1 64 METAMORPHOSIS 



example. We will briefly consider the information that has been 

 obtained on this subject. 



The development of the embryo in the egg of the blowfly is 

 unusually rapid, occupying only a period of twenty to twenty- 

 four hours. After its first moult the blowfly larva grows rapidly 

 during a period of about ten to fourteen days, during which it 

 undergoes moults, the number of which appears not to be 

 definitely ascertained. After becoming full-fed the larva loses 

 its active state, and passes for a period into a condition of com- 

 parative quiescence, being spoken of in this state as a resting 

 larva. This quiet period occurs in most full-grown larvae, and 

 is remarkable for the great variation that may occur in its 

 duration, it being in many Insects subject to prolongation for 

 months, in some cases possibly even for years, though in favour- 

 able circumstances it may be very short. Lowne informs us that 

 in the blowfly this period of the life is occupied by very great 

 changes in the internal organs, which are undergoing very exten- 

 sive processes of destruction and rebuilding. After some days 

 the outer skin of the resting larva shrivels, and is detached from 

 the internal living substances, round which it hardens and forms 

 the sort of cocoon or capsule that is so well known. This 

 using of the cast larval skin as a cocoon is, however, limited to 

 certain of the two-winged flies, and perhaps a few other Insects, 

 and so must be considered an exceptional condition. The capsule 

 conceals from view a most remarkable state, known to the old 

 naturalist Reaumur as the " spheroidal condition," but called by 

 more recent writers the pronymph. The pronymphal state 

 may be looked on as being to a great extent a return of the 

 animal to the condition of- an egg, the creature becoming an 

 accumulation of soft creamy matter enclosed in a delicate skin. 

 This spheroidal condition, however, really begins in the resting 

 larva, and Van Eees and others think that the delicate membrane 

 enclosing the substance of the pronymph is really the hypodermis 

 of the integument of the larva. Although this seems probable, 

 from the resemblance this condition would in that case present 

 to the phenomena usual in ecdysis, it is not generally admitted, 

 and there is much difficulty in settling the point. Lowne is of 

 a contrary opinion, looking on the limiting membrane as a sub- 

 sequent formation ; he calls it the paraderm. The process of 

 forming the various organs goes on in the pronymph, till the 



