252 ME. W. F. KIEBY — A EEVISIOX OF 



d. The Foee Wings. 

 These are very important organs in the classification of Odonata. The first point of 

 interest on the costa is a notch about the middle of the wing, called the nodus, from 

 which descends a thick nervure which divides the costal region into two halves perpen- 

 dicularly. The upper part of the space between the base and the nodus is again 

 divided longitudinally into the upper and lower antenodal costal spaces by a nervure 

 which does not extend beyond the nodus, and has been called the internodal radius ; it 

 appears to me to correspond to the so-called " costal nervure " in Papilio, and would be 

 better so designated. Between the base and the nodus are the antenodal cross nervures, 

 varying in number from five to upwards of thirty, according to the genus and species. 

 These generally cross both spaces, although those on opposite sides of the costal nervure 

 do not always exactly coincide ; but in many genera the last crosses the first space 

 only, leaving the second empty below it. This character, though perfectly constant in 

 some genera, is variable in others, and must therefore be used with caution. When 

 the last cross nervure is not continuous it is often very oblique. I have only seen two 

 specimens in which the first two nervures on one side were discontinuous, and I have 

 seen no Corduliinse in which the last antenodal cross-nervure was not continuous. 

 When the cross nervures are very numerous and crowded they occasionally become 

 Y-shaped, V-shaped, or anastomosed. An extra cross nervure is not unfrequently 

 interposed between two others on either the upper or lower space, and not continued 

 on the other. The lower antenodal subcostal space is bounded below by a nervure 

 which runs from the base to the apex of the wing, and which is usually called the 

 principal radius, but which would be more correctly termed the subcostal nervure. 

 Towards the apex of the wing is an opaque spot, usually of an oblong form, bounded 

 by two thick nervures of a darker colour than itself, and called the pterostigma. The 

 space between the nodus and the pterostigma is the upper postnodal costal space, and 

 is likewise traversed by cross nervures, one or more at the commencement of which are 

 always discontinuous in the Libellulinse. Returning to the base, we come next to the 

 upper basal cell, a rather broad space below the subcostal nervure, which is never 

 divided by nervures in any species of Libellulinse. It much resembles the discoidal cell 

 of Lepidoptera, to which (or perhaps rather to the upper part of which, as Mr. C. O. 

 Waterhouse has suggested) it evidently corresponds. The upper basal cell is bounded 

 on the outside by an oblique cross nervure called the arculus 1 . On the opposite side 

 rise two nervures called the sectors of the arculus. They generally rise in a stalk from 

 the arculus at or near its lower extremity, but sometimes diverge from a point at or 

 close to their origin; they are rarely, if ever, so completely separated at the base as is 



1 This nervure really corresponds with, the upper discocellular nervule in Lepidoptera. I have retained the 

 name arculus, 'which is used by Selys-Longchamps, Hagen, Brauer, &c, because it is necessary to have a short 

 and convenient term for it. In Charpentier's terminology, however, the term arculus seems to be applied to 

 the short basal curve of the lower sector of the arculus, and to this the term arculus really applies much better 

 than to the former. 



