262 ELEMENTARY ZOOLOGY. 



272. Development. Some of the arthropods hatch at 

 once into the adult form, but the metamorphosis is much 

 more common. The egg has considerable food to start 

 with, and it is usually laid in positions where the young 

 can get food as soon as it is hatched. The first or larval 

 stage in insects is worm-like, with legs poorly developed. 

 It is at this stage that the insects show their kinship with 

 the annulata most clearly. During this stage the animals 

 eat enormously, grow, and store up fat. It is in this 

 stage, therefore, that they are most injurious to human 

 interests. Grubs, maggots, caterpillars, army worms, 

 tomato-worms, measuring-worms, wiggle-tails, are some 

 of the names applied to the various larvae of insects. The 

 larval stage may continue for only a few days or for several 

 years. During a period of rest, known as the pupa 

 stage, the organs of the larva are resorbed and the new 

 organs of the adult are developed from the stored mate- 

 rials in the body. Often this change is profound, as 

 from a crawling, leaf-eating caterpillar to a nectar-suck- 

 ing moth with large wings. The adult is known as an 

 imago. The adult life may be only long enough to deposit 

 eggs, as in the May-flies ;-or may be several years, as in the 

 case of some queens among the social insects. 



The metamorphosis in some of the Crustacea (Figs. 

 82 and 83) is quite as striking as among the insects, but 

 it follows a somewhat different course. There is no rest- 

 ing stage ; but the change from larva to adult is made by 

 a series of smaller changes that occur during successive 

 moultings. In moulting the skeleton is separated gradually 

 from the skin that has produced it. The animal bursts 

 it at some point, and gradually frees itself by crawling 

 out the rent. It is soft and helpless in this stage and 

 usually hides. It grows rapidly, may change its shape 



