418 EXTERNAL ANATOMY OF INSECTS. 
imbibe juices, it is membranous. In form and shape it 
varies greatly, being sometimes nearly square, at others 
almost round; in some insects representing a parallelo- 
gram, in others a triangle, and in many itis oblong. In 
some instances it is long and narrow, but more generally 
short and wide. It is often large, but occasionally very 
minute. In the majority it has an intire margin, but it is 
not seldom emarginate or bilobed, or even dentate. Its 
surface is commonly even, but it is sometimes uneven, 
sometimes flat, at others convex, and in a few species 
armed with a short horn or tubercle?. As to its pubes- 
cence, it is often naked, but now and then fringed or 
clothed with down or hairs, or sprinkled with bristles. 
It consists in almost every instance of a single piece; but 
an exception to this occurs in Halictus, a little bee, in the 
females of which it is furnished with a slender appen~- 
dage*.—The direction of the upper-lip is various. It is 
rarely horizontal, or in the same line with the nose, often 
vertical; it sometimes forms an obtuse angle with the 
anterior part of the head, and occasionally an acute one, 
when it is more or less inflexed. The use of this part is 
ordinarily to close the mouth from above, to assist in re- 
taining the food while undergoing the process of masti- 
cation ; but in many Hymenopterous insects its principal 
use seems to be, to keep the ¢rophi properly folded; and 
in some cases where it is inflexed, as in the leaf-cutter 
bees (Megachile), to defend their base, while the mandi- 
bles are employed, from injury by their action °. 
* Kirby Mon. Ap. Angl. i. t. v. Apis*. b. f. 18. d. 
> Ibid. t. ii. Melitta**. b. f. 4,5. Pirate XXVI. Fic. 30. 
° Pirate XXVI- Fic. 31. Mon. Ap. Angl. i. t, x. Apis**. c. 2.3. 
f-A3.2. 
