EXTERNAL ANATOMY OF INSECYS. 405 
ii. The most general law relative to the fgure or shape 
of the head seems to be, that it should approach to that of 
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. 
ili. 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 antennz. 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 by the above instruments. 
