406 EXTERNAL ANATOMY OF INSECTS. 
iv. With regard to its superjicies, the head of insects 
is generally more or less uneven, though in some cases it 
presents no inequalities. In‘many of the Petalocerous 
tribes, and a few other individuals, in one sex at least, 
as has been before observed *, it is armed with long 
horns, or prominent tubercles ; it is often covered with 
numerous puncta, or pores; and some of its parts, as the 
nose, after-nose, &c. in particular groups, marked out 
by an impressed line >. In many Hymenoptera, Diptera, 
&e. its upper surface is convex, and the lower concave ; 
in others both surfaces are convex. 
v. It is the most general rule, as to its proportion, that 
it shall be smaller than either trunk or abdomen; but in 
some instances, as inthe South American ant, Moco- 
doma cephalotes, it is much larger than either. 
° 
vi. By the direction of the head, I mean its inclina- 
tion with respect to the prothorax. The most natural 
direction, or that which obtains most generally, is for it 
to form an angle more or less obtuse with the part just 
mentioned. ‘This seems to obtain particularly in Coleo- 
ptera ; but in some, as Mylabris, it is inflexed, forming 
an acute angle with it. In the Heteropterous Hemiptera 
itis generally in the same line with the body, or hori- 
zontal; and in many Diptera it is vertical. 
vii. We now come to a circumstance which will de- 
* See above, p. 308—. 
» In the hornet and other wasps, this line on the inside of the 
head furnishes a foundation for a septum, which in the sides of the 
nose is very high, and connects also with the hind part of the head. 
