406 EXTERNAL ANATOMY OV INSECTS. 



ii. The most general law relative to thejigure or shape 

 of the head seems to be, that it should approach to that ol 

 an equilateral triangle, with its angles rounded, and the 

 vertex being the mouth ; and that the vertical diameter 

 should be less than the horizontal, whether longitudinal 

 or transverse. But the infractions of this law are nume- 

 rous and various. Thus, in some insects an isosceles tri- 

 angle is represented by the head, the length being greater 

 than the breadth ; in others, instead of being flat it is 

 compressed, so that the horizontal diameter is less than • 

 the vertical; in others, again, it is orbicular, or round and 

 depressed ; in others nearly spherical : occasionally it is 

 rather cylindrical. In many instances it is very long; 

 in others the width exceeds the length. Though often 

 narrowest before, in some cases the reverse takes place. 

 Its anterior end is often attenuated into a long or short 

 snout or rostrum, and its posterior into a long or short 

 neck. Its contour, though usually regular, is sometimes 

 either cut into lobes, or scooped out into sinuosities. 

 But to enumerate minutely all the variations of form 

 which take place in the head of insects would be end- 

 less; I shall therefore proceed to the next particular. 



iii. The composition of the head is very simple ; for, 

 exclusive of its organs, it consists only of a single piece 

 or box, without suture or segment, with an aperture at 

 the end below to receive the instruments of manducation, 

 others for the eyes and stemmata when present, and also 

 for the antennas. In the Arachnida, &c, in which the 

 head is not separated from the thorax, it is merely a 

 plate, the under-side or cavity of which is occupied and 

 filled bv the above instruments. 



