98 Beekeeping 



Later embryonic development. 



In a later stage (Fig. 51, c), the embryonic band on the 

 ventral side of the egg has widened and in the next stage 

 here illustrated (Fig. 51, d) the band completely envelops 

 the egg. In the stage shown in Fig. 51, c the mouth {Mth) 

 and anus (An) have appeared as pits. These continue to 

 grow into the egg and ultimately join with certain cells on the 

 interior to complete the alimentary canal. The portions 

 formed by the two invaginations from the outside are the 

 fore- and hind-intestine, while the part arising from the 

 interior is the mid-intestine. The Malpighian tubes {MT), 

 the excretory organs, arise as outgrowths from the anterior 

 end of the hind-intestine. The pits {S-p) which are the rudi- 

 ments of the spiracles, deepen and send branches forward, 

 backward and downward to meet corresponding outgrowths 

 from other pits, finally forming the tracheal trunks with 

 their commissures and branches. The silk glands (SlkGl), 

 which function only in the larva, project backward as long 

 tubes. 



Segmentation. 



The most striking feature of the late embryo is the fact 

 that it is constricted into a series of segments (metameres 

 or somites) which are plainly recognized in the larva. These 

 segments are characteristic of all insects and part of the 

 metameres of the abdomen are still plainly marked off in 

 the adult. From the fact that segmentation is recognizable 

 in various parts of adult insects and is present in insect 

 embryos, it is assumed that this form is characteristic of the 

 primitive organism from which all insects have arisen. The 

 typical appendages are arranged in pairs on the segments 

 but in their later development these appendages are modified 

 according to their fate. The stigmata and the gangUa of 

 the nervous system are also arranged segmentally at first, 

 but this primitive arrangement is later partially lost. The 

 segmentation of various species studied does not wholly 

 agree, but it is usually assumed that the first six or seven 



