ENTOMOLOGY IX OUTLINE STRUCTURE OF INSECTS. 



15 



as the dragon-fly, the house-fly, and others, they form the greater part 

 of the head. The eye of an insect is immovable, fast in its socket. It 

 is hemispherical or. curvilinear in form, and covered with facets or flat 

 surfaces. They are called compound eyes, and are of many colors blue, 

 black, emerald green, or deep golden, as in the lace-winged fly. Really, 



FIG. 8. Agglomerate eyes of a 

 male coccid, Leachia fusci- 

 penni.* (After Signoret.) 



FIG. 9. Facets of a com 

 pound eye of 

 plns. Highly magnified. 



each facet is a separate eye, hexagonal in 



shape, with a cornea, lens, pigment-coating, ' llll 



and nervous filament. The facets face in w/% 



every direction and enable the insect to see FIG. 10. Portion of compound eye 



-j i I j.'u j.i_ ' XL of fly (Caliiphora vomitoria), radi- 



on all sides with greater ease than if they a i section e cornea- i iris pig- 

 were single lensed and movable. In SOme ment; , nerve fibers; nc, nerve 



cases these facets are very numerous, the eye ' ygXSJ" '' tr 



of a small beetle, the Mordella, having over 



25,000, the common swallow-tailed butterfly, the Papilio, having 17,000, 



the dragon-fly 12,000, the house-fly 4,000, while the eyes of some ants 



are limited to 50. Besides these compound eyes, most insects have two 



or three simple eyes, 

 known as ocelli or 

 stemmata. These 

 are usually arranged 

 in triangular form 

 (thus .*.), and are 

 round and convex 

 in shape. These ex- 

 tra eyes are not pres- 

 ent in all insects, 

 nor are their exact 

 functions known. 



They are possessed of great refractive power and are supposed to be of 



use in the examination of near-by objects. 



Mouth. The mouth is a very complicated piece of mechanism, 

 and is furnished with various organs, enabling the insect to take its 

 nourishment as its habits require. There are two general classes of 



FIG. 11. Ocelli and compound eyes of the honey-bee (Api 

 mellifera). A, queen; B, drone. (After Cheshire.) 



