METAMORPHOSIS IN LEPIDOPTERA. 31 



whilst the wings become external organs, and conspicuous on each side 

 of the mesothorax ; the simple ocelli of the larva are replaced by the 

 large compound eye, whilst the prolegs disappear from the abdominal 

 segments and the genital organs become externally visible ; the pupal 

 cremaster is developed from the suranal plate of the larva, and so on. 

 However great the external changes may be, those of the internal organs 

 are equally modified, and these changes are usually more particularly 

 evident in the digestive and nervous systems. The pupal form is, in 

 these particulars, much nearer to the imago than to the larva, and the 

 pupal digestive organs are much more nearly approximated to the 

 imaginal, although the " reservoir " of the imago is not indicated in 

 the pupa. The head, genital organs, and urinary vessels are nearly 

 the same. The imaginal discs of the wings and legs are to be seen 

 when the larva is in the quiescent stage, whilst Gonin finds in Pieris 

 that the ventral discs of the three thoracic segments are each repre- 

 sented by several distinct folds attached to the femoro-tibial bud and 

 to the tarsal joints. The imaginal discs may serve, in the case of the 

 Lepidoptera, either for the formation of new organs or for the growth 

 and transformation of organs already existing. The imaginal discs of 

 the wings do not participate in the larval moults, and their surface 

 only forms a cuticle towards the end of the last larval stage and 

 becomes pronounced with the assumption of the pupal form. At this 

 stage, the network of fine tracheae of the wing-bud is drawn out with 

 the internal cuticle of the large tracheal (the permanent tracheae appear 

 at the time of the third larval moult as large rectilinear trunks, but are 

 not filled with air until the time of pupation). There are from eight 

 to ten of these tracheae in each wing, and they give rise in the pupa to 

 a new system of fine tracheae (tracheoles) Avhich replaces that of the 

 larva (Gonin). 



The development of the pupal legs, antennae, wings, maxillae, 

 maxillary and labial palpi, takes place principally during the final larval 

 stage, and their growth becomes especially rapid in the quiescent 

 period preceding pupation. The pupal organs are, more or less, how- 

 ever, only one stage of the series that leads to the final development of 

 the imaginal structures, and hence, although external organs, their 

 development is continuous within the pupal shell, and they may thus 

 be best considered when dealing with the internal structure of the 

 lepidopterous pupa. 



Gonin has set himself to explain why the antennae, maxillae, legs, 

 and wings occupy the position they do in the lepidopterous pupa. He 

 states that " when pupation is normal, the integument splits open on 

 the back of the thorax and the larval skin is drawn from before back- 

 wards, and that, owing to the feeble adherence which the chitinous 

 secretion gives it, it draws along with it the underlying organs. The 

 legs, antennae, and two halves of the maxillae, retained by their ends 

 in a small chitinous case, can only disengage themselves from it when, 

 in elongating, they have acquired a sufficient tension. The curves are 

 thus straightened out and the folds unbend. The chitinous mask of 

 the head, in withdrawing with the larval skin, follows the ventral line; 

 the tongue and labial palpi free themselves from its median part ; the 

 antenna' disengage themselves from the two lateral scales. Between 

 these different appendages a space is left on the surface of the head for 

 the eyes and on the thorax for the legs. These are not completely 



