THE BUTTERFLY: STRUCTURE OF I'lIE ^^•IXGS. 41 



present, precostal and internal.* The preeostal, cf).stal, subniedian, and 

 internal veins are invariably simple and terminate at the margin or even 

 disa[)[)ear before reaehing it.f Tlie snbcostal and median veins, on the 

 other hand, arc as invariably branehed, and with their offshoots support 

 nearly the entire wing ; the subcostal vein curves downward and the median 

 upward so as to meet, or nearly meet, al)out the middle of the wing, and 

 to inclose between them a large space called the discoidal cell ; all tJie 

 branches of the median vein are thrown off from its lower side before iniion 

 with the subcostal vein ; the princi])al branches of the subcostal vein, on the 

 other hand, are thrown off from its u})per side, but as the vein curves 

 downward at the extremity of the cell, another set is thrown off, at least 

 in the front wings, from the lower side ; and it is these branches, rather 

 than the subcostal ^•ein proper, which unite with the median vein to close 

 the cell. None of the median nor any of the inferior subcostal branches 

 are ever forked ; but at the apex of the front wing, where the play of neu- 

 ration is usually the greatest, the last superior subcostal branch is occa- 

 sionally forked. The neuration of the wings, then, consists essentially of 

 u})per and lower simple straight veins, and a pair of middle veins which 

 unite with or approach each other near the centre of the wing ; and from 

 the outer edge of the cell or loop thus formed throw off to the border a 

 number of branches. The veins are more closely crowded next the front 

 edge of the front wings to give greater solidity to the parts which meet with 

 the greatest air resistance in flying. No cross-veins proper exist in butter- 

 flies, excepting that one occasionally (especially in Papilioninae) connects 

 the median and submedian veins next the base of the fore wings. All 

 these nervures and their branches, when they do not run into one another, 

 terminate at the border of the wing, and by their extension determine to a 

 great extent its form ; for though the membrane often recedes between the 

 tips of the nervures so as to give a scalloped margin to the wing, this 

 never takes place to an excessive extent ; while the thrusting forward of 

 the subcostal nervures of the front wing necessitates a more or less falci- 

 form outline ; or, the great extension of a single nervure of the hind wing, 

 as particularly of the third median nervule in the Papilioninae, or the first 

 median in the Lycaeninae, permits a tail-like appendage of great beauty. 



In connection with the wings it will be well to mention the epidermal 

 covering characteristic of the order to which the butterflies belong, since it 

 is upon the wings more than upon any other part of the body that they 

 take the form of scales from which the name Lepidoptera has been derived. 

 The scales are depressed sacs of a more or less rounded, quadrate or trian- 

 gular form, striate upon the upper surface, usiudly rounded, also deeply 



* Spang-licrg calls them (in the most recent t The intcrn;il. in the front wings, ami es- 



paper on the sulyect) costal, subcostal, radial. pccially in Hesperidae, sometimes terminates 

 ulnar, anal and axillary. by running into the submedian. 



