FIELD BOOK OF PONDS AND STREAMS 



The outside of an insect. — The skeleton of an insect entirely 

 covers the outside of its body. It is chitinous or horny, thin 

 and flexible at the joints, and even thinner over the tips of the 



,^'' 



►*) 



Fig. 138. — Diagram of stonefly nymph with legs 

 of one side removed to show their segments: i, head 

 with simple and compound eyes; 2, prothorax; 3, 

 mesothorax; and 4, metathorax with developing 

 wingpads; 5, abdomen; 6, legs removed (a, coxa; h, 

 trochanter; c^ femur; <f, tibia; e, tarsi and claws). 



palpi and the antennae or ' ' feelers. " It is made of the hardened 

 fluid secretion of the skin cells beneath it and is molted or 

 cast off at intervals, a new one being formed beneath it. 

 Every hair and spine and claw is a part of it and is molted 

 with it. 



All insects are built upon the same plan. This plan can 

 be easily traced in the stonefly already selected to show a 

 simple type of insect structure (Needham, Bibliography, p. 

 416). The body of an insect is divided into three main parts, 

 head, thorax, and abdomen (Fig. 138). 



On the head are the simple and the compound eyes, the 

 antennae, and the mouth parts. The simple eyes are small and 

 inconspicuous and it is only the compound eyes which are 

 usually seen. Antennae vary greatly in length even in dif- 

 ferent stages in the growth of the same insect, as in the nymph 



180 



