THE DEVELOPMENT OF THE NYMPH. 293 



the whole range of zoological science in the present half of 

 the nineteenth century. 



The only observations of importance made previously to the 

 appearance of Weismann's exposition occur in De Reaumur's 

 seventh memoir in the fourth volume of his ' History of 

 Insects '; and as these appear to me to possess a very high 

 interest in relation to the most recent views propounded by 

 Van Rees, I shall give an abridged translation of Reaumur's 

 statements. He says : 



' I have spoken elsewhere of those worms which are nourished 

 in the intestines of the horse, and which only leave them when 

 they are about to be transformed into pupte. These insects 

 remain within the pupa -shell much longer than the Flesh 

 flies, and are much longer in becoming flies. I opened the 

 shell of some eight days after the transformation, and was 

 able to withdraw the whole insect from the shell without 

 injury. In these I could observe neither wings nor legs, nor 

 any of the parts proper to a nymph; they presented the 

 appearance of elongated ovoids. . . . Probably all those flies 

 which form the pupa-shell from the larval skin undergo this 

 metamorphosis previously to becoming nymphs. I term it the 

 spheroidal or ellipsoidal metamorphosis.' 



He continues : ' With much address and patience, one may 

 convince himself that this occurs in the Flesh flies,' and ' the 

 worms therefore pass through a metamorphosis which is addi- 

 tional to that which caterpillars and the larvae of most of the 

 four-winged flies undergo.' If we open the pupa-case of one 

 of the Flesh flies five or six days after the transformation of 

 the worm, we find a well-formed white nymph, provided with all 

 the parts of a fly; the legs and wings, although enclosed in 

 sheaths, are very distinct. The sheaths are thin, and conceal 

 nothing. The proboscis of the fly is seen lying upon the 

 corselet and the lips, the aguillon and its sheath are distinct ; 

 the head is large and well developed, and the compound eyes 

 are very recognisable. But how has our insect quitted its 

 second form, the spheroid stage, to take this third form, that of 

 the nymph ? 



