METAMORPHOSIS IN LEPIDOPTERA. 3 



undoubtedly due to the varying conditions of the environment (used 

 in its widest sense, vide, Proc. South Load. Ent. Society, 1898, pp. 70 

 et seq.), to which many insects are subject, and to the fact that the 

 holometabolic insects lead, during the periods of existence folloAving 

 the egg stage, three distinct and different lives, under quite dissimilar 

 surroundings, amongst different enemies, and exhibiting entirely 

 different habits. That these divergences should be accompanied by 

 distinct modifications of the existent structures is to be expected, and 

 thus we find that the larval organs are modified in the pupa, and the 

 pupal organs in the imago, to such an extent that their functions are 

 often radically different, yet there can be no doubt that the necessary 

 change in function has brought about the evident change in structure, 

 in other words, that modification has accompanied the different needs 

 of the animal. 



The term larva is strictly applicable to the stage following the egg 

 only in the holometabolic orders of insects, although it is often popularly 

 applied to the corresponding stage of the heterometabolic orders. In 

 the older and more generalised orders the larval and pupal stages are 

 not differentiated, and the term "nymph" is now generally used to 

 designate in them all the stages of existence between the egg and the 

 imago. The term is defined by Eaton as " applying to the young of 

 insects which live an active life, quitting the egg at a tolerably 

 advanced stage of morphological development, and having the mouth- 

 parts formed after the same type of construction as those of the adult 

 insect." In this sense the term is used by McLachlan, Cabot, Calvert, 

 Sharp, Packard, &c. Other entomologists, however, still use the 

 term larva for the early stages of the Heterometabola, and restrict the 

 term "nymph" to the stadium preceding the imago. Brauer actually 

 applies the term "nymph" to the holometabolous pupa. 



Every insect after leaving the egg casts its skin a number of times. 

 Each of these castings constitutes a moult or ecdysis. The number of 

 moults varies for different species, although usually fixed for the same 

 species, except in those cases in which it differs in the sexes. With- 

 out entering into a comparison between the gradual succession of 

 ecdyses that the Orthopteran nymph, as representing the Heterome- 

 tabola, undergoes, without any striking change of form until it reaches 

 the imaginal condition, and the succession of larval changes under- 

 gone by the Lepidoptera, as representing the Holometabola, before the 

 pupal stage is reached (when a most remarkable change of form occurs, 

 equalled only by the change from the pupa to the imaginal condition) we 

 may state that the great difference in the two series is that the Lepi- 

 doptera have interposed, between the penultimate and the final ecdyses, 

 a completely quiescent condition, whilst the Orthoptera have no such 

 quiescent condition, their only period of rest being confined to a short 

 time immediately before the final form is assumed, and apparently of 

 the same character as the rest previously indulged in at the preceding 

 ecdyses. Not that all those insects that have a pupa* present an 

 absolutely immobile form as do the more specialised obtect lepidopterous 

 pupa', for those of certain Trichoptera, though quiescent at first, 

 become active just before the final change, and many lepidopterous 

 pupae-incompletaB are capable of considerable movement. 



The intervals between the ecdyses are called "stadia," the first 

 "stadium" extending from the hatching period to the first moult, 



