THE EYES 



249 



THE SENSOKY OBGANS 

 a. The eyes and insect vision 



Of the eyes of insects there are two kinds, the simple and the 

 compound. Of the former there are usually three, arranged in a 

 triangle near the top of the head, between the 

 compound eyes (Fig. 259, B). The compound or 

 facetted eyes, which are usually round and promi- 

 nent, differ much in size and in the number of 

 facets. A 



The number of facets varies from 12 in Lepisma, 

 though in a Brazilian beetle (Lathridius) there are only 

 seven unequal facets, to 50 in the ant, and up to 4000 

 in the house-fly, 12,000 in Acherontia atropos, 17,000 in 

 Papilio, 20,000 in the dragon-fly (^Eschna), 25,000 in a 

 beetle (Mordella), while in Sphinx convolvuli, the number 

 reaches 27,000. The size of the facets seems to bear some 

 relation to that of the insect, but even in the smallest 

 species none have been observed less than 55 ^ of an inch 

 in diameter. Day-flying Lepidoptera have smaller facets 

 than moths (Lubbock). 



Ct 



D 



-re 



FIG. 259. Different 

 forms of compound eyes. 



no A, a bug (Pyrrhocoris). 



Fio. 260. Section through the ocellus of a young Dyticus larva: < worker bee. C, drone. 

 ct, cuticula ; I, corneal lens ; yh, cells of the vitreous body, being modified A male Bibio, a ? ol ?R"5 

 hrpodennal cells (fiy); at, rods; re, retinal cells ; no, optic nerve. insect. From Judeich 

 After Grenacher, from Lang. and Nitsche. 



The simple, or single-lensed eye (ocellus). Morphologically the 

 simple eye is a modified portion of the ectoderm, the pigment 

 enclosing the retinal cells arising from specialized hypodermal cells, 

 and covered by a specialized transparent portion of the cuticula, 

 forming the corneal lens. The apparatus is supplied with a nerve, 

 the fibres of which end in a rod or solid nerve-ending, as in other 

 sensory organs. 



As seen in the ocellus of Dyticus (Fig. 260), under the corneal 

 lens the hypodermis forms a sort of pit, and the cells are modified 



