INSECT A. 15 



calcareous shell would have been too heavy as well as 

 too stiff and unwieldy. The insect's armor is, there- 

 fore, composed wholly of the single substance chitine, 

 the lightest and toughest material excreted by the 

 epidermis, and capable of fully protecting, while not 

 embarrassing by its weight, the body of this essentially 

 aerial type. It is not a wonderful material of new and 

 mysterious origin, but probably an adaptation of the 

 outer part or horny cuticula, a layer similar to that 

 found in the Worms and Crustacea. 



Upon cutting away the prothorax, the upper or 

 tergal part of the mesothorax (PI. I., Fig. 3, //') bear- 

 ing the first pair of wings, is exposed.^ 



The mesothorax is divided into the scutum (/-) and 

 scutellum {ts-^. The sides consist of two parts, the epi- 

 sternum (PI. I., Fig. 7, //-) and epimerum (//i-'^), which 

 extend downward and backward. The sternum (PI. I., 

 Fig. 5,^') is a flat, stiff piece. In Guide No. VII., Fig. 

 8, D^ is a drawing of a typical crustacean ring in which 

 the epimeral plates are above the episternal. In the locust 

 the epimera have been crowded downward and backward, 

 so that they no longer lie above, but behind the episterna. 



The mesothorax is separated from the third and 

 last segment, the metathorax, with some difficulty, as 



^ Scholars will understand the structure of the locust far bet- 

 ter after they have separated the parts; and if several in the 

 class fasten these parts on cardboard, the preparations are 

 useful for reference, and valuable additions to the school cabi- 

 net. This process enables them to see, that, although each ring 

 of the thorax is apparently subdivided by sutures into two or 

 three rings, the whole region can be more readily divided into 

 three rings, and has but three pairs of appendages. 



