432 EXTERNAL ANATOMY OF INSECTS. 



well as vertically — so those of insects may have some 

 motion vertically as well as horizontally ; and it seems 

 necessary for some of their operations that they should. 

 I am not anatomist enough to speak with confidence on 

 the subject, but the ball and socket articulation at the 

 low r er part of the mandible, and the curving one at the 

 upper, though a kind of ginglymus, seems to imply a de- 

 gree 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 Ca- 

 pricorn beetle, and others of that genus, they are nearly 

 straight; and in Rhina barbirostris b i a most remarkable 

 Brazilian weevil, their curvature is outwards. In Pholi- 

 dotus lepidotus MacLeay, and Lucanus JElephas, two in- 

 sects of the stag-beetle tribe, they are bent downwards ; 

 and in Lucanus nebidosus K. (Myssonotus MacLeay) they 

 turn upwards c . They are usually widest at the base, 

 and gi'ow 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 mandibles principally for purposes 

 connected with their economy, they are often more broad 



a Oliv. Ins, no. 66. Prionus. t. xiii.y. 54. 

 b Ibid. no. S3. Curculio. t. Vf.f. 37- 

 c Linn. Trans, xii, /. xxi.f. 12. 



