188 [August, 



mediastinal vein, and lastl}^, the presence of three rows of strong 

 bristles across the frons, each consistins^ of four bristles, the first row 

 at the level of the ocelli, the second in the middle, and the third just 

 above, the antennje. On the present occasion I purpose to deal only 

 with the species that fall under Becker's Group I. But 1 must first 

 acknowledge the important help T have received from Mr. Collin, 

 whose readiness to come to my assistance in every difficulty and to 

 check my conclusions has been invaluable. Without it I might not 

 have ventured upon the undertaking. My thanks are also due to Dr. 

 Sharp for the loan of some interesting species from the Cambridge 

 Museum. 



At the time of writing (1901) Becker was acquainted with 37 

 species belonging to this group. Of these we have at present in 

 Britain 26, all but four of which (autumnalis, ahhreviata, femorata 

 and coritiifronsi) have fallen to my own net in Herefordshire, whilst 

 we have besides four new and undescribed species, all taken and two 

 of them exclusively in my own district, which brings up the total to 

 80 British species. Few of them are common. The abundance, 

 under certain conditions, of curvinervis has already been alluded to, 

 equally abundant too is the ubiquitous concinna ; some eight or ten 

 more are to a greater or less degree fairly common, but the rest are 

 decidedly scarce and will probably remain so till our ignorance of 

 their economy is removed. 



To some extent I have adopted Becker's table, making use of the 

 venation for the primary lines of division, but where he turns to the 

 colour of the halteres for the further grouping of the species, T have 

 selected instead the position of the spines on the middle tibiae — a 

 character quite as easy of recognition, less liable to variation, and at 

 the same time leading, I think, to a more natural arrangement of the 

 species. Not that I would in any way underrate the value of colour 

 in the halteres. Indeed, it was the belief in its reliability that has in 

 more than one instance been the first step in leading to the discrimina- 

 tion of two closely allied species. For example, unispinosa occurs in 

 two places in Becker's table — among the species with yellow halteres 

 and among those with black. The insect is constantly turning up 

 here throughout the season, and I must have taken first and last a 

 large number. On sorting my material I found that the variation was 

 limited to the females ; in the males the colour was invariably black. 

 This led to a closer examination, and then it was seen that the insects 

 with dark halteres had quite a different venation to the others and 

 were the true unispinosa, whilst those with yellow or whitish halteres 



