HESPERIDI: THE GENUS THANAOS. 1451 



scales seem to be move feebly attached, since the wings arc very easily 

 abraded. The bunch of short, conical filaments which nearly fills the 

 opening of the prothoracic spiracles can be protruded to a considerable 

 distance so as to form a prominent tubercle-like swelling, which I have not 

 noted as possible in otiier ncncra of Hcsperidi. 



Some structural peculiarities of the butterfly. One of the most 

 remarkable features in the structure of the butterflies of this group is the 

 extensive and asymmetrical development of the abdominal .ippendages of 

 the male, which was first pointed out in the common European species 

 by Kambur, but until recently has escaped further observation. The same 

 as\Tnmet:y is found in some of the neighboring genera, such as Achlyodes, 

 etc., and doubtless in some other groups, since I have detected it in a Heli- 

 conideous butterfly. It does not, however, occur in any other New Eng- 

 land l)utterflies, except in a very inconspicuous degree in some of our 

 other Hesperidae. The asymmetry is not only found in the lateral valves, 

 which are sometimes of widely disproportionate size, and serve well, also, 

 to distinguish the different species, but often also above, in the central 

 hook, the posterior view of which shows it to be sometimes remarkably 

 lop-sided. 



One reason, doubtless, that this strange asymmetry has escaped obser- 

 vation, is the entire concealment of the parts by scales, to which may be 

 added the want of attention to these appendages in butterflies ;* the upper 

 organ is protected by an extensive posterior expansion of the terminal seg- 

 ment of the abdomen, which forms a projecting hood, and which is also 

 provided at tip with a heavy fringe of excessively long scales ; the clasps are 

 themselves furnished externally with a heavy coating of pretty long scales, 

 which effectually hides the sculpture of the parts ; although the disparity 

 in length of the two clasps is readily seen, when it is so marked as it is in 

 T. brizo. 



In endeavoring to assign a reason for this excessive development and re- 

 markable asymmetry of the external genital organs, one cannot but be 

 struck by the fact that the males of this genus are far more commonly met 

 with than the females ; whether this is due to the comparative scarcity of 

 the latter, or to the greater seclusion of their haunts — all the species are 

 sylvan — I am not prepared to say ; the females, however, though more 

 radically svlvan, are constantly taken in the same stations as the males, so 

 that I am inclined to adojit the former hjqiothesis and to believe that, not- 

 withstanding the simplicity of the external genital apparatus in the female, 

 the excessive development of these parts in the male is in correlation with 

 their superior numbers, ensuring, beyond doubt, the impregnation of every 

 female ; I do not, however, see how asymmetry gives any superior ad- 



•Now that Gosse, Buchanan White, and tcmatic value of these parts, this reproach 

 Godmau have directed attention to the sys- will no longer be possible. 



