84 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IO3 



begins in the metathorax. The larger dorsal channel gives passage 

 to the tracheal trunks, the oesophagus, and the aorta ; the ventral 

 channel contains the nerve cords. 



VII. THE ABDOMEN 



Structural changes in an animal that bring about conditions in 

 conflict with standardized nomenclature create embarrassing situa- 

 tions for the descriptive anatomist. The transfer of the fourth body 

 segment to the thorax in the Clistogastra, for example, leaves us 

 without a suitable term for the third region of the body in this 

 group of Hymenoptera, since the body region called the "abdomen" 

 in all other insects includes the transposed segment of the Clisto- 

 gastra. The clistogastrous abdomen has been termed the postabdomen, 

 and the gastcr, but neither name is satisfactory ; the first is used also 

 for the last four or five segments in certain insects in which the 

 posterior genital region of the abdomen is structurally differentiated 

 from the anterior visceral region, and the word "gaster" ordinarily 

 refers to the stomach. The most practical solution of the difficulty 

 for hymenopterists is to use the term abdomen in a functional rather 

 than a morphological sense for the third body region of the insect. 

 The abdomen in the Clistogastra, therefore, is not entirely homologous 

 with that of the Chalastogastra, and to keep in view the identity of 

 the segments in the two groups the primary abdominal segments may 

 be designated / to X regardless of their associations. Segment / in 

 the Clistogastra is the thoracic propodeum, segment // is the segment 

 of the abdominal petiole, segment X is the anus-bearing lobe or tube 

 known as the proctiger. In many Hymenoptera the true anus lies 

 within a terminal invagination of the proctiger, which possibly 

 represents segment XI. 



General structure of the abdomen. — The segments of the pregenital 

 region of the abdomen in the honey bee have well-developed tergal 

 and sternal plates, but there are no lateral, or "pleural," sclerites, and 

 the spiracles lie in the sides of the terga (fig. 27 A). The tergal 

 plates, except the first, are provided each with a large anterior tergal 

 apodeme on each side (figs. 26 D, 28 A, 29 A, a); the sternal plates 

 have each a pair of anterior sternal apodeuies (figs. 26 D, 28 A, B, 

 29 A, b), and on each side a long lateral sternal apodeme (c). The 

 sternal plates of segments /// to VI widely underlap each the sternum 

 of the following segment, forming pockets which in the worker con- 

 ceal the areas of the wax glands on sterna IV to VII. 



