NO. 2 



HONEY BEE — SNODGRASS 



8l 



Such insects have little mobility of the abdomen on the thorax, even 



though the two parts of the body may be separated by a constriction. 



A comparison of the structural relation between the thorax and 



the abdomen in clistogastrous Hymenoptera (fig. 26 B) with that in 



N3 PN3 



-14-9 



Fig. 26. — Relation between the propodeum and the abdominal petiole; lateral 

 muscles of an abdominal segment. 



A, diagram of thoracico-abdominal relation in a generalized winged insect : 

 the dorsal intersegmental muscles of metathorax (ds) are wing motors and 

 do not move the abdomen. B, specialized condition in clistogastrous Hymenop- 

 tera : dorsal and ventral muscles (d, v) of first abdominal segment (pro- 

 podeum) become effective motors of rest of abdomen. C, usual structure in 

 higher winged insects : abdomen but little movable on thorax. D, anterior end- 

 view of abdominal segment IV oi drone, showing mechanism of dorsoventral 

 expansion by contraction of reversed anterior lateral muscles. E, right half 

 of abdominal sternum IV of drone, with sternal and lateral muscles. 



a, anterior tergal apodeme; b, anterior sternal apodeme; c, lateral sternal 

 apodeme; d, dorsal intersegmental muscles; v, ventral intersegmental muscles. 



either a generalized winged insect (A) or a more specialized form (C) 

 will show clearly the advantage derived by the Clistogastra from hav- 

 ing the first abdominal segment solidly joined to the thorax, and a 

 constricted point of movement established between the first and 



