HYMENOPTERA 



247 



and the abdomen is joined to the thorax by a slender petiole, or 



stalk, as in the fam- 

 ilies named below, 

 from which they 

 are distinguished by 



lacking the dark 

 spot, or stigma, 

 toward the end of 

 the anterior margin 

 of the fore-wings. 

 The}' have short, 

 thick bodies, and 

 the abdomen is com- 

 monly compressed, 

 so that the segments 

 appear to be more 

 or less telescoped. 

 The mossy rose-gall 

 (Rhodites rosae\ 

 which forms a large, 



FlG. 389. Mossy rose-gall (Rhodites rosae) 

 (After Comstock) 



mosslike gall on the stems of roses, and the 

 spongy oak-apple (Amphibolips spongificd) y 

 which looks like a puff-ball on the leaves and 

 stems of oaks, are well-known examples. The 

 adult flies may be easily reared by removing 

 the galls from the plants when fully matured 

 and placing them in any suitable receptacles. 

 Only a few species are of economic importance 

 on cultivated crops, among which may be men- 

 tioned the pithy blackberry-gall (DiastropJius 

 nebulosus), an irregular swelling two to three 

 inches long on blackberry stems, inside which 

 will be found numerous larvae. 



3. PARASITIC HYMENOPTERA 



FIG. 390. Spongy oak- 

 Most of the small, slender, wasplike hymen- app i e 



optera, which are distinguishable from the true spongifica} 



wasps by the two-segmented trochanters of the (Photograph by weed) 



