10 GENUS DINEUEA. 



nervure in D. stilata is, from an evolutionary point of 

 view, of much interest, for it shows us how " genera " 

 have been formed. At present at least three-fourths 

 of the specimens of stilata have only one radial cellule. 

 In Prussia apparently all the specimens are in this 

 condition, and in all likelihood in the course of some 

 generations the transverse radial nervure will have 

 disappeared, and then the species will require to be 

 transferred from its present position in our systematic 

 works to be arranged in Nematus among the species 

 of ihepavidus group. We find many similar instances 

 in different groups Selandriadcs, Hylotomina of the 

 partial or almost complete obliteration of the first 

 transverse cubital nervure, as is shown by its being 

 only present at the sides in many species of Nematus, 

 or by its being only occasionally present in some 

 species of Hylotomina. It is, therefore, quite clear that 

 many of our so-called genera have been formed by 

 certain nervures having been by some cause obli- 

 terated. I believe that cause to be the presence of 

 of " bullse " on the nervures. There is always a bulla 

 on the transverse radial and first transverse cubical 

 nervures ; the effect of the presence of the bulla is that 

 the nervure is divided in the middle, which makes it 

 weaker, and also, no doubt, interferes with the circu- 

 lation of fluid through it. If that be the case, then, 

 such a nervure will be of little use to the insect, and 

 not fulfilling any useful purpose will in course of time 

 disappear entirely. That this is the explanation of 

 the disappearance of many nervures found in some 

 sawflies and not in others is proved by such nervures 

 being always bullated. The cross nervure in the 

 costal and lanceolate cellules are cases in point ; and 

 another one is that of the transverse brachial nervure 

 of Lyda a nervure only found in that genus. In some 

 species it is well nigh complete, in others there is 

 only a tiny branch to indicate its presence, and in a 

 few species it has entirely disappeared. What may 

 be the cause of the bailee is a point which has not yet 



