GENUS SYNERGUS. 61 



cessive. The sculpture of the thorax might be held 

 at first sight to afford some criterion of specific dis- 

 tinctness; but experience and observation show that 

 the difference in this respect is very great indeed. In 

 species like S. incrassatus, for example, the normal 

 coarsely coriaceous sculpture of the thorax may be 

 alutaceous merely, particularly in smaller-sized speci- 

 mens. Indeed, the intensity of sculpture appears to 

 have some relationship to the amount of nourishment 

 the larva has had, abundant food yielding large speci- 

 mens, poor and insufficient food small and stunted 

 examples. It is not easy to find even structural 

 characters which are constant. Thus the length of 

 the second antennal joint in normal specimens of 8. 

 melanopus gives a fairly good specific character ; but in 

 abnormal examples it does not, as they have it as in 

 S. Beinhardi. The form of the radial cellule is not of 

 much importance, while the colour of the nervures 

 is very inconstant. Further, the frontal laminae in 

 stunted examples become very indistinct, however 

 well marked they may be in larger forms. On the 

 other hand, the species oviposit with tolerable con- 

 stancy in the same galls ; and in truth it is only by 

 rearing the species that undoubtedly typical examples 

 are to be had. 



Mayr in his excellent monograph describes twenty- 

 two European species ; besides which there are a few 

 species described by Hartig which he was unable to 

 determine to his satisfaction. The genus is, as might 

 be expected, common in North America, and is also 

 known from Mexico. 



A. Second abdominal segment at the apex, broadly and 

 closely punctured. 



1 (2) Second antennal joint in $ shorter than broad, in $ as broad 



as long; head reddish, black on the top (typical form only). 



Melanopus. 



2 (1) Second antennal joint in <$ and $ longer than broad. 



Beinhardi. 



