ICHNEUMON-FLIES, PART 2\ ACAENITINAE 545 



Coleocentrus but more specialized, and the rest of the tribe comprises 

 a group of closely related genera with few similarities to Coleocentrus 

 and Mesoclistus. The differences between Coleocentrus and Mesoclistus 

 on the one hand, and the rest of the tribe on the other, are contrasted 

 in the first couplet of the key to the genera, below. It may be that 

 these two groups of genera are not actually related, but there does 

 not seem to be enough evidence to warrant putting them in separate 

 subfamilies. 



The distributional center of the subfamily is in the Old World 

 tropics. Among the genera occurring in the Nearctic region, Coleo- 

 centrus and Mesoclistus are Holarctic, Arotes is Holarctic and with 

 at least two species in Central America, Spilopteron is predominantly 

 eastern Palaearctic and eastern Nearctic, and Yezoceryx is Indo- 

 Australian and eastern Palaearctic, with one species in the eastern 

 Nearctic. No members of the tribe are Neotropic except for a few 

 species of Arotes hi Central America, and the general distribution of 

 the group gives the impression of origin and development in the Old 

 World and recent invasion of the New World by way of the Bering 

 Strait. 



Key to the Nearctic genera of Acaenitinae 



1. Front and middle tarsal claws simple; irons without a strong median longitu- 



dinal carina; areolet present except in a few species of Coleocentrus; trans- 

 verse ridges on apex of ovipositor distinct (fig. 331, i); fifth segment of hind 

 tarsus shorter than, or a little longer than the second segment .... 2 

 Front and middle tarsal claws with an acute accessory tooth; frons with 

 a strong median longitudinal carina; areolet absent; transverse ridges on 

 apex of ovipositor weak or absent (fig. 331, j); fifth segment of hind tarsus 

 conspicuously longer than second segment 3 



2. Clypeus with a median subapical tubercle; spiracle of propodeum 0.35 the 



distance from its base; first abdominal sternite not fused with its tergite; 

 combined 8th and 9th abdominal tergites elongate; epipleura of second and 

 third tergites poorly defined and not turned under; prepectal carina weak 



or absent 1. Coleocentrus (p. 546) 



Clypeus without a median subapical tubercle; spiracle of propodeum 0.45 the 

 distance from its base; first abdominal sternite not fused with its tergite; 

 combined 8th and 9th abdominal tergites short; epipleura of second and 

 third tergites well defined and turned under; prepectal carina strong. 



2. Mesoclistus (p. 560) 



3. Ventral swelling of first sternite without hairs or rarely with one or a very few 



hairs, intercubitus opposite or basad of second recurrent; exposed part of 

 labrum about 0.3 as long as wide; second lateral area of propodeum, when 

 defined, subtriangular; front wing in Nearctic species without an apical 



dark spot 5. Yezoceryx (p. 578) 



Ventral swelling of first sternite with numerous long erect hairs; intercubitus 

 opposite or distad of second recurrent; exposed part of labrum about 0.6 as 

 long as wide; second lateral area of propodeum subtrapezoidal; front wing in 

 Nearctic species with an apical dark spot 4 



