THE MAKING OF THE PERFECT FLY 29 



a horse's bedding, or appearing in small clusters in the 

 corners of the manure-heap. Inside the pupa-shell, the 

 gradually maturing imago has withdrawn itself into a 

 tiny rolled oval body ; and this body develops six legs, 

 two wings, a head separated from the thorax by a short 

 neck, all packed within the chrysalis-case, but sheathed 

 by a protecting membrane like the mummy-clothes. 

 Thus the chrysalis remains for five days, taking no food, 

 but occupying the whole time in changing from a 

 maggot to a fly. This pupa-stage may be delayed for 

 several days according to the temperature, like the other 

 stages of the metamorphosis. Howard states that the 

 average pupa-life is five days, but Newstead gives the 

 period as from five days to seven days where there is 

 heat produced by the fermentation of the manure sur- 

 roundings ; while the period may be prolonged to 

 fourteen, twenty-eight, or even more days when the 

 weather is cold. So the fly may be said to be affected 

 by the climate in all stages of its life, and will hibernate 

 during all or any period of its metamorphosis or during 

 its subsequent career if the weather is cold. But when 

 the spring is come and summer advancing, hibernation 

 ceases and its life begins. 



Within the brown barrel-shaped mummy-case or 

 shell (puparium) the perfect fly is formed. The head, 

 with its two compound eyes, develops inside the pupa- 

 rium. Between and in front of the eyes there appears 

 a membranous bag or sac which the fly can inflate at 

 will. According to Hewitt, when the time has come 

 for the perfect insect to emerge from the chrysalis, this 

 bag is inflated and the inflation bursts the pupa-case. 

 A circular split occurs in the shell, and the fly pushes 

 itself out from within. First the head, with its antenna?, 

 proboscis, and eyes, is protruded ; then the forelegs are 



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