BRANCH ARTHROPODS; CLASS INSECTS: THE INSECTS 159 



is really composed of six or seven body segments greatly 

 modified and firmly fused together. Note that it bears 

 a pair of large compound eyes and three much smaller 

 simple eyes or ocelli. 



TECHNICAL NOTE. Strip off a bit of the outer covering of a 

 compound eye, mount on a glass slide and examine under the 

 microscope. 



Note that, as in the crayfish, each compound eye is 

 composed externally of many small hexagonal facets, the 

 outer covering, the cornea, being simply the cuticular cover- 

 ing of the body, in this place transparent and divided into 

 small facets. Besides the eyes, the head bears also several 

 movable appendages, namely the antennce, and the 

 mouth-parts. Note the number, place of insertion, and 

 segmented character of the antennae. These antennae are 

 sense-organs and are used for feeling, smelling, and, in 

 some insects, for hearing. Note that the mouth-parts 

 consist of an upper, broad, flap-like piece, the *labrum; of 

 a pair of brown, strongly chitinized, toothed jaws or 

 mandibles; of a second pair of jaw-like structures, the 

 maxillce, each of which is composed of several parts ; and 

 of an under, freely-movable flap, the labium, also com- 

 posed of several pieces. Each maxilla bears a slender 

 feeler or palpus composed of five segments. The labium 

 bears a pair of similar palpi, which are, however, only 

 three-segmented. The mandibles and maxillae, which 

 are the insect jaws, move laterally, riot vertically as with 

 most animals. 



Make drawings of the lateral aspect of the head ; of a 

 bit of the cornea ; of the dissected out mouth-parts. 



Of the three segments of the thoracic region of the 

 body, the most anterior one is called the prothorax. It 

 is freely movable and has a large hood or saddle-shaped 



* The labrum differs from the other mouth -parts in not being composed of 

 a pair of body appendages ; it is simply a fold or flap of the skin of the head. 



