174 [Aiigvist, 



the mesal surface in the extensor row three prominent bristles . . . 

 and on the lateral surface in the flexor row one . . ; middle tibia 

 has on the anterior surface one ... on the posterior surface 

 three . . , and on the flexor surface one . . . ; hind tibia 

 has on the lateral surface in the flexor row two . . . and on the 

 mesal surface in the extensor row one." (5). Schnabl, who has 

 paid considerable attention to the chaetotaxy in Aricia (Cont. a la 

 Faune Dipterologique, St. Petersbourg, 1887) devotes more than five 

 pages to the description of the legs and their bristles, and in his 

 account of A. perdita (p. 400) thus describes the posterior tibiae : — 

 " Soies externo-anterieiires 3, dont 1 au dessus, la 2^ au milieu ; soies 

 externo-'posterieures 2 grandes, dont 1 au dessus du milieu, I'autre au 

 dessous de cette derniere ; une rangee de soies interno-medianes sur 

 le \ median du tibia composee de soies i)eu longues et rarement dis- 

 posees ; au hord posteripur un eperon court, un peu plus long que les 

 s. externo-posterieures, dans le I inferieurs du tibia." Lastly (G) 

 Yeurall, in describing Boliclwpus Inticola (Ent. Mo. Mag., 1904, p. 

 198) says : — " Middle tibiae with three bristles above towards behind 

 and three others alternating lower down above towards front, also 

 one bristle beneath below the middle." 



In the first place I would suggest the use of the four simple 

 terms anterior, posterior, dorsal, and ventral, whose meaning is sufii- 

 ciently obvious, and which moreover are capable of easy combination 

 with each other. Being of Latin origin they can be used in a diafjnosis 

 given in that language with facility, and the only point which can 

 offer any difiiculty is that of exactly defining their application. Now, 

 if the leg of a fly be stretched out to the utmost, so that the tarsus 

 and tibia are as nearly as possible in a line with the femur and the 

 whole leg horizontal, then all the surfaces which face upwards I call 

 dorsal, those facing downwards ventral, those facing towards the head 

 anterior, and those facing in the opposite direction posterior. A 

 surface between any of the foregoing may be denoted by a combination 

 of the two concerned, and thus we get the terms antero-dorsal,postero- 

 ventral, and so on. Thus a series of eight surfaces of attachment 

 may be easily differentiated, and these are, I believe, quite sufficient 

 for all practical purposes. Taking them in order, and working round 

 in the same direction as the hands of a watch we get the following 

 succession, commencing at. the top: — dorsal, antero-dorsal, anterior, 

 antero-ventral, ventral, postero-ventral, posterior, and postero-dorsal. 

 If the leg of a specimen happens to be bent, then the ventral surfaces 

 of the femur and tibia are those which would come into opposition 



