COMPARATIVE PHYblOLOGY OF VISION. 175 



532. In the snail, the eye is situated at the extremity 

 of the tentacula or feeler. 



533. Eyes of Insects. -Nearly all the insects are fur- 

 nished with organs of vision either in the larva or perfect 

 state, and many of them in both. 



Many insects are furnished with two kinds of eyes, one 

 kind being situated on each side of the head, and so large 

 as not to escape common observation. These are called 

 compound eyes. The others are three in number, and are 

 situated on the top of the head, obliquely behind, and be- 

 tween these. These are called stemmata. They are either 

 in a row, or in the form of a triangle. 



534. The structure of the stemmata have been minutely 

 examined, by Professor Muller, who has ascertained that 

 they contained a hard crystalline lens, a vitreous humor, 

 and a choroid coat, the whole being covered externally by 

 a hard convex coat. In wasps, bees, and bugs, these parts 

 are distinguished by the naked eye, and so far as external 

 form and appearance are concerned, may be satisfactorily 

 examined by a common magnifier. 



535. In the spider the stemmata are of considerable 

 size, their number being generally eight, and their situa- 

 tion on the top of the head, where they are disposed w T ith 

 much regard to symmetry. 



536. The compound eyes of insects are among the most 

 complex and curious organs which the animal kingdom 

 presents. In some tribes, as in the wasp and dragon-fly, 

 they cover a large portion of each side of the head, and 

 although when only slightly examined they present a 

 smooth outside, and appear each as a single eye, yet they 

 are formed of a vast number of separate cylinders or elon- 

 gated cones closely packed together, each being a distinct 

 eye, and capable of perfect vision. The exterior of each 

 tube is a hexagon, a form which admits of the closest ar- 

 rangement, like the cells of a honey-comb. 



537. The number of these cylinders differ much in dif- 

 ferent insects. In the ant they are only 50 ; in some of 



What are the two kinds of eyes with which insects are furnished 

 called ? What are their situations ? Give a description of the compound 

 eye of an insert 



