“This disposition is, we believe, not known. It gives the key to the experiments of Réaumur and of Newport.
“Even when we cut off the limb of the caterpillar at its base, we only remove the tarsus of the imago; the femur and the tibia remain intact. From an evident homology Réaumur has erroneously concluded that there is an identity. His opinion, classical up to this day, that the limb of the butterfly is entirely contained in the leg of the caterpillar, has been found to be inexact and should be abandoned.”
Embryonic cells and the phagocytes.—Up to the last larval stage the legs do not offer, says Gonin, any vestige of an imaginal germ, but they contain a great number of embryonic cells (Fig. 145, ec). They are almost always collected around a nerve or trachea; sometimes they are independent, and sometimes retained in the peritoneal sheath, seeming to arise by proliferation from this sheath. Some thus contribute to the lengthening of the tracheal branches or nerves, and the others, becoming detached, form leucocytes or phagocytes. They are very numerous in the legs, at the beginning of the 4th stage, but are disseminated some days later throughout the whole cavity of the body. At the time of histolysis they attack the larval tissues and increase in volume at their expense; in return they serve for the nutrition of the imaginal parts and exercise no destructive action on them. Van Rees agrees with Kowalevsky in comparing these attacks of the embryonic cells, sometimes victorious and sometimes impotent, to the war which the leucocytes wage against both the attenuated and the virulent bacteria.
Formation of the femur and of the tibia, transformation of the tarsus.—Capillary tracheæ appear in the leg at the same time as in the wing. They arise from the end of a tracheal trunk near the base of the limb on the dorsal and convex side. After the 3d moult the hypodermis thickens near this place; in a few days a pad is formed there and then a large imaginal bud with a circular invagination. These buds were noticed by Lyonet, who supposed them to be “les principes des jambes de la phalène.” Nerves and a tracheal branch penetrate into the femoro-tibial bud and form a small bay or constriction which marks the point of junction of the femur with the tibia, and the body-cavity remains in direct communication with the end of the limb.
Fig. 608.—Feet of the Pieris butterfly withdrawing from those of the larva.
Fig. 609.—Imaginal feet of Pieris uncovered with great care to preserve the position which they had in the larva: ta, tarsus; t, tibia; g, knee; f, femur; h, coxa.
The tarsus undergoes a series of changes; the surface is folded in a very complicated way; at the level of each articulation, but only in the internal and concave region of the leg, is developed a deep fold; on one side there is a hypodermic thickening, on the other a simple leaf of the envelope, which afterwards joins at its base with the parietal hypodermis, and then two leaves are destroyed with the large cells of the setæ. The internal part and end of the tarsus are then reconstituted with the elimination of the débris, while the external and convex region undergoes direct regeneration.
The coxa and trochanter are derived from the base of the larval leg, and only the 1st pair are well separated from the base of the thorax. One or two days before pupation the femoro-tibial bud, after having, until now, preserved its antero-posterior direction, is placed transversely as regards the larva, then becoming directed obliquely forward. This rotatory movement of the coxa may be attributed to the great extension of the fore wings, which push before them the two first pairs of legs. The last pair in their turn are simply covered by the hind wings and are but slightly displaced. This new position of the legs is that of the imago: the knee of the 1st pair is situated in front of the tarsus; that of the 2d a little outward; that of the 3d pair is directed backward. (Gonin.)