58 



THE ANATOMY OF THE HOXEY BEE. 



( lios. 20 and 21. 2) ai)i)areiitl_v helonoinnr to the sterna, it mig^ht be 

 supposed that the troehantins have fused with the hitter phites. 



The posterior part of the thoracic mass (fig. 21) consists of the 

 first abdominal terg-um (IT), which fits into the deeply concave pos- 

 terior edges of the nietathorax and forms the peduncle (Pd) that 

 carries the rest of the abdomen (fig. 32). It consists of a single large, 

 strongly convex sclerite (figs. 21 and 23 A, IT) bearing the first 

 abdominal spiracles laterally {ISp) and having its surface divided 

 into several areas by incomplete sutures. 



Many entomologists find it difficult to believe that this plate, which 

 so apparently belongs to the thorax, is really derived from the abdo- 

 men. But the proof is forthcoming from a number of sources. In 

 the first place, the thorax is complete without it and the abdomen is 

 incomplete without it, the latter having otherwise only nine seg- 

 ments. Again, if the plate is reckoned as a part of the thorax we 



WP 



WP. 



B 



Fig. 24. — A, uppor part of left mesopletinim of workor, oxtornal ; P., inner view of same. 



should have the anomaly of a thorax with three pairs of spiracles — 

 there being the normal two on each side situated, as they always are, 

 between the true thoracic segments. Furthermore, comparative anat- 

 omy shows us that in some of the sawflies (Tenthredinidse) the first 

 abdominal tergum, while separated by a wide membranous space 

 from the second, is not at all incorporated into the thorax. In a horn- 

 tail such as Sirex (Siricida?) the entire first abdominal segment is 

 fused to the posterior edge of the nietathorax and is only loosely 

 joined to the next abdominal segment by membrane. This insect 

 affords, therefore, a most complete demonstration of the transference 

 of this segment from the rest of the abdomen to the thorax. Finallyj 

 we have absolute proof of its abdominal origin based on a knowledge 

 of development, for it has been shown by Packard from a study of the 

 bumblebee that the first abdominal segment of the larva is trans- 

 ferred during the pupal metamorphosis to the thorax and forms the 



