ODYNERUS. 215 



applies itself exactly against it, and it results that the superior 

 face of this segment meets the anterior face almost at a right 

 angle. This face is very short, sometimes four or five times wider 

 than long (sometimes, however, it has but little width). 



This structure is always the same, theoretically speaking, but it 

 becomes less and less marked in the extreme types. So, when 

 the first segment is pctiolate and narrow, it only fits itself against 

 the middle of the metathorax which offers then a sort of furrow 

 to receive it, while the lateral borders of the metathorax, remain- 

 ing disengaged, become rounded (0. arcuatus, tuber culatus). 

 When the form becomes very club-shaped and the abdomen sessile, 

 the first segment is wide, triangular, sessile (0. Antucensis, 

 villosus), and fits itself exactly upon the whole width of the meta- 

 thorax ; then this is flattened or excavated in its whole width, 

 and its lateral borders become quite trenchant. Finally, when the 

 first segment becomes rounded and sessile and when it no longer 

 fits itself as strongly against the metathorax, the borders of the 

 metathorax become blunted and rounded (0. vespiformis) and 

 one sees it produce a form quite similar to that of the Vespa. 



It is easy to understand from what precedes, that the abdomen 

 will have two forms according to the way in which one regards 

 it. 1 If it is fitted against the abdomen it will appear sessile, for 

 one sees only its very short superior face ; if it is open it will 

 appear more or less pediculate, for, the anterior face being tri- 

 angular, it forms, when it is let down, a sort of petiole having a 

 bell-shaped form, which would not happen if the anterior face 

 was more or less circular. 



One sees then that. the petiole is only the result of a deceitful 

 appearance of the abdomen placed in an abnormal position,-' very 

 different in this from that which the Eumenes offers, where the 

 petiole exists separately and cannot be disguised by any position 

 of the abdomen whatever. It proceeds from this fact that the 

 lengthened types of the Division Hypodynerus belong truly to 

 Odynerus, and not to Eumenes, as Spinola decided, nor to Nor- 

 tonia, as one might be tempted to judge at first sight. 



The Hypodynerus offers all degrees of prolongation of body 



1 Take, for example, the 0. labiatus or the 0. humeralis as average typos. 



2 The normal position of the abdomen is when it is applied against the 

 metathorax. 



