38 ENTOMOLOGY. 



invaginations of the outer germ-layer. The development of 

 the salivary glands precedes that of the urinary tubes; which, 

 with the genital glands, are originally offshoots of the 

 primitive digestive tract. Finally the heart is formed. 



When the insect hatches, it either cuts its way through 

 the egg-shell by a temporary egg-cutter, as in the flea; or 

 the expansion of the head and thorax and the convulsive 

 movements of the body, as in the grasshopper, burst the 

 shell asunder. The serous membrane is left in the shell, 

 but in the case of grasshoppers the larva on hatching is 

 still enveloped in the amnion. This is soon cast as a thin 

 pellicle. 



Metamorphosis of Insects — The Larva. — The life of the 

 insect may be divided into four stages, represented by the 

 egg, the larva, the pupa, and the imago or adult. The 

 change from the egg to the adult is called a metamorphosis. 

 The larva (Latin larva, a mask) was so called because it 

 was thought to mask the form of the perfect insect. The 

 larva of a moth or butterfly is called a caterpillar; that of a 

 beetle, a grub; and that of a fly, a maggot. The larva? of 

 other groups have no distinctive common names. 



The principal change from the larval to the adult locust 

 or grasshopper is the acquisition of wings. In such insects, 

 then, as the Ortlioptera and Hemiptera, in which the adults 

 differ from the newly hatched larva mainly in the possession 

 of wings, metamorphosis is said to be incomplete. Its 

 development is direct. In the beetle, fly, butterfly, or bee, 

 the metamorphosis is complete ; the caterpillar, for example, 

 is a biting insect, is voracious, and leads a different life 

 from the quiescent, sleeping pupa, or chrysalis, which takes 

 no food; on the other hand, the imago, or butterfly, has 

 mandibles, which are rudimentary and incapable of biting, 

 while the maxilla, or ''tongue,'' which was rudimentary in . 

 the caterpillar, becomes now greatly developed; and the but- 

 terfly takes liquid food and but little of it, while its surround- 

 ings and mode of life are entirely changed with its acquisi- 

 tion of wings. Thus the butterfly leads three different 



