172 THE EMBRYOLOGY OF THE HONEY BEE 



growths of the last pair of tracheal sacs similarly unite and fuse 

 with each other beneath the hind-intestine to form the posterior 

 tracheal loop. 



The outgrowths forming the various parts of the tracheal 

 system are at first stout and thick-walled, but as they lengthen 

 they decrease rapidly in diameter until they attain their final 

 form and distribution. 



The chitinous lining of the tracheae is formed between Stages 

 XIII and XIV. 



In addition to the ten pairs of tracheal invaginations found in 

 the trunk, a pair of tracheal invaginations occur also on the first 

 maxillary segment. These make their appearance at the same 

 time Stage VIII as the other tracheal invaginations, and are 

 situated on the lateral surface of the anterior half of the segment, 

 above the base of the rudiment of the second maxilla of each side, 

 and only a few sections caudad of the boundary between the first 

 and second maxillary segments. The location on the segment of 

 this pair of tracheal invaginations is therefore the same as that of 

 the remaining ten. Like the other tracheal invaginations, those 

 of the second maxillary segment are at first shallow and cup- 

 shaped depressions. They soon lose this form and, developing 

 with surprising rapidity, become sacs with a narrow mouth, 

 directed obliquely caudad. The bottom of the sac quickly spreads 

 between the ectoderm and yolk (Fig. 67, Trlnv) and at the same 

 time sends off four branches or diverticula. One of these is 

 directed caudad, one dorsad and the other two cephalad. Of 

 these last two branches one passes above the mouth of the invagi- 

 nation and one below it. All of these changes take place between 

 Stages VIII and IX. Even before the close of Stage IX the 

 posterior branch of each of these tracheal sacs may be traced 

 back to a juncture with the anterior prolongation of the tracheal 

 sac of the corresponding side of the second trunk (thoracic) seg- 

 ment. The anterior ends of the tracheal trunks therefore owe 

 their origin to the tracheal invaginations of the second maxillary 

 segment. The mouths of the invaginations have by this time con- 

 tracted to narrow ducts while over them (Fig. 67) on each side is 

 seen a well marked fold of ectoderm obviously caused by the 

 formation of the tracheal sac, which raises the overlying ectoderm, 

 on account of the resistance offered by the yolk. This fold is 



