THE INTEENAL STRUCTUEE OF THE LEPIDOPTEROUS PUPA. 69 



base, one only cuts away that part that becomes the tarsus of the 

 imago, the femur and tibia remain intact. Eeaumur, therefore, was 

 wrong in the assumption that the whole imaginal leg was contained in 

 the larval one (Gonin on Pieris brassicae). 



In the above description, Gonin pictures the leg as it exists towards 

 pupation, when the enlarged leg has left its narrow quarters in the 

 larval leg, from which, however, it takes its origin. He appears to fall 

 into the remarkable error that was originally made with regard to the 

 larval head, when certain authorities averred, from their observation of 

 larval moulting, that the head was developed within the prothorax. 

 We have already noticed (ante, p. 18) the inconclusive character of 

 experiments of ablation, and we still consider our explanation of the 

 phenomena as probably the correct one, viz., that the imaginal bud 

 grows out of the larval leg, and finds a lodgment during the progress of 

 development for some of its elements in the thorax at the base of the 

 leg. It will be observed that the stage at which the development of 

 the leg has reached, at the end of larval, and commencement of pupal, 

 life, is such that the greater part of the earlier stages of the develop- 

 ment of the imaginal legs has already taken place when the process of 

 pupation is completed. These organs simply complete their develop- 

 ment in the pupal stage. The development of the tarsus may now be 

 considered separately. 



The tarsus undergoes a series of transformations. In the last 

 larval stadium the surface is folded in a very complicated manner. At 

 the level of each of the corneous joints, but only on the inner and con- 

 cave aspect of the foot, a deep fold is developed ; there is a hypodermal 

 thickening of the one part, a simple membrane enveloping the other. 

 We think that this new duplication is to permit a complete renovation 

 of the interior of the entire fold, whilst preserving for some days longer 

 the muscular insertions and the connections of the surface with the 

 sensory hairs. These organs, supported by the central face, must be 

 useful to the larva during its preparation for pupation. The envelope 

 unites later, by its base, with the parietal hypoderm, and these two 

 membranes are destroyed together with the large cells of the hairs. 

 The internal part and the extremity of the tarsus are, therefore, merely 

 reorganised, and waste removed, whilst the external (and convex) region 

 undergoes complete reproduction. From the base of the larval leg the 

 hip and the trochanter are derived, being well differentiated from the 

 base of the thorax only in the first pair. A day or two before pupation 

 the femoro-tibial bud, which has preserved till now its antero-posterior 

 direction, is placed transversely in relation to the larva, and then 

 deviates obliquely forward. This see-saw movement around the hip is, 

 perhaps, to be attributed to the enormous extension of the anterior 

 wing, which pushes the first two pairs of legs before it. The last pair, 

 on the contrary, finds itself simply covered by the hindwing, and under- 

 goes only a slight displacement. This new arrangement of the limbs 

 is already that belonging to the imaginal state. The knee of the first 

 pair is placed a little before the tarsus, that of the second pair faces a 

 little more to the outside, that of the third pair is directed backward 

 (Gonin on Pieris brasxicae). 



The first important stages in the development of the imaginal 

 head practically occur in late larval life, the imaginal discs for the 

 antennae, ocelli, maxillae, &c, undergoing most of their development, 



