INTRODUCTION. 



fork at the base of 1 b ; the median nervure with its three branches 

 (2, 3, 4) ; the lower radial (5) ; the upper radial (6) ; the subcostal 

 with five branches (7, 8, 9, 10, 11) ; and the costal nervure (12) ; 

 but almost any of these may be absent. 



The hind wing typically has eight veins, the difference from the 

 fore wing consisting in the absence of four of the subcostals ; but 

 vein 11 is probably represented by the bar between veins 7 and 8, 

 present in Spliingidce, Eupterotidce, Zygcenidce, &c. In the ffepialidce 

 and Micropterygidce, however, twelve veins are present as in the fore 

 wing. Besides the true veins, cross veins, or veinlets between the 

 costal nervure and costa, or between vein 1 b and the inner margin 

 of the fore wing, may be present in lowly organized forms, or even in 

 higher ones. The present form of neuration arose from a system 

 of simple longitudinal veins, which by the union of their basal 

 portions formed the present branching system ; thus the disco- 

 cellulars which close the cell of each wing were formed by the 

 junction of the basal portion of two veins, the basal stem being 

 then aborted, leaving only an outwardly directed fork of which the 

 radials are the extremities ; in the cases where the radials are said 

 to arise from the discocellulars, and not from the angles of the 



Fig. 5. 



A. Fore wing of Cossid (slightly diagrammatic). 



c.n. Veinlet in cell, the fork of which forms the discocellulars d. 



s.n. Stalk of veins 9, 10 ; 10 anastomosing with 7 and 8 to form the areola. 



B. Hind wing of Hepialid (slightly diagrammatic). 



7, a, b, c, d. Five subcostal nervules. 



cell, the basal portions of one or two other longitudinal veins have 

 also been aborted : the whole process in its various stages is seen 

 to perfection in such lowly forms as many Cossids, Psychids, 



