WALTER] CLASPING ORGANS IN HYMENOPTERA 69 



ular and variable, not only in different species, but also within the 

 same species. In the forms with a single row the distal hooks usually 

 lie in a straight line parallel to the margin of the wing ; sometimes, 

 as in many Ichneumonids, they form a gentle curve open towards the 

 anterior margin of the wing. The intervals between the individual 

 hooks differ greatly. In the genera Bombus and Xylocopa they are 

 very close together, comb-like, in contact with each other at the base ; 

 in the other Anthophilids and the remaining families they are 50 to 

 200 microns apart. The strongest and largest hooks generally stand 

 farthest apart, so in the Vespids the first ones and in the Ichneu- 

 monids the middle ones, but no uniformity can be recognized in this 

 respect. The number of the distal hooks in the different species 

 varies exceedingly, from two (Proctotrupes) to about 50 (Sirex). 

 Even among different individuals of the same species the number of 

 hooks is very variable. This, as well as the availability of the num- 

 ber and arrangement of the hooks for systematic purposes, will be 

 dealt with below in the systematic part. 



The distal hooks have the shape of rather flat bands. Only at the 

 base, and just above it, the cross-sections are circular (pi. x, fig, 31) ; 

 towards the tip, they soon become elongated, elliptical. The shape 

 of a distal hook can be readily made out by the aid of fig. 10 on 

 plate viii (surface view T ) and fig. 22 on plate ix (transverse section). 

 The hook describes an arc, open towards the wing surface. This arc 

 does not, however, extend in a plane, but describes half to three- 

 quarters of a spiral, the axis of which is parallel to the costal vein. 

 The basal portion of the hook is inclined towards the costal vein. 

 With this it incloses an angle of 40 to 60 degrees and with the wing 

 surface an angle of 90 to 120 degrees. When the latter approaches 

 120 degrees the central parts of the hooks project beyond the anterior 

 margin of the wing, particularly when this margin is strongly turned 

 down, as in the flower-wasps (pi. vn, figs. 7, 9 ; pi. vm, figs. 11, 12, 13). 

 The stouter hooks of the series are particularly strongly bent. The 

 distance between their ends and the wing surface is less than the space 

 taken up by the corresponding part of the groove of the fore wing 

 (made clearer by lines in pi. ix, fig. 22). As the transverse sections 

 (pi. ix, figs. 19-25), show, the hooks are uniformly curved and, how- 

 ever much twisted, never, as the surface view might lead one to 

 assume, abruptly bent at any point. If a hook were straightened 

 out, its form would be lanceolate. It is broadest in the middle and 

 narrows towards the apex, finally tapering to a point. A tubular 

 cavity extending the entire length of the hook is always clearly dis- 

 cernible. Conforming to the shape of the hook, this cavity, in trans- 



