296 THE DEVELOPMENT OF THE NYMPH. 



1. THE FORMATION OF THE PRONYMPH FROM THE 



LARVA. 



From the commencement of the pupa stage to the end of the second or 



middle of the third day* 



The Paraderm. I shall show hereafter that the whole integu- 

 ment of the nymph is developed from the epiblast of the 

 imaginal discs, but after the larval tissues have undergone 

 histolysis (see p. 22), and are converted into a cream-like 

 pseudo-yelk, the imaginal discs, which are as yet concealed in 

 their provisional capsules, are united with each other by a 

 cellular membrane, which encloses the pseudo-yelk. Van Rees 

 regards this as the larval hypodermis ; but the larval hypo- 

 dermis has long before undergone complete histolysis, and the 

 cellular covering of the pseudo-yelk is a new formation of 

 parablastic origin. It is a temporary structure, destined to be 

 replaced by the epiblast of the discs. I have therefore termed 

 it the paradcrm. 



The Pronymph. The parablastic sac enclosing the pseudo- 

 yelk is Reaumur's ovoid stage, my pronyrnph. When the 

 paraderm has been replaced by the ectoderm of the discs, 

 the nymph is fully formed, and takes the place of the pro- 

 nymph. Moreover, a cuticular layer is shed at this period, 

 constituting the pupa-sheath of Weismann, which corresponds 

 with the hard covering of the nymph in the Lepidoptera and 

 other insects with obtectate nymphs (see p. 20). 



The changes which occur before the formation of the pupa- 

 sheath correspond nearly with those which take place in the 

 caterpillar before the ecdysis of the last larval skin, whilst 

 those which occur after its formation correspond with the 

 changes which take place in the chrysalis, or nymph stage of 

 the Lepidoptera. 



If these views are correct, Reaumur's idea that the Diptera 



* The dates from the commencement of the pupa stage given in this 

 work are much longer than those given by Van Rees, but correspond pretty 

 closely with those of Weismann's memoir. They are only approximate, as 

 so much depends on temperature. 



