484 



THE SENSORIAL FUNCTIONS. 



tera, and the aquatic Neuroptera, have com- 

 pound as well as simple eyes. Perfect insects, 

 with the few exceptions above noticed, have 

 always compound eyes, generally two in num- 

 ber, placed on the sides of the head : and they 

 are often accompanied by stemmata situated 

 between, or behind them, on the upper part of 

 the head. These stemmata, when met with, are 

 generally three in number, and are either placed 

 in a row, or form a triangle. Their structure 

 has been minutely examined by Professor Muller, 

 who found them to contain a hard and spherical 

 crystalline lens, a vitreous humour, and a choroid 

 coat, with its accompanying black pigment ; the 

 whole being covered externally by a convex 

 cornea. The stemmata of a caterpillar, which 

 has eight of these eyes, are shown in Fig. 419, 



^o 



connected together by a circular choroid mem- 

 brane (x x) common to the whole ; together with 

 the separate branches (oo) of the optic nerve 

 (n) belonging to each. 



All the x4rachnida possess eyes of this latter 

 description ; and from their greater size afford 



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