122 



ENTOMOLOGY 



sented by three pairs of fundaments, and in Thysanura and Collembola a 

 fourth pair is present to form the superlinguae (Fig. 196, si), already re- 

 ferred to. Next in order are the three pairs of thoracic legs (Fig. 195) 

 and then, in many cases, paired abdominal appendages (Figs. 195, 197), 

 indicating an ancestral myriopod-like condition; some of these abdom- 

 inal limbs disappear in the embryo but others develop into abdominal 



prolegs (Lepidoptera and Tenthredinidae) , 

 external genital organs (Orthoptera, Hy- 

 menoptera, etc.) or other structures. The 

 study of these embryonic fundaments sheds 

 much light upon the morphology of the ap- 

 pendages and the subject of segmentation. 



Two Types of Germ Bands. The germ 

 band described above belongs to the simple 

 overgrown type, exemplified in Clytra, in which 

 the germ band retains its original position 

 and the amnion and serosa arise by a pro- 

 cess of overgrowth (Figs. 191, 192), as dis- 

 tinguished from the invaginated type, illus- 

 trated in Odonata, in which the germ band 

 invaginates into the egg, as in Fig. 198, until 

 the ventral surface of the embryo becomes 

 turned around and faces the dorsal side of 

 the egg. In this event, a subsequent process 

 of revolution occurs, by means of which the 

 ventral surface of the embryo resumes its 

 original position (Fig. 199). 



Dorsal Closure. As was said, the germ 

 band forms the ventral part of the insect. 

 To complete the general form of the body the 

 margins of the germ band extend outward 

 and upward (Fig. 200) until they finally close 

 over to form the dorsal wall of the insect. 

 Besides this simple method, however, there 

 are several other ways in which the dorsal closure may be effected. 



Nervous System. Soon after gastrulation, the ventral nervous 

 system arises as a pair of parallel cords from cells (Fig. 201, n) which have 

 been derived by direct proliferation from those of the germ band, and are 

 therefore ectodermal in origin. This primitive double nerve cord be- 

 comes constricted at intervals into segments, or neuromeres, which cor- 



FIG. 197. Embryo of CEcan- 

 thus, ventral aspect . a , antenna ; 

 a l -a 6 , abdominal appendages; e, 

 end of abdomen; /, labrum; li, 

 left fundament of labium; Ip, 

 labial palpus; / x -/ 3 , thoracic legs; 

 m, mandible; mp, maxillary 

 palpus; mx, maxilla; p, pro- 

 cephalic lobe; pr, proctodaeum. 

 After AYERS. 



