ICHNEUMON-FLIES — gelinae: mesostenini 435 



fauna, as follows: 1. The Transfuga group is Holarctic in distribution, 

 including in our fauna the species albinotatus, liogaster, temporalis, 

 melanurus, gracilis, clitellatus, sicarius, and thoracicus. These have 

 the clypeus rather narrow, second trochanter of hind leg short, and 

 punctures on second tergite of female rather dense (except in liogaster 

 and temporalis). 2. The Longicaudis group is mosth^ Mexican and 

 Central American, with three species rangmg northward into the 

 United States. These are eisenii, angustus, and longicaudis. This 

 group is characterized by the unusually wide clypeus, usuall}^ more 

 or less elongate second trochanter of hind leg, and punctures on 

 second tergite of female very sparse. It would be possible to recognize 

 the group as a distinct genus (Derocentrus) , but because of unresolved 

 questions about the limits of various species groups of the tropics in 

 both hemispheres, it is included in Mesostenus. 



Previously the genus Mesostenus had very broad limits, containing 

 nearh^ all the species of the tribe with the areolet small. About 60 

 years ago, Cameron started the trend of breaking "Mesostenus^' 

 into a large number of genera. Later, Cushman (1929, Proc. U.S. 

 Nat. Mus., vol. 74, art. 16, p. 1) expressed the conviction that a small 

 areolet is not a natural character in the tribe Mesostenini, and that 

 even the grouping of genera on the size of the areolet resulted often 

 in artificial assemblages. Subsequent work has fully confirmed this 

 view. 



The hosts of Mesostenus are the cocoons of various Lepidoptera, 

 usually species with rather thin cocoons that are covered with frass, 

 debris, or occurring in short tunnels. The parasite cocoon is spun 

 within the host cocoon. It is thin, whitish or light brown, and some- 

 what translucent. 



Keys to the Nearctic species of Mesostenus 



MALES 



(The male of melanurus is unknown.) 



1. Hind coxa entirely black; segments 2-4 or 3-4 of hind tarsus entirely white 



(except sometimes in the European subspecies of albinotatus) 2 



Hind coxa fulvous or mostly fulvous, or rarely black, in which case segments 

 2-4 of hind tarsus are brown 4 



2. Collar of pronotum white; prepectal carina distinct and strong up to subtegular 



ridge; temple moderately convex .... 3. temporalis, new species 



Collar of pronotum black; prepectal carina obsolete dorsally, not distinct up to 



subtegular ridge; temple only weakly convex 3 



3. Tergites 6 and 7 black 1. alliinotatus Gravenhorst 



Tergites 6 and 7 ferruginous 2. liogaster, new species 



4. Metapleurum and propodeum black marked with white ; second hind trochanter 



0.35 to 1.2 as long as first hind trochanter (second trochanter measured at its 

 shortest place on upper edge, beginning at pointed apex of first trochanter; 



