THE PROBLEMS OF TRANSFORMATION 259 



to notice how greatly the pupae of the Endopterygota vary 

 in their powers of movement. Where, on account of con- 

 ditions of life, such as those of the gnat -pupa (p. 197), swim- 

 ming in the water and using the surface-film for breathing, 

 a considerable degree of mobility is necessary, the pupa may 

 be quite active throughout its term of existence. The caddis- 

 pupa remains quiescent and submerged within the shortened 

 larval house, but in the end it has to bite its way out with its 

 strong mandibles and rise through the water that the fly may 

 emerge into the air. The pupa of a snake-fly (Raphidia), of 

 primitive standing among the Neuroptera, has a period of 

 activity before the final moult. And, it has been indicated 

 (p. 140) how among the Lepidoptera, the pupa has a consider- 

 able power of motion in the lower-grade families, making its 

 way partly out of the cocoon before the emergence of the moth, 

 while in the higher families it is quiescent except for restricted 

 twitchings of certain abdominal segments. Among such 

 comparatively primitive terrestrial Diptera as the crane-flies 

 (Tipulidae) we find that the pupa may work its way upward 

 through the soil, so that the developed fly can emerge into 

 the air, leaving the empty and shrivelled pupa-cuticle 

 protruding from the surface of the earth wherein the larva 

 found shelter and food. In the highly-specialized muscoid 

 Diptera, on the contrary, the pupa remains quiescent within 

 the hardened larval cuticle (or puparium) from which as well 

 as from the pupal cuticle the fly, when developed, has to 

 make its escape. 



Considering the general indications afforded by these facts 

 it seems clear that the pupa is a specialization from an originally 

 active pre-imaginal stage in the life-history of primitive 

 insects, and that we see manifestations of this activity apparent 

 to some extent in certain primitive families, or in cases where 

 such a mode of behaviour is suitable to the special life-con- 

 ditions of the pupal period. The quiescence of the pupa 

 is most marked where there is the greatest divergence between 

 imago and larva, and it has already been pointed out that such 

 a resting stage is essential for the reconstruction that must be 

 effected, on account of this divergence, at the end of larval 

 life in preparation for the strikingly different structure and 

 habits of the winged adult. 



