4 THE L IFE-HIS TOR V OF THE HL O W-FL Y. 



it will be found to contain nothing apparently but a white cream- 

 like fluid ; but on careful microscopic examination some of the 

 imaginal discs will be detected, and many of the muscles of the 

 larva still remain at its posterior end. The imaginal discs are 

 really very numerous ; fourteen were known to Weismann, and 

 about fifty have been discovered since, besides many scattered 

 groups of cells (histoblasts), from which the internal organs 

 originate ; so that there are in all more than sixty separate 

 discs, which subsequently unite with each other, and form the 

 embryonic fly within the pupa-case. This embryonic fly is 

 known as the nymph. 



The Nymph corresponds with the chrysalis of a lepidopterous 

 insect, and has its wings, legs, and proboscis folded mummy- 

 fashion over its ventral surface. 



It is entirely formed from the imaginal discs. All the organs 

 of the larva, without exception, undergo disintegration, and are 

 converted into a creamy fluid — the pseudo-yelk, by which the 

 imaginal discs are nourished. 



The nymph is also developed in a very peculiar manner. 

 Its head is at first enclosed within its thorax, and its thorax 

 is enclosed within its abdomen. Subsequently, the abdomen 

 is drawn back, exposing the thorax, and the thorax is drawn 

 back, exposing the head. 



The Imago, or perfect insect, is the fully developed nymph, 

 and escapes from the pupa-case at the end of from twelve to 

 fourteen days in summer; but in winter the pupa stage may 

 last for months, as all development is arrested by a temperature 

 below 45"" Fahr. The preservation of the species in winter 

 depends mainly upon this circumstance. The fly escapes from 

 the pupa-case by pushing off the operculum, or cap. This is 

 effected by the distension of a large bladder-like organ on the 

 insect's forehead— the frontal sac. When it first emerges it is 

 of a pale ash-gray colour, and very soft ; its wings are moist, 

 thick, and crumpled, and the large frontal sac projects from 

 its forehead. The proboscis and many of the internal organs 

 are in a half-developed condition. In two or three hours the 

 newly-escaped insect, which rapidly increases in size by dis- 



