THE SUBFAMILY LIBELLULIX^. 253 



the case in many species of Corduliinse. The upper sector of the arculus curves round 

 to the hind margin ; the lower sector falls at or near the outer angle of the triangle (of 

 which we shall speak presently), and then runs subparallel to the upper sector. 



Before the upper sector of the arculus reaches the level of the nodus, it throws off 

 two nervures, rising in a stalk from its upper surface. The upper nervure runs upwards, 

 and passes below the nodus and pterostigma to the extremity of the wing, parallel with 

 the subcostal nervure ; it may be called the subcostal radius; and the space beyond the 

 nodus between this and the subcostal nervure forms the lower postnodal costal space. 

 This space is always more or less free from cross nervures at the base, and the post- 

 nodal cross nervures are never so regular in the lower space as in the upper, and are 

 not unfrequently bisected beyond the level of the pterostigma, rarely before. 



The thick nervure (or nodal cross nervure) descending from the nodus curves a little 

 outwards, crosses the subcostal nervure, and falls on the subnodal sector, which rises 

 from the same stalk as the subcostal radius ; it, however, throws off or, rather, is con- 

 tinued in a slender nervure (the nodal sector) to the hind margin ; and it sometimes 

 throws off a second short branch below, near its origin, to the subnodal sector. 



The nodal sector is sometimes nearly straight at its base, but more frequently a little 

 undulated or curved downwards ; it then runs forwards nearly straight, or is arched a 

 little at or before the middle, and at its extremity curves downwards towards the bird 

 margin. But in several important genera {Orthetntm, Leptetriim, Orthemis, &c.) 

 beyond the arch near the middle, or without forming a distinct arch, it again dips 

 downwards in a long and sometimes rather deep curve, another alary peculiarity which 

 I have not observed in any Corduliinaj. The subnodal sector follows the course of the 

 nodal sector to a certain extent, but is always straighter, rarely following the course of 

 the arch. There is generally a single row of cells between (broadest, of course, beneath 

 the arch) ; but these are usually bisected towards the hind margin, occasionally increasing 

 to three or even four rows of cells before reaching it. 



Below the upper basal cell lies the lower basal cell, a long narrow space extending 

 considerably beyond the other, and ending more or less acutely at the base of the 

 triangle ; it is generally crossed by a single nervure before the middle ; in several 

 genera, however, it is crossed by more than one, though less frequently than the lower 

 basal cell of the hind wings. 



In many genera one or, rarely, more cross nervures descend from the lower sector of 

 the arculus to the lower basal cell, or to the base of the triangle ; these are called 

 supratriangular nervures. 



We now come to one of the most important features in the wings of Odonata — the 

 triangle. The base lies between the extremity of the lower basal cell and the lower 

 sector of the arculus, and is usually more or less oblique. 



In some genera containing small species it happens either constantly [Nannophya &c.), 

 or as an individual peculiarity {Biplacodes and Baphismia), that the lower sector of the 

 arculus falls on the base of the triangle before its extremity. It then usually happens 



