( Ixxxvi ) 



Besides the vnuiiKil ventral area there is the eighth tergite which forms 

 jiart of the rojnilafory apjiaratus of the female. This tergite, covered by the 

 seventh, is never sjnnose : it varies in size and shape (PI. XV. f. 9. 13 ; 

 PI. XVI. f. (i. 0—11. 10: ri. XVll. f. .J— S), and is in many cases different in 

 the species of the same genns, occasionally even in geographical forms of the 

 same species (PI. XVll I.' f. b. S). The most remarkable eighth tergites are 

 those represented by PI. XVII. f. 10 and PI. XVIIl. f. 7. In the former there 

 exists ajiparently a gland (o), from which a groove leads anad to a deep cavity. 



The geographical variability of the sexual armature is very instructive for 

 the beginner in the stndy of comparative morphology. The similarity between 

 the structures of the various subspecies is evident in spite of the differences ; 

 the modification.s are easy to perceive, and there is no difficulty in recognising 

 the homology. The meaning of evolution and of relationship is very clear when 

 a series of subspecies is compared with one another and with the nearest allied 

 species. 



No less instructive is the asymmetrical development of the armature found 

 in a number of Sphingidae. From one jioint of view the asymmetry is even 

 more interesting than the geographical variability. For it is in the case of 

 geographical modifications not rarely impossible to say with certainty which of 

 the modifications is the oldest, while there can be no doubt about the asymme- 

 trical armature being a development from a symmetrical one. Therefore it is J 

 easy to make out by comparison of all the allied species which was the ancestral ^ 

 form of the armature. We have here not only the line of development, but also '' 

 the direction followed (see p. Ixxiv.). Apart from the penis, the armature of \ 

 which is nearly always asymmetrical, we meet with asymmetry in the claspers ! 

 and harpe and in the tenth segment of the male. There is outside the Sesiinae i 

 only one case of obvious asymmetry in the lKirj)e, which occurs in a species of i 

 Poli/l>tt/chus, a genus oi Ambiilicinu<\ In Poh/ptijckus (IcntKtHx (PI. XXXIV. f. 0) i 

 the harpe has a long ventral [iroeess {pc) ; the processes of the right and left 

 harpe are the same in length, and both harpes are quite separate from one another. 

 In a closely allied species, Polypti/ckus trilineatiis, a remarkable modification 

 is observed. The ventral processes {pv) of the right and left harpe are fused 

 proximally in the Philipi]ine form of trilinralus (PI. XXXIV. f. 7) ; the process .; 

 of the right harpe (left in figure) is small and slender as compared with the 

 process of the left harpe. In the North Indian form, P. trilineatiis tmdatus 

 (PI. XXV. f. 'Z, pr), the two processes are fused together to a fishtail-shaped 

 structure, and in the Ceylonese subspecies, P. trilinmtus lutcatiis (PL XXXIV. 

 f. 10), the process {pr) appears as a single asymmetrical one, which does not 

 show a trace of its being the jiroduct of the fusion of two processes, excei)t at 

 the base, where the two halves extend upward on each side, surrounding the 

 penis-sheath. This combination of geographical variation and asymmetrical 

 development is very interesting. It will be seen, moreover, from the figures 

 quoted, that the upper, shorter, process {pm) of Pob/ptychus deiUatiis is of difl'erent 

 length right and left in /'. trilinentus (pdl and pdr). 



