174 WILLIAM BRODBECK HERMS 



twenty-four to thirty-six hours after pupation, and on the third 

 day reorganization from the imaginal discs is already in rapid 

 progress. 



d. The imago stage, which is reached on the emergence of the 

 full-grown fly from the pupa case. At this stage the reproduc- 

 tive products are not fully mature, except possibly in the male. 

 In the female the ova are not ripe for from nine to twenty-one 

 days in Lucilia caesar and from twelve to thirty-six days in Calli- 

 phora vomitoria. This difference in the time required for ripen- 

 ing the reproductive products influences the success with which 

 breeding may be carried on inter se in the same brood. 



e. The adult stage, which may be said to be that part of the 

 imago's existence that is characterized by sexual maturity. In 

 the male imago this is arrived at after perhaps a few hours and 

 in the female after from nine to thirty-six days. 



The question has frequently been asked : How long does a fly 

 live? The following obervations may aid in answering this ques- 

 tion. It should be remembered, of course, that flies may hiber- 

 nate in the early imago or even the adult stage for several months, 

 though they may at any time be rendered active by exceptional 

 warmth. The writer saw specimens of Lucilia caesar active out 

 of doors in Cambridge in the middle of January. This hibernation 

 causes the life of a fly to be greatly prolonged, possibly to six 

 months, but the life time of an individual should be considered 

 primarily from the standpoint of its active life. My records show 

 considerable variation in this stage for both males and females of 

 the two observed species. The observations as recorded do not 

 warrant the belief that the female fly has. a longer lifetime than 

 the male. It may be seen from table 1 that the usual length of 

 life of the imago is about thirty days, regardless of sex. The 

 oldest individual (Calliphora vomitoria) lived as an imago 63 

 days. 



