EXTERNAL ANATOMY OF INSECTS. 489 
head, forming its sides and posterior angles, and inclu- 
ding the hinder part of the eyes, the vertex, and the oc- 
ciput. They seldom exhibit any tangible character, ex- 
cept in certain ants (A¢ta), in which their angle termi- 
nates in one or two strong spines, giving the animal a 
most ferocious aspect; and in that remarkable genus 
Corydalis they are armed below with a tooth or point, 
which was not overlooked by De Geer ?. 
vill. Oculi ».—I must now call your attention to organs 
of more importance and interest, and which indeed in- 
clude a world of wonders: I mean the eyes (Oculz) of 
insects. These differ widely from those of vertebrate 
animals, being incapable of motion. They may be re- 
garded as of three descriptions—simple, conglomerate, 
and compound. 
1. Simple Eyes*.—We will consider them as to their 
number, structure, shape, colour, magnitude, situation, and 
arrangement. 
As to their number, they vary from two to sixteen. 
In the flea, the louse, the harvest-man (Phalangium), 
there are only a pair ; in the bird-louse of the goose 
(Nirmus Anseris), and probably in others of the same 
genus, there are four‘; in some spiders (Scytodes, Dys- 
dera, and Segestria*), and some scorpions ‘, there are 
* De Geer iii. 561. ¢. xxvii. f. 1. > Pirates VI. VII. X XVI. h. 
¢ Prats VII. Fic. 8, 9. XXVI. Fic. 43. h. 
4 Viz. one on each side above, and one below. 
© Walckenaer Aranéides, t. v. f. 50, 52. t. vill. f. 82. 
£ Treviranus (Arachnid. 4.) says that Scorpio Huropaus has only 
two eyes. He appears to have overlooked the two on the anterior 
side of a tubercle at each angle of the head, where they are large, 
but not conspicuous, at least in my specimen. 
