EXTERNAL STRUCTURE. 



13 



wheu these were iudicated original names were nearly always 

 applied to them with the result that no fixed system came into 

 vogue until Professor Thomson revised the whole subject and 

 founded the first satisfactory nomenclature of uervures (c/. 

 Morley, Trans. Ent. 8oc. 1909, p. 439). I have described tliese 

 nervures as follows : — The front margin of the wing is always 

 bounded by a strong sustaining costa, which may or maj^ not be 

 united with the parallel and adjacent post-costa. The corneous 

 stigma is a strong muscle to sustain the more slender veins and 

 assist in foldiug the wing. From some part of it the radius or 

 radial nervure runs in a curve to or near the apex of the wing. 

 From the base of the wing, two other elongate nervures extend 



Fig. S. — Wings of an Icbiieiimou. 



A. Fore wing; a, apex; cells: — 1, 2, 3, basal; 4, 5, 6, cubital'; 7, S.* 9, 



discoidal ; 10, anal ; 11, apical or radial. 



B, Hind wing; //, humeral nervure ; cells: — 1. costal ; 2, 3, basal ; 4, aual ; 



5. radial; 6, cubital; 7, 8, first and second discoidal. 



to beyond its centre ; of these the posticus or posterior nervure 

 simply sustains the whole lower margin to the sinus at the anal 

 angle, while the medius or median nervure is that in the centre 

 and runs straight to the first recurrent vein ; the portion of the 

 nervure beyond this has usually been regarded as distinct, under 

 the name of the discoidal vein, etc., but 1 find tliat in the very great 

 majority of cases it is undoubtedly a continuation of the medius. 

 Through the centre of the medius runs a transverse nervure, 

 termed the basalis or basal nor\ure, which is not always con- 

 tinuous but sometimes coalescent for a greater or less distance 

 with the medius. From the junction of the medius with the first 

 recurrent vein a nervure runs (in IciiN£UAroNiD,i) upwards to the 



