114 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



Graber, Heider, Heymons, and other workers, on the other hand, main- 

 tain that only among some highly specialized insects as in the honeybee 

 is a tubular coelom developed without first forming the metameric 

 coelomic sacs. The coelomic sacs in the Orthoptera are closely correlated 

 with the formation of the appendages, the first anlage of the sac being 

 fully restricted to the cavity in the appendage. Formed in a similar 

 manner are the sacs in Sialis lutaria and Eutermes (Strindberg, 1915rf, 

 19136) and in the Ephemeridae (Heymons, 1896a). 



Among more highly specialized insects the coelomic cavity does not 

 extend into the appendages. The cavity of the appendages is filled with 

 a compact, irregularly disposed mesoderm in the Coleoptera, the coelomic 

 cavities being restricted to the laterodorsal part of the germ band. 

 Similarly in the Lepidoptera, Homoptera, Heteroptera, and Hymenoptera 

 the coelomic cavities are restricted to the sides of the germ band. In 

 Pieris according to Eastham (1930) they are circular in cross section but 

 fusiform longitudinally, extending far into the intersegmental region, 

 only just failing to be continuous from one segment to another. In 

 the honeybee no coelomic sacs, as such, are distinguishable, the two 

 flattened mesodermal tubes on each side being continuous longitudinally 

 according to Nelson. Their splanchnic and somatic layers are well 

 defined and thick. In Chalicodoma, the mason bee, the thin-walled 

 portion of the mesoderm is distinctly divided into segmentally arranged 

 sacs, which however are so flattened dorsoventrally that visceral and 

 somatic layers are in contact with one another, except at their lateral 

 margins. In the margins in a longitudinal direction the sacs run into 

 each other, forming a tube. 



Still more modified are the conditions among certain Homoptera 

 (aphids) and Heteroptera (Pyrrhocoris) where Will (1889) and Seidel 

 (1924) found that the sacs remain open toward the yolk. Lepisma, 

 among the Apterygota, according to Heymons (18976), has large sacs 

 formed in a manner similar to those of the Orthoptera. 



THE MUSCULATURE 



The muscles of the body and appendages arise from the somatic 

 layer of the mesoderm; those of the ahmentary canal, from the splanchnic 

 layer. In some forms of the eutracheates such as the Orthoptera in 

 which the coelomic cavities are large, the muscles develop as tubular 

 evaginations which constrict from the somatic layer and sooner or later 

 lose their lumen. With the more highly specialized insects, on the other 

 hand, in which the coelomic cavities are narrow and poorly developed or 

 are wholly lacking, the muscles arise from isolated migrant cells from the 

 coelomic wall as in Forficula, or they develop directly from the parts 



