EXTERNAL ANATOMY OF INSECTS. 431 
I am not anatomist enough to speak with confidence on 
the subject; but the ball and socket articulation at the 
lower part of the mandible, and the curving one at the 
upper, though a kind of ginglymus, seems to imply a 
degree of rotatory movement, however slight. 
I must next say something upon the general shape of 
these organs. Almost universally they incline to a tri- 
quetrous or three-sided figure, with their external sur- 
face convex, sometimes partially so, and their internal 
concave. Most frequently they are arched, curving in- 
wards; but sometimes, as in Prionus octangularis*, a 
Capricorn beetle, and others of that genus, they are 
nearly straight; and in Rhina barbirostris», a most re- 
markable Brazilian weevil, their curvature is outwards. 
In Pholidotus lepidosus, and Lucanus Elephas, two insects 
of the stag-beetle tribe, they are bent downwards; and 
in Ryssonotus nebulosus they turn upwards*. They are 
usually widest at the base, and grow gradually more 
slender to the apex; but in the hornet (Vespa Crabro) 
the reverse takes place, and they increase in width from 
the base to the apex; and in the hive-bee, and others of 
that tribe, they are dilated both at base and apex, being 
narrowest in the middle; others are nearly of the same 
width every where. In those insects that use their man- 
dibles principally for purposes connected with their 
economy, they are often more broad in proportion to 
their thickness, than they are in those which use them 
principally for mastication. In the locust tribes (Zo- 
" Oliv. Ins. no. 66. Prionus. ¢. xiii. f. 54, 
> Ibid. no. 83. Curculio. t. iv. f. 37. 
© Linn. Trans. xii, t. xxi. f. 12. 
