OF THE MILK-WEED BUTTERFLY. 13 



This latter organ is a dark, horny, slightly curved tube, about 6 mm. in length. The 

 tip expands into a small trumpet shaped mouth, which is twisted more or less asymmetri- 

 cally. At rest, the tip lies between, and a little beyond the triangular flaps of the anal 

 somite, just below the anus, and is inclosed in a membranous sheath (ps.), to permit of 

 its protrusion beyond the body. This protrusion is worked by two extensor muscles (e.p.), 

 one on each side, whose attachments and mode of action will become sufficiently evident 

 by the inspection of fig. 16. 



The ventral arch of the ninth somite sends a process into the abdomen immediately 

 underneath the penis, for the support of the latter, and the attachment of a large band of 

 muscular fibres (r.p.) which embraces the penis, holds it in place, and accomplishes its re- 

 traction. 



The External Male Armature iisr the Lepidoptera. The secondary male organs 

 of the Lepidoptera are constructed on a simple plan, which is modified after various 

 fashions ia the different groups, and these modifications are often serviceable as characters 

 of classificatory importance. 



Their typical structure is well illustrated by the Satyrid but> 

 terflies, (see the adjoining figure), in which the dorsum of the 

 anal or ninth abdominal segment is produced into a simple term- 

 inal hook directed downwards, recalling the telson of Crusta- 

 cea. The sternum of the same somite is reduced to a narrow, 

 U-shaped piece, which may be called the ventral arch. To this 

 ventral arch are articulated two, more or less pincer-like, clasp- ^ , , „ 



. )) A n 1 11 External armature of Saty- 



mg organs, termed simply " claspers. As already stated, the rws a?ope, male; (/.dorsum, and 

 anus lies just beneath the hook, and the penis projects below L"; r^iT". tf^ rTght 

 the anus. Anal cerci are never present in the Lepidoptera. ^"^^ left claspers. 



Remarkably enough, the eighth abdominal somite sometimes closely imitates the 

 ninth. For instance in the yellow butterflies (Pierids), its dorsum is produced into a hook 

 exactly like that of the ninth segment; while on the other hand in the subject of this 

 paper, the posterior border of the sternum is extended into two false claspers (figs. 13, 14, 

 etc.,/.c.) in close imitation of the genuine organs, except that they are not articulated.^ 



The terminal hook is often bifid, sometimes very long, or at others curiously armed 

 with spines, etc. The clasps are equally various in shape, and armature, and in Nison- 

 iades ^ very remarkable for their asymmetrical development. 



External Male Armature in Aechippus. In the Milk-weed Butterfly, the hook is 

 entirely obsolete, but the dorsum of the ninth somite projects into two rather triangular 

 lateral flaps, just as in the female, except that they are longer and slenderer. Between 

 the flaps the segment is emarginated above. The ventral arch (see fig. 15, at the numeral 

 9) is wider than usual, and sends a stout cylindrical process (pr.) into the abdomen, for the 

 support of the penis and the attachment of its retractor muscle as already described. 



1 The real claspers are, presumably, true arthropodan ap- claspers, and yet not homologo~us with true abdominal ap- 

 pendages, but the existence of the false claspers suggests a pendages. 



mere possibility of independent development. For if a ^ ggg Scudder and Burgess, Proc. Bost. Soc. Nat. Hist., 



joint could arise between the false clasp and its segment, we xiii, 282. 

 should have organs indistinguishable from the ordinary 



