METAMORPHOSIS 1 65 



" nymph " has completed its development, the creature having 

 then again taken on a definite form which apparently corre- 

 sponds to the pupa of Hymenoptera. Great doubt, however, 

 exists as to this equivalence, and indeed as to any exact corre- 

 spondence between the metamorphic stadia of different Insects, 

 a view which long since was expressed by Sir John Lubbock ^ 

 and Packard. The term nymph is used in this case not because 

 there is any resemblance to the condition similarly named in 

 Insects with less complete metamorphosis, but because the term 

 pupa is applied to the outer case together with the contained 

 nymph. The transformation of the nymph into the perfect blow- 

 fl.y occupies a period very variable according to the temperature. 



Histolysis. — The processes by which the internal organs of 

 the maggot are converted into those of the fly are of two kinds, 

 — histolysis or breaking down, histogenesis or building up, of 

 tissue. The intermediary agents in histolysis are phagocytes, 

 cells similar to the leucocytes or white corpuscles of the blood : 

 the intermediary agents in histogenesis are portions of tissue 

 existing in the larval state incorporated with the different organs, 

 or preserving a connexion therewith even when they are to a 

 great extent separated therefrom. In this latter case they are 

 called imaginal discs, though Professor Miall prefers to term them 

 imaginal folds.^ The two processes of histolysis and histogenesis, 

 though to some extent mutually dependent (for the material to be 

 built up has to be largely obtained by previous destruction), do not 

 go on pari passu, though they are to a great extent contemporaneous. 

 In the resting larva histolysis is predominant, while in the nymph 

 histogenesis is more extensive. Microscopic observation shows 

 that the phenomena connected with the histolysis of the mus- 

 cular tissue are scarcely distinguishable from those of an inflam- 

 matory process, and Viallanes ^ dilates on this fact in an instruc- 

 tive manner. The phagocytes attach themselves to, or enter, the 

 tissues which are to be disintegrated, and becoming distended, 

 assume a granular appearance. By this pseudo-inflammatory pro- 

 cess the larval structures are broken down into a creamy substance ; 

 the buds, or germs, from which the new organs are to be devel- 

 oped being exempt from the destruction. These buds, of which 

 about sixty or upwards have already been detected, undergo 



1 Trans. Linn. Soc. xxiv. 1863, p. 65. = Trans. Linn. Soc. "Zool." v. 1892, p. 267. 

 3 Ann. Sci. Nat., Series 6, "Zool." xiv. 1882, p. 150. 



