EEV. A. E. EATOX ON EECENT EPHEMERID^ OR MAYFLIES. 7 



nearer the middle of the wing (PL XII.), the sector and the adjacent adventitious nervures 

 either remaining apart from both or forming a union with either of them. When the 

 costa is not rounded oif at the extreme base, it almost always describes a salient ana-le 

 in or before the middle of the anterior margin, after which it becomes approximated to 

 the subcosta (2) ; and this last, when not straight nor evenly curved, is strongly arched 

 towards its proximal extremity. The radius (3) takes a nearly direct course to "^he further 

 border of the wing, near the apex, so that a relatively wide space is left between it and 

 the subcosta : in Bcetisca it is interrupted, or obsolescent. The sector and adventitious 

 nervures (4-4^) are suppressed in scantily nerved wings, but vary in number and in their 

 combinations in other instances. The sector alone is present in some species of Campsurus 

 (PL V. 8 i) ; but in most genera there are at least two adventitious nervures associated 

 with it, the hinder oue of which visually unites with the sector, so as to form a fork, 

 including its fellow. Another arrangement occurs sometimes in Polijniitarct/s (PL VI. 

 10 a) where three such nervures are interposed; In. Palmgeiiia {^\. I.) and Bcetisca 

 (PL XXI.) there are perhaps five of them, whilst in most of the genera from Coloburus 

 onwards, although the number of the adventitious nervures appears at tirst to be two, it 

 seems reasonable upon closer inspection to recognize four of them, of which the third 

 unites with the cubitus (5) to form a fork enclosing the fourth (1^), in the same manner 

 as the second and sector enclose the tirst. 



Second Group. — The defection of the cubitus and sector from this group is compen- 

 sated for by the transference of the anal (8) nervure to it. When adventitious nervures 

 are interposed between the prgebrachial and the pobrachial (they are absent in Habro- 

 phlebia, PL XIII.), they are more frequently associated with the former than with the 

 latter nervure ; and it sometimes happens that the hindermost adventitious nervure (6') 

 in genera related to Siphlurus, assumes equality with, or even predominates over, the 

 prtebrachial (6). The adventitious ueuration intervening between the pobrachial (7) 

 and anal (8) is of meagre extent when it is not suppressed. 



Third Group.— The axillary nervures (9), usually left behind by the anal (8), gene- 

 rally occupy a very limited space in the hind wing ; they attain their highest develop- 

 ment in Cliirotonetes and Oniscigaster (PL XIX. & XXI.). 



TJie legs present great diiferences in their condition, in the relative lengths of the 

 several pairs, and in the proportions of the component parts of corresponding pairs. Some 

 of these differences are sexuaL others are generical. Sometimes all of the legs are fuuc- 

 tionless, — flaccid, filamentary rudiments of the tibiae and tarsi, or else atrophied miniatures 

 of the same, definitely shaped, but thoroughly infirm, remaining attached to the femora ; 

 in other instances such is the condition of only the two hinder pairs, and then the anterior 

 pair may be either stout and short, or slender and long in either the male only, or in both 

 sexes. The fore legs are always longer in the male than in tne female (usually very 

 much so), and are generally longer than either of the hinder pairs ; but in the male of 

 OUgonenria the fore leg is shorter than the intermediate. The hind legs are usually as 

 long as, or shorter than, the intermediate ; but in Adeiiophlebia the middle pair is the 

 shortest of all. The prolongation of the fore leg is chiefly due to the lengthening eitlier of 



