INSECTS 205 



respects. The chief changes with many insects occur in the 

 last two moults. As insects have no bones or inside skeleton 

 it is usually considered that the outside skin is really a 

 skeleton. In this sense they may be said to have an outside 

 skeleton. Muscles of the insect are attached to this skeleton; 

 and sometimes the texture is very hard and horn-like. 



Complete and Incomplete Changes. — All insects may be 

 classified on the basis of the amount of change occurring 

 in their structure during the last two moults. (1) Those 

 making the least change in structure and appearance during 

 the last two moults are said to have incomplete changes. 

 Examples of this are the true bugs and the grasshoppers. 

 (2) Those making very great changes in structure and out- 

 side appearance at the times of the last two moults are said 

 to have complete changes; this is seen in the wasps, bees, 

 butterflies, moths, beetles, flies, and mosquitoes. 



Four Stages in the Life. — Insects with complete changes 

 may be said to have four stages in their lives: (1) Egg stage, 

 (2) larva stage, (3) pupa stage, (4) adult stage (Fig. 123). 



The larva is the growing stage. It is during this stage 

 that most of the eating is done ; and with some insects enough 

 is eaten during this stage to last them through the other 

 stages of life. Some forms of larva will consume several 

 times their own weight of food in a single day. 



The pupa is the resting or sleeping stage for many insects. 

 The larva has enclosed itself in a case of some kind; some- 

 times it spins a silken covering called a cocoon, as in the case 

 of the silk worm and others. During this quiet resting stage 

 the insects go through a great many changes. The digestive 

 organs are very much changed; wings are grown and ready 

 to unfold; legs are present instead of mere claws or pads; 

 compound eyes are developed; often a very different mouth 

 is formed; antennae or feelers are grown upon the head. The 

 pupa stage sometimes lasts over winter, and in other insects 

 or other broods it may last only a few weeks. 



