head ; and 
in Dermestes this part is excavated into an anterior and 
posterior one which admits the antennz and arms when 
folded for repose. 
2. The margin of the prothoraz is a ridge, either de- 
fining its sides or whole circumference. In many cases 
this margin is broad and dilated, but in others it is merely 
a thread or bead that separates the shield from the ora. 
Though generally terminating the upper surface, it some- 
times, as in Staphylinus, dips below it. In many insects, 
however, as I just observed, the thoracic shield has no 
lateral margin whatever. 
3. Various and singular are the appendages with which 
the prothorax of numerous insects is furnished. Many of 
these are sexual distinctions, and have been before de- 
scribed to you*; but there are others common to both 
sexes, the most remarkable of which I shall notice.— 
Some are distinguished by a long horn which overhangs 
the head, as Membracis cultrata, ensata, &c.°; in others 
it stands upright, as in Centrotus spinosus*; C. Tau- 
rus has a pair of thoracic horns like those of a bull, 
only dorsal"; in Ledra aurita they are flat, and repre- 
sent ears®; in some species of Tingis (JT. Echiz, Pyri, 
&c.) a kind of reticulated hood, resembling lace, is ele- 
vated from the anterior part of the prothorax, which re- 
ceives and shelters the head‘. In Centrotus globularis 
and clavatus, especially the former, the part in question 
* See above, p. 326—. 
» Coquebert Ilustr. Ic. ii. t. xviii. f. 2, 4. 
° Stoll Cigales t. xxi. f. 116. 4 [bid. t. xi. f. 53. 
¢ Pirate II. Fie. 4. f Prate XIII. Fie. 18. a. 
