HYMENOPTERA 



32'; 



Shortly after the appearance of the head fold of the serosa, seg- 

 mentation of the embryo occurs. Twenty-one segments are found, 

 including an anal segment, or telson. Appendages appear on the 

 antennal, gnathal, and three thoracic segments, but none is found on the 

 abdomen. Prior to hatching, the antennal rudiments as well as those on 

 the thorax are reduced to epidermal thickenings. In the honeybee, as 

 mth other insects, the second maxillae fuse to form the labium. 



The nervous system forms in general as with insects of other orders. 

 In the abdomen 11 pairs of gangha develop, the last 3 pairs fusing into a 

 single one late in embryonic life. The neuroblasts divide unequally and 

 teloblastically, giving rise to several cells smaller than themselves. The 



ABC 



Fig. 283. — Apis. Diagrana of three stages in the development of the serosa (ser) in lateral 

 aspect. {From Nelson.) 



last-mentioned cells divide equally, the products becoming differentiated 

 to form the ganglion cells. The optic ganglia are not, however, produced 

 by the agency of neuroblasts but are formed as simple infoldings of the 

 ectoderm. Beginning at the anterior margin of the mandibular segment 

 and extending to the last segment of the trunk is a narrow median strip 

 of ectoderm, the median cord. In the intrasegmental region this con- 

 tributes the central portions of the ganglia; in the intersegmental regions, 

 it constitutes a series of thickenings of the epidermis. The supra- 

 esophageal commissure is formed, at least in part, from the median 

 ectoderm. The outer neurilemma is formed from cells that have the 

 same origin as the ganglion cells and that migrate to the external surface 

 of the brain and ventral cord. An inner neurilemma is lacking. 



The tracheal system is formed from 11 pairs of invaginations of the 

 lateral ectoderm. The first of these, situated on the second maxillary 

 segment, by the formation of four diverticula produces the anterior ends 



