187&] 149 



ological factors concerned : 1st — the tracheal ligament, or tlie shed 

 tracheae from the last or ninth pair of spiracles, which uniformly 

 become blind or obsolete in the chrysalis ; 2nd — the rectal ligament, 

 or shed intestinal canal ; 3rd — the " Osborne " or retaining membrane 

 {membrana retinens), which is but a stretched part of the membranous 

 corium that accumulates around the rectum and in the anal prolegs, 

 and that is intimately connected with the rectal ligment. 



" In the second category, we have the structural featvires of the 

 chrysalis, these are : First— the cremaster proper, which is the homo- 

 logue of the anal plate of the larva, and the form of which is fore- 

 shadowed in that of said anal plate This cremaster is 



surmounted at the apex, and sometimes along the ventral ridges, by 

 what may be called the cremastral hook-pad, thickly studded with 

 minute but stout hooks 



"Secondly — we have the sustainers (sz^s^ew^ores), two projections 

 which homologize with the soles {plantcB) of the anal prolegs, and 

 which take on various forms, but are always directed forward so as to 

 easily catch hold of the retaining membrane 



''^Thirdly — we have what may be called the sxistentor ridges, usually 

 connected with the sustainers, and embracing them on the outside, and 

 extending backward to the inside of the ventral cremastral ridges. 

 These sustentor ridges are homologous with the limb of 

 the anal prolegs, and the exposed edge with the posterior border of 

 said limb 



"Fourthly — between them is what may be called the rectal piece, 

 consisting of a piece more or less well marked and elevated, especially 

 around the closed rectum. 



" It is principally by the leverage obtained by the hooking of the 

 sustainers in the retaining membrane, which acts as a swinging fulcrum, 

 that the chrysalis is prevented from falling after the cremaster is 

 withdrawn from the larval skin. It is also principally by this same 

 process that it is enabled to reach the silk with the cremastral hook- 

 pad. Tet the rectal ligament plays a most important part, and, in 

 some species, a more important part even, in my estimation, than the 

 membrane itself. The tracheal ligaments, which, from a study of 

 specimens plunged in alcohol when the larval skin was about half shed, 

 I was at first inclined to believe important auxiliaries, are, I am now 

 satisfied, of very little or no service in most cases. The rectal ligament 

 is a constant physiological factor, and its importance cannot be as- 

 certained by attempts to sever the membrane at the critical moment, 

 because in such attempts the ligament is more or less drawn out be- 

 yond the power of the sphincter muscles in the chrysalis to control it." 



