DEVELOPMENT OP INSECTS. 



331 



%. 304.— Embryo of Sphinx 

 Tnuch more advanced, h, heart ; 

 ■fir, gangrli'in ; i, intestine ; tti, 

 rudinienta y muscular bands mn- 

 jiinir to ihuhearr ; s, stigma and 

 beginning of a trachea {t) ; d.a. 

 gland. This and JPigs. 293, 295 

 after Kowaleveky. 



^1 



<lL 



appendages bud out from the under side of the primitive 

 band, and antennse, jaws, legs, ovipositor (or sting), and the 

 abdominal feet of caterpillars are at 

 first all alike. Soon the appendages 

 begin to assume the form seen in 

 the larva, and just before the insect 

 hatches the last steps in tiie elabora- 

 tion of the larval form are taken. 



As to the development of the in- 

 ternal organs, the ner- 

 vous system first origi- 

 nates ; the alimentary 

 canal is next formed ; 

 and at about this time 

 the stigmata and air- 

 tubes arise as invagina- 

 tions of the outer germ- 

 layer. The development 

 of the salivary glands precedes that of the uri- 

 nary 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-cut- 

 ter, as in the flea, or the expansion of the 

 iiead 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 

 Jarva on hatching is still enveloped in the am- 

 nion. This is soon cast as a thin pellicle. 



The principal change from the larval to the 

 adult locust or grasshopper is the acquisition of 

 wings. In such insects, then, as the Orthoptera 

 and Hemiptera, in which the adults differ from 

 the newly hatched larva mainly in the posses- 

 sion of wings, metamorphosis is said to be in- 

 complete. In the beetle, butterfly, or bee, the metamorphosis 

 is complete j the caterpillar, for example, is a biting insect. 



Pig. 295.- 

 Primitive 

 band or germ 

 of a Spbinx 

 moth, wjtli^e 

 eegmeiits "in- 

 dicated, and 

 tlieir rudimen- 

 tary append- 

 ages, c, upper 

 lip ; at, anten- 

 nae ; md, man- 

 d i b ) e 8 ; mx, 

 mx', first and 

 second maxil- 

 lae ; I, I', V 

 legs ; al, abd» 

 zninal legs. 



