INTRODUCTION. ix 



together during flight the fore and hind wings. Examined with a 

 good lens, the fore wing is seen to have a fold along its posterior 

 margin, while on the anterior or costal margin of the hind wing a 

 row of hook-shaped bristles or hairs can be easily detected. When 

 the ^^•iugs are expanded these hooks, or hamuli as they are called, 

 catch on firmly to the fold in the fore wing, and the fore and hind 

 wing on each side are enabled to act in concert, having the appear- 

 ance and all the firmness of a single expanded membrane. The 

 names of the uervures and cells in the wings are included in the 

 explanation nnder figures 10 and 11. The number and disposition 

 of these nervures and cells are of considerable importance in the 

 classification of the Hymenoptera. 



The abdomen (fig. 12) is attached to the thorax direct or through 

 the median segment, the articulation being either broad and not 

 very flexible, as in the lower forms, or narrow and possessed of 

 the utmost flexibility, as in the bulk of the families into which the 

 Hymenoptera are divided. So remarkable is this flexibility, that a 

 bee or a wasp is not only able to twist its abdomen and sting in 

 all directions posteriorly, but many species can, and do, double the 

 abdomen underneath the thorax, and protrude the sting in front 

 of the head. 



As a rule the abdomen, in the higher Hymenoptera, consists of 

 six visible segments in the female and of seven segments in the 

 male, each segment formed by a dorsal and a ventral plate (fig. 12, 

 a-a^, h-b"). 



The epipygium, or dorsal portion of the apical abdominal 

 segment, and the lujpopygium, or ventral portion of the same, 

 together form what is called the pygidiam. Occasionally the term 

 " pygidial area " is used, and refers to a portion of the epipygium 

 which in certain forms is margined and llat, not convex *. 



The Hymenoptera fall naturally into two great divisions or 

 suborders, characterized by the difference in the articulation of the 

 abdomen with the thorax referred to above, and by more profound 

 difl'erences in the form and life-history of the larvae. The external 

 difference in the imago may be tabulated as follows : — 



I. Suborder SESSILIVENTEES. 



Median segment (1st true abdominal segment) not completely 

 amalgamated with the thorax, the joint between the thorax and 

 abdomen broad. 



* For an accurate and full account of the external and internal anatomy of 

 the Hymenoptera, see the ' Cambridge Natural History,' Vol. V., Part Insecta, 

 by Dr. D. Sharp. 



