154 



ENTOMOLOGY 



unite to form the main lateral trunks, from which arise the 

 countless branches of the tracheal system. 



Mesoderm. From the inner layer which was derived 

 from the germ band by gastrulation (Figs. 189-191) are 

 formed the important germ layers known as mesoderm and en- 



FIG. 199. 



Diagrammatic transverse sections to illustrate formation of dorsal wall in a beetle, 

 Lcptinotarsa. a, amnion (breaking up in C) ; g, germ band; s, serosa. After 

 WHEELER, from the Journal of Morphology. 



toderm. Most of the layer becomes mesoderm, and this splits 

 on either side into chambers, or ccelom sacs (Fig. 200, c), a pair 

 to each segment. In Orthoptera these coelom sacs are large 

 and extend into the embryonic appendages, but in Coleoptera, 

 Lepidoptera and Hymenoptera they are small. These sacs 



FIG. 200. 



Tansverse section of germ layers of Clytra. c, crelom sac; n, 

 neuroblasts (primitive nervous cells). After LECAILLON. 



may share in the formation of the definite body-cavity, though 

 the last arises independently, from spaces that form between 

 the yolk and the mesodermal tissues. From the coelom sacs 

 develop the muscles, fat-body, dorsal vessel, blood corpuscles, 

 ovaries and testes; the external sexual organs, however, as 

 well as the vagina and ejaculatory duct, are ectodermal in 

 origin. 



