COMPARATIVE PHYSIOLOGY OF VISION. 179 



In the Snail, the eye is situated at the extremity of 

 the tentacula or feeler. 



Eyes of Insects. Nearly all the Insects are furnished 

 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 num- 

 ber and are situated on the top of the head, obliquely 

 behind, and between these. These are called stemmata. 

 They are either in a row, or in the form of a triangle. 



The structure of the stemmata has 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. 



In the spider the stemmata are of considerable size, 

 their number being generally eight, and their situation 

 on the top of the head, where they are disposed with 

 much regard to symmetry. 



The compound eyes of Insects are among the most 

 complex and curious organs which the animal kingdom 

 presents. In some tribes, as in the Wasps 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 elongated cones closely packed together, each being 

 a distinct eye, and capable of perfect vision. The exte- 

 rior of each tube is a hexagon, a form which admits of 

 the closest arrangement, like the cells of a honey-comb. 



The number of these cylinders differ much in differ- 

 ent Insects. In the Ant they are only 50 ; in some of 



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

 ed 1 What are their situations ? Give a description of the compound 

 eye of an insect. 



