340 THE DEVELOPMENT OF THE NYMPH. 



draws itself out of the pupa-sheath as it escapes from the pupa- 

 case. 



The Pro-imago. It is evident that the separation of the pupa- 

 sheath and of the intima of the tracheae is a virtual ecdysis. 

 From this period the nymph becomes gradually transformed 

 into the imago. I regard the nymph as completely formed as 

 soon as this ecdysis has occurred, but as all the subsequent 

 events are gradual developmental processes, it is impossible to 

 say that either the nymph stage ends or the imaginal stage 

 commences at any time, unless the actual ecdysis of the pupa- 

 sheath and larval skin be regarded as the commencement 

 of the imago state. To avoid circumlocution, it will be exceed- 

 ingly convenient to term the nymph, from the shedding of the 

 pupa-sheath to the escape from the pupa, the pro-imago, and 

 to regard all those parts which exist at the time of the separa- 

 tion of the pupa-sheath as parts of the nymph, and all those 

 subsequently developed as parts of the pro-imago. 



For example, I shall speak of the tracheal system as it exists 

 before this period of virtual ecdysis as the tracheal system of the 

 nymph, and the new tracheal system which is subsequently 

 formed as the tracheal system of the pro-imago. 



k. The Dorsal Vessel and Ccelom. 



The dorsal vessel of the imago is apparently developed 

 directly from that of the larva. At every stage of development 

 I have always found the dorsal vessel intact. 



Herold [135] observed that in the Lepidoptera the pulsations 

 of the dorsal vessel continue throughout the whole period of 

 the pupa sleep. Newport confirms this in Sphynx ligustri, as 

 he asserts that the vessel pulsates throughout the whole pupa 

 stage, although its beats almost cease during hibernation, a 

 period, however, in which development is also at a standstill. 

 Kiinckel d'Herculais saw the heart beating in the pupa of 

 Eristalis until the eighth or ninth day, after which he states 

 that it stops or beats feebly for a day or two, but that after- 

 wards it pulsates regularly. 



