ANATOMY AND MORPHOLOGY OF INSECTS. 23 



in which the developmental changes of the internal organs at 

 least present a more usual character. 



In the most specialised Metabola all the larval organs and 

 tissues are rapidly broken up, and converted into a cream-like 

 pseudo-yelk, which serves as a pabulum, or food-material, for 

 the development of the nymph and imago. 



The nymph is practically a new embryo formed entirely from 

 the imaginal discs and histoblasts of the larva, and the imago 

 may be regarded as the fully-developed nymph after it has 

 undergone a final or, in rare cases, a penultimate ecdysis. 



In many of the Metabola the process of histolysis and 

 regeneration occur simultaneously, and the nymph is already 

 considerably developed before any of the larval tissues are 

 entirely broken up, but in the Diptera (Muscid^) it sometimes 

 happens that all the larval tissues are converted into a granular 

 pseudo-yelk before the nymph is formed, so that on opening the 

 pupa it is found to contain an ovoid capsule filled with pseudo- 

 yelk, in which the imaginal discs still remain un-united with 

 each other. This stage I shall designate the pro-nymph. De 

 Reaumur was the first to observe that a distinct stage occurs 

 in the development of the Muscidae not found in the other Meta- 

 bola. He wrote : ' The worms which become flies with two 

 wings make a shell of their own skin and pass through an 

 extra metamorphosis, which caterpillars do not exhibit, that of 

 an elongated ellipsoid, before they become nymphs' [1, torn. 

 iv., pp. 296, 297]. . . 



Relation of Metamorphosis to Ecdysis.— Dr. Weismann, writmg 

 in 1864, said: 'In recent times it is well known that the 

 change from the larva to the pupa state has been regarded as 

 a moult, differing only in degree. But in the pupa formation 

 of the Muscida; the nature of the change precludes this 

 ' view ' [2, p. 162]. More recent observations nevertheless tend to 

 show, I think very distinctly, that ecdysis and metamorphosis 

 differ in degree rather than in their essential character. The 

 researches of Barrois show that a perfectly parallel series of 

 phenomena occur in the development of the Neinertid worms. 

 Discs are formed as invaginations of the epiblast, which by 



