200 AN INTRODUCTION TO ENTOMOLOGY 



the antennae of the adult are developed from histoblasts within the 

 head and during the latter part of the larval life are folded like the 



bellows of a closed accor- 

 dian; at the close of this 

 period they become eva- 

 ginated, but the definitive 

 form is not assumed until 

 the emergence of the adult. 

 A similar course of devel- 

 / m^ opment of the antennae 

 takes place in vermiform 

 larvae (Fig. 218). 



d. THE DEVELOPMENT OF 



Fig. 218. — .Sagittal section through headof old THE MOUTH-PARTS 

 lan^a of 5/;«u/^'mw, showing forming imaginal 



head paits within. Ic, larval cuticula; id. Great differences exist 



imagi-al head-wall; la, larval antenna; ia, among insects with refer- 



imagmal antenna; ie, imagmal eye; Imd, ° 



larval mandible; imd, imaginal mandible; ence tO the comparative 



h:x larval maxilla; imx, imaginal maxilla; structure of their mouth- 

 Ih, larval labmm; tit, imagmal labium (rrom 



Kellogg). parts in their immature 



and adult instars. In 

 some insects the immature instars have essentially the same type of 

 mouth-parts as the adults ; in most of these cases, the mouth-parts are 

 of the biting types, but in the Homoptera and Heteroptera both 

 nymphs and adults have them fitted for sucking; in many other 

 insects, the mouth-parts of the larvae are fitted for biting while those of 

 adults are fitted for sucking; and in still others, as certain maggots, the 

 development of the mouth-parts is so retarded that they are first 

 functional in the adult insect. Correlated with these differences are 

 differences in the method of development of these organs. 



In those insects that have a gradual or incomplete metamorphosis 

 and in the Neuroptera, the Coleoptera, and the Hymenoptera in part, 

 the mouth-parts of the immature and adult instars are essentially of 

 the same type. In these insects the mouth-parts of each instar are 

 developed within the corresponding mouth-parts of the preceding 

 instar. At each ecdysis there is a molting of the old cuticula, a 

 stretching of the new one before it is hardened, a result of the growth 

 in size of the appendages, and sometimes an increase in the number 

 of the segments of the appendage. In a word, the mouth-parts of the 

 adult are developed from those of the immature instar in a compara- 

 tively direct manner. In some cases, however, where the mouth- 



