MESODERMAL DERIVATIVES 115 



of the meseiichyme-like mesodermal tissue in the Coleoptera, Diptera, 

 Hymenoptera, etc. With the exception of the ventral longitudinals 

 most body muscles arise from the lateral coelomic walls. The ventral 

 longitudinal muscles arise where the somatic and splanchnic layers meet. 

 These muscles are the first to develop, and in the Orthoptera according 

 to Heymons (18956) they develop from special coelomic diverticula. 

 From this part the intersegmental transverse muscles also arise. Muscles 

 of the appendages develop from the coelomic sacs which have entered 

 the ectodermal evaginations in the Orthoptera and related forms, in the 

 higher groups from the undifferentiated mesoderm. 



The musculature of the stomodaeum and proctodaeum in general 

 arises from mesoderm that has entered the primary head and anal seg- 

 ments independent of the coelomic sacs. The mid-gut muscles, as has 

 already been stated, form from the splanchnic mesoderm — in Forficula 

 by delamination, in Blattella by migration of cells. 



The history of the derivatives of the coelomic sacs of the various 

 segments has been studied in a number of insects, especially among the 

 Orthoptera by Heymons (18956) and more recently in Carausius morosus 

 by Wiesmann (1926) and in Locusta migratoria by Roonwal (1937) as 

 well as more briefly in other insects by several writers. From the labral 

 sacs the labral musculature arises in Locusta. In Carausius the pre- 

 antennal sac gives rise to the stomodaeal musculature. The relatively 

 large antennal sacs in Locusta give rise not only to the anterior and pos- 

 terior portions of the cephalic aorta but also to the investment of the 

 pharyngeal ganglion and of the corpora allata, the fatty tissue, and the 

 antennary muscles. A part of the mandibular sacs of Locusta and 

 Carausius gives rise to mandibular muscles; an anterior pouch of these 

 sacs forms the antennary muscles in Carausius. Maxillary muscles 

 in the Orthoptera generally develop from the first maxillary sacs. In 

 Carausius the median diverticulum of the second maxillary sac forms 

 the longitudinal ventral muscle strand; the ventral diverticulum, the 

 maxillary muscles; and the laterodorsal diverticulum, the longitudinal 

 muscles, the tortion muscles of the head, splanchnic muscles, and some 

 nonmuscle tissue. 



The growth of all coelomic sacs from the second maxillary to the sac 

 in the caudal extremity of the body ends in the breaking down of the 

 partition walls that separate the coelomic cavities, with the result that a 

 continuous mesodermic tube is formed on each side of the body in the 

 chilopods (Heymons, 1901), Orthoptera (Heymons, 1895a, etal.), Coleop- 

 tera (Lecaillon, 1898), Hymenoptera, and other forms. The diversity 

 among the different orders of the Eutracheata of muscle development 

 in thorax and abdomen is so great that generalizations will not be 

 attempted here. Abstracts of the detailed accounts of the development 



