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§ VIL On the Costal Fold of the Hesperidae . 1 
Plates F and G. 
When writing of the sexual differences of the Hesperidae, Westwood 2 
says that “ in some groups the fore margin of the fore-wings is recurved in 
the males, the enclosed space being thickly clothed with pale-coloured down.” 
Herrich-Schaeffer gave to this recurved margin of the fore-wings the name 
of “ costal fold,” making use of it as a distinctive character of the genera 
in which it occurs. 3 
I was convinced that until the costal fold of all species known to 
possess it had been submitted to a microscopic examination and compared 
with one another, it would be impossible to form an opinion as to its 
function ; also that, from a purely systematic point of view, this structure, 
being at once varied and diverse in very similar species, was well deserving 
of close study. 
Of the many hundreds of Hesperidae furnished with a costal fold, I 
have only been able to secure less than a dozen—too few to permit me to 
draw any general conclusions from my observations : I publish them more 
for the purpose of arresting the attention of entomologists than for their 
own value. 
In the Hesperidae , as in many nocturnal Lepidoptera, the anterior 
margin of the fore-wings is occupied by a nervure unnamed by Lepido- 
pterists, which, as I shall frequently mention it, I call the marginal nervure 
(M in Figs. 2, 7, 13, 20, 24, 26, Plates F, G). 
The species in which I have examined the costal fold are the following: 4 
Telegonus midas , Cram. (PL F, Figs. 1-5).—If the costal margin of the 
fore-wings be divided into five equal parts, the costal fold occupies the 
second and third, counting from the base, and is about 15 mm. long by 
T5 mm. broad. The fold extends along the margin of the wing, and 
resembles a figure bounded by two arcs of a circle and divided into two 
parts by their common chord (Figs. 1, 2). These two parts are composed 
of the reflexed margin of the wing, bounded by the marginal nervure, and 
that part of the wing which it covers. Both the arcs bounding this figure 
1 Archivos do Museu National do Bio de Janeiro , III. (1878), pp. 41-50. By Dr. 
Fritz Muller, Travelling Naturalist for the National Museum. 
2 Doubleday, Westwood, Gen. of Diurnal Lepidopt., 1852, p. 506.—F.M. 
3 Herrich-Schaeffer, Prodrom. Syst. Lepidopt., fasc. iii., 1868, p. 52.—F.M. 
4 I follow the nomenclature of Kirby’s Cat. Diurnal Lepidopt ., 1871.—F.M. 
