PROTECTIVE COLORATION AND DEFENSIVE STRUCTURES OF LARV^. 95 



exhales a distinct odour, the latter differing entirely, according to the 

 species, scarcely perceptible, according to Scudder, in Laertias, and 

 varying through all degrees of offensiveness, to a truly sickening stench 

 in Iphiclide*. The organ itself is really a development of the integu- 

 ment, with glandular cells at its base, their secretion being probably 

 discharged through the cuticular pores, the odorous secretion accumu- 

 lating in the invaginating horns, and being freed by their exsertion. 

 When it is withdrawn, the osmaterium lies with one of its horns on 

 either side of the body, extending backward to the 1st abdominal seg- 

 ment, according to Klemensiewicz, but to the 3rd, according to 

 Studer, both of whom studied the organ in the larva of the same 

 species, Papilio machaon. Both of these authorities agree that a 

 delicate muscle is attached to the tip of each horn, which has its in- 

 sertion, according to Studer, on the dorsal, but according to Klemen- 

 siewicz, on the ventral, side of the body. It is by this muscle that 

 the osmateria are withdrawn, and this explains, not only why one 

 branch may move quite independently of the other, but also why the 

 imagination begins at the tip. The osmaterium is probably protruded 

 by the muscular contraction of the walls of the body, forcing the 

 contained fluids into the tube, and thus pressing out the reversed 

 osmaterium. 



Probably the most frequently studied of the eversible glands of 

 larvre are the well-known flagella of Centra vinula. These flagella 

 are very long, and can be thrown out and withdrawn by the larva with 

 great rapidity. In the process of evagination, the filament, which lies 

 in a receptacle, unrolls from the base, so that the apex is the last part 

 to appear. When invagination takes place, the apex first disappears fol- 

 lowed by the remainder. When the flagellum is half its original 

 length, the apex, which is passing in, is on a level with the base of the 

 structure when it is fully evaginated, the remainder disappearing until 

 the flagellum has been completely drawn inside the base. Poulton 

 writes : " At the base of each flagellum there is a small transparent 

 area extending round the whole circumference, and through this, as 

 through a window, the processes of evagination and invagination can 

 be readily watched. When the invaginating flagellum has shortened 

 to half its length, the tip has, of course, been drawn inwards as far 

 as the transparent base, and a pink line is seen in the axis of the 

 latter, rapidly lengthening inwards, until the whole axis is pink. As 

 invagination becomes complete the pink axis disappears inwards as the 

 transparent part is itself invaginated. The same phenomena are also 

 seen in evagination in the reverse order. The protrusion and with- 

 drawal of the claspers (prolegs) seem to be essentially due to the same, 

 process" (Trans. Ent. Soc. Lond., 1885, p. 322). When, by any 

 means, the rim of the receptacle (base of flagellum when everted) is 

 injured, the flagellum often becomes permanently introverted, and it 

 can then be seen within the receptacle. It can also be everted by 

 forcing blood into it. The arrangement of the nerves and muscles 

 relating to these organs is figured by Poulton (IbiiL, 1887, pi. x., 

 fig. 9). f The power of contraction in the retractor muscles of the 

 flagella is enormous. 



Speaking of the manner in which the anal prolegs have been modi- 

 fied into stemapoda, with their attendant flagella in allied genera, 

 Packard says : The hypertrophy of the anal legs once initiated became 



