21 (18) Apical margin of second segment not red. 

 21a(21b) Basal abdominal segment nearly entirely red. 



O. microdemas P. 

 21b(21a) Basal abdominal segment with an apical red band, sometimes 

 interrupted. O. monobiiis var. 



Propodeum black. 



Propodeum rugose. O. erythrostacies P. 



Propodeum smooth or punctured. 



Propodeum smooth, impunctate or nearly so. O. montivagus P. 

 Propodeum densely punctured in the concavity. 



O. monobius P. 

 Prothorax not red-marked. 

 Second abdominal segment with a red apical band. 



O. sandwichensis , de S. {O. rubritinctus, Sm) 

 Second segment without a red apical band. O. petrobius, P. 



Abdomen without red markings, and the mesopleura without a 

 red spot beneath the tegulae. 



31 (40) Costae of second ventral segment of abdomen largely effaced 



and indistinct, or altogether wanting, the transverse 

 sulcature often smooth and shining, the abdomen never with 

 two yellow bands. 



32 (33) A median red frontal spot on the face, and the prothorax always 



red-marked, second segment of abdomen above simple, not 

 at all strongly convex or raised from the base, beneath 

 greath^ and abruptly raised behind the sulcature. 



O. deinogaster, P. 



33 (32) No red frontal spot on face, the prothorax at most indistinctly 



reddish along the hind margin, second segment above 



always more or less strongly raised from the base or strongly 



convex in profile; beneath of various forms. 

 Second ventral segment raised from the sulcature to a height 



very rauch greater than that of its basal portion. 

 Propodeum smooth, shining; apex of clypeus red 



O. purpurifer, P. 

 Propodeum not smooth and shining, but punctate or subrugose 



apex of clypeus not red. O. instabilis, P. 



Second ventral segment of abdomen (seen in profile) raised 



behind the sulcature only to about the height of its basal 



portion or a little higher. 

 Second ventral segment with a rather wide depression behind 



the sulcature. O. ecostatus, P. 



Second ventral segment with a narrow depression. 



O. laevisulcatus, P. 

 Transverse sulcature of second ventral segment with distinct 



costae, never smooth and shining; if the costae are more or 



less obsolete the abdomen has two yellow bands. 

 41 (42) A median frontal spot and the tegulae always, other markings 



sometimes red. O. camelinus, P. 



