RELATION OF LARVAL TO ADULT LEGS 655 



without precise limits with the extremity concealed in the larval 

 legs. The three divisions of the latter do not appear to have any 

 relation Avith the five joints of the perfect state. Under the micro- 

 scope the rudiment appears very strongly plaited at the level of the 

 tarsus, much less so in the other regions. A. large trachea penetrates 

 into the femur with some capillaries ; reaching the knee it bends into 

 the tibia at a sharp curve, but does not become truly sinuous in 

 approaching the extremity. It is then the tarsus especially which 

 is susceptible of elongation ; it may, on being withdrawn, give rise 

 to the illusion that the whole organ is disengaged from the larval 

 leg. 



" This disposition is, we believe, not known. It gives the key to 

 the experiments of Reaumur 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 Reaumur has erroneously con- 

 cluded 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 aban- 

 doned." 



Embryonic cells and the phagocytes. --Up to the last larval stage the 

 legs do not offer, says Goniu, 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 tracheae 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 



