61 



The perfect fly issues by pushing ott the convex end of the puparium, which splits at 

 the suture between two of the old larval joints, and releases a circular cap in the shape of 

 a watch-glass. 



The duration of the egg and larval periods of these aphis-eating flies is short; the egg 

 hatches in forty-eight hours after it is laid, and the larva becomes full grown and forms its 

 pupa in five or six days. About ten days, the average time of insects having many broods, 

 are passed in pupa. The reason of this extremely rapid development in the first two stages 

 — the egg and larva — becomes obvious when we consider how brief is the existence of the 

 aphis itself, and how suddenly its colonies appear and disappear — for the life of a colony 

 of aphis is also very short. Upon the orange the aphis can feed onlj^ on the verj^ tender 

 young leaves: in a short time these harden, and then the colony must scatter; biit fre- 

 quently long before that time their numbers are reduced almost to extermination by ene- 

 mies and parasites. As the syrphus larvre cannot follow the winged insects, they must 

 make the best of their limited o])portunities and ffeed quickly or perish of starvation. It 

 is curious to mark how nature in the case of these insects has responded to the necessi- 

 ties of the situation and given their larvpe restless activity, great rapacity, and destructive 

 powers, notwithstanding their slow locomotion, and also a remarkably brief egg period, so 

 that this wingless, blind, and almost legless maggot is enabled to compete with more per- 

 fectly organized rivals in the food struggle which takes place over every aphis colony. 



Broods, etc. — The larva3 of these sj-rphus flies feed only upon aphis, and depend vipon 

 them for their existence. They therefore appear and disappear with the colonies of the 

 latter, and the broods may be supposed to follow rapidly one upon another during the sea- 

 sons of growth, when the appearance of new shoots upon the orange gives support to 

 numerous colonies of aphis. 



The seasons of growth in the orange, after the renewal of the folage in the early spring, 

 depend in a great measure upon the prevalence of rains, and varj^ from year to year, but 

 are usually three or four in number during the year. The colonies of aphis, and likewise 

 their syrphus enemies, are most abundant in June and September. 



Three representatives of the family Sijrphida are found among aphis on the orange. 

 They belong to the genus Baccha. 



