SLIME SLUGS, MYRIAPODS AND INSECTS 127 



THE EXTERNAL STRUCTURE or THE HONEY-BEE 



Body -wall. The body of a bee, which is a well-developed 

 insect type, is, let us note first of all, entirely covered by a firm 

 body wall or hardened skin. This body-wall is composed of 

 two layers, an inner, very thin and soft, cellular layer, the cells 

 being arranged side by side to form a skin membrane, only 

 one layer of cells in thickness, and an outer, thicker, non-cellu- 

 lar cuticular layer, composed of material secreted by the skin 

 cells and perhaps partly of the hardened outer ends of these 

 cells themselves. This thick, firm, colored cuticle is made up of 



FIG. 50. Honey-bee, Apis mellifica. (Nearly 3 times natural size.) 



successive fine laminae well fused together, and is composed 

 chiefly of a complex sustance called chitin. It is this chitinized 

 cuticle which gives the body-wall of insects its hardness, and 

 makes of it not only a firm protecting cover for the soft parts 

 within but also an exoskeleton to which the muscles are 

 attached. The chitinized cuticle, although extending continu- 

 ously over the body, is flexible in certain places, as between the 

 head and thorax, thorax and abdomen, various abdominal 

 segments and at the articulations of antennae, mouth-parts, 

 legs, wings and between the various antennal, mouth-part 

 and leg joints, etc. This is necessary, of course, for the 

 effective movement of all these parts. Such flexible places 

 in the cuticle are called sutures, while the firmer, fixed parts 

 are called sclerites. 



