March, 1903.1 



53 



In certain cases the hind-wing lias wliat. may be called a " continuous external 

 neumtion," that is to say, from k, the apex of the costa, a succession of veins and 

 nerves run at a little distance from the margin of the wing to g, the apex of the 

 humerus. Tims practically the whole neurated portion of the wing is encircled from 

 a to e by one continuous nervure, consisting of two whole longitudinal veins (the 

 costa and the humerus) and also apparently of portions of other nerves and veins 

 — more or less displaced from their normal directions. 



This sort of wing is not only remarkable in itself, but puzzling in the strangely 

 "sporadic " manner of its occurrence among the Saw-flies. 



It appears in widely different groups— sometimes characterizing a whole Genus, 

 sometimes only one or more species in a Genus ; and what is very singular, it seems 

 to be found only in the males in all cases, but in these to be quite constant. 



Thus it appears in all species of the Genera PericHsta and Perineura, in all 

 British species but one {Coqueberti.) of Tenthredop.ii.i, and in one species each of 

 Eriocampoides and Taxonus — but as aforesaid only in tlie males ! 



How such a phenomenon is to be accounted for I cannot pretend to conjecture. 

 Doubtless it must somehow benefit the insects exhibiting it, and certainly it benefits 

 the systematist who is called upon to name them ! 



Fig. 6. — Hind-wing witu continuous exteknal neubation 



OF A TeNTHEEUOPSIS (J. 



/ k 



A very few words more amy be said us to the wings. 



(1) As to the word " cell," it means strictly a portion of an area completely 

 surrounded by veins and nerves ; but many writers apply it more loosely, e. g., to a 

 space bounded partly by veins and nerves and partly only by a veinless and nerveless 

 margin. It is used thus when writers speak of " four cubital cells," an " open 

 lanceolate cell," an " anal cell," &c. 



(2) The term " Interstitial " is used when two or more distinct nerves (e. g., a 

 radial and a cubital) strike one vein at the same point, so that they form a continuous 

 straight or crooked line. 



(3) The terms before and after in descriptions of neuration mean respectively 

 nearer to and further from the base of the wing. 



(4) Though the course of all the " veins " on the whole, i. e., measured from 

 end to end, is longitudinal, certain portions of them may be so deflected from this 

 course as to run actually transversely. This may at first cause some trouble ; but 

 one soon learns to see each " vein " as a whole, however it may zigzag at this point 



