352 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



the structure has migrated mesad and sHghtly caudad, coming to He in 

 front of the prothoracic legs, and has lost its attachment with the surface. 

 Later, its cells can no longer be distinguished from those of the adjacent 

 mesoderm. It seems probable that it is the rudimentary cervical gland 

 which is so highly developed in the caterpillars of Schizura concinna, 

 Dicranura vinula, and other notodontids, though vestigial in many 

 other lepidopterous larvae. In the silkworm, according to Toyama, it 

 persists in the larva as branched glands in the first thoracic segment. 



In lepidopterous larvae the first thoracic segment bears a distinct 

 spiracle, and in some cases a vestigial one may be demonstrated between 

 segments two and three. In the embiyo at 65 hours on the anterior 

 margins of thoracic segments two and three the invaginations for the 

 tracheae have advanced sufficiently to permit the recognition of a short 

 anterior and a posterior branch. Their walls are single-layered and 

 indistinguishable from the adjacent ectoderm (Fig. 3085, ^r. i). There is 

 no indication of a tracheal invagination on the prothorax. Invagina- 

 tions for the abdominal tracheae are equally advanced at this stage, 

 located on the anterior part of segments one to eight, the last one being 

 the largest. The ninth and tenth segments, as well as the telson, are 

 not provided with tracheal invaginations. By the seventy-seventh hour 

 the most anterior spiracle has migrated on to the prothorax; the second" 

 one is no longer apparent. 



The oenocytes, of ectodermal origin and associated with the tracheal 

 invaginations, lie in clusters ventrad of the spiracles. In the 84-hour 

 stage they resemble the fat cells in size but are not vacuolate; conse- 

 quently, they stain more deeply. They are conspicuous in the newly 

 hatched larva. 



The trichogen cells are by far the largest and most conspicuous 

 ectodermal cells in the embryo. They cannot well be confused with the 

 oenocytes, being larger and more dorsad in position. At 59 hours they 

 are not yet differentiated, but at 65 hours they may be found in both 

 thoracic and abdominal segments (Fig. SOSB,trich). They are so large 

 that they extend far below the level of the neighboring ectodermal cells. 

 In some sections, as in the one figured, they are cut tangentially, so that 

 they appear to be located among the underlying mesodermal cells. 

 The seta develops shortly before hatching. 



Shortly after the beginning of body segmentation, the rudiments of 

 the appendages appear. At 45 hours the mouth parts and the thoracic 

 legs are as long as broad, with the lumen distinctly marked. At 65 

 hours, mouth parts and thoracic legs show distinct segmentation. 

 Twelve hours later, the thoracic legs are three-segmented; the maxillae 

 and labium, two-segmented. The labrum, at first simple, in the 65-hour- 

 old embryo becomes bilobed. The future abdominal prolegs, which at 



