328 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



of the main tracheal trunks, including the anterior tracheal commissure 

 and also the trachea supplying the head. The 10 pairs remaining are 

 situated on the second and third thoracic and the first eight abdominal 

 segments. These also form four diverticula each. The anterior and 

 posterior diverticula become united along each side of the embryo to 

 form the longitudinal tracheal trunks. The ventral diverticula fuse with 

 those of the opposite side of the same segments to form the tracheal 

 commissures. The dorsal diverticula form branches supplying the 

 dorsal region of the larva. The openings of the tracheal invaginations 

 remain as the spiracles. 



The tentorium is formed from two pairs of ectodermal invaginations. 

 The first pair of these is situated in front of the bases of the mandibles; 

 the second, behind the bases of the first maxillae. The invaginations 

 belonging to the first pair grow caudad and mesad ; those belonging to the 

 second pair, cephalad and mesad. All four meet in the median plane to 

 form a structure having the form of an X, extending across the head 

 capsule between the esophagus and the subesophageal ganglion. An 

 invagination situated immediately caudad of the base of the mandibular 

 rudiments produces the apodeme for the adductor muscle of the mandible. 



The openings of the silk glands appear first just behind the bases 

 of the second maxillae. As these appendages move toward each other 

 and fuse to form the labium, the gland apertures also approach each 

 other and unite into a common median orifice which is finally situated on 

 the labium in the larva. The invaginations themselves lengthen to 

 form long slender glandular tubes extending the length of the trunk. 



The oenocytes, of which there are eight sets, are situated on the first 

 eight abdominal segments in line with the openings of the tracheal 

 invaginations. They are produced by immigration of cells from localized 

 areas of the lateral ectoderm. 



The mesoderm, soon after its formation, becomes differentiated 

 laterally into two layers: an outer somatic and an inner splanchnic layer; 

 along the ventral mid-line it remains single-layered. Separate coelomic 

 sacs are not present, the somatic and splanchnic layers of each side being 

 continuous longitudinally throughout the trunk. At the lateral margins 

 of the mesoderm the two layers are continuous and in this region are 

 composed of long columnar cells. The median single-layered section of 

 the mesoderm breaks up into rounded blood cells. The somatic layer 

 forms the trunk muscles, both longitudinal and oblique, the pericardial 

 fat cells, and the dorsal diaphragm including the pericardial cells. The 

 splanchnic layers sends off from its dorsal border a mesal layer which 

 forms the muscular layer of the mid-gut. The remainder of this layer 

 is principally concerned in the formation of the two main divisions of 

 the fat body. This is exceptional, since in most insects the fat body 



