182 INSECTS AFFECTING THE ORANGE. 



of a pair of terminal prop-legs, aided by a viscid secretion which it 

 voids, and which in drying glues it to the surface of the plant. The 

 body becomes distended and thickened, losing in length what it gains 

 in girth. The skin of the larva is not split or shed, but hardens and 

 forms the puparium, which protects the true pupa within. In the pupa- 

 riurn the shape of the larva is profoundly altered, the body joints are 

 obliterated, the anterior end becomes swollen and broadly rounded, and 

 the form tapers suddenly behind. 



The perfect fly issues by pushing off the convex end of the puparium, 

 which splits at the suture between two of the old larval joints, and re- 

 leases a circular cap, in the shape of a watch-glass. 



The duration of the egg and larva periods in 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 with 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 col- 

 onies appear and disappear; for the life of a colony of Aphis is also 

 very short. Upon the Orange the Aphis can feed only upon the very 

 tender young leaves; in a short time these harden, and then the colony 

 must scatter; but frequently, long before that time, their numbers are 

 reduced almost to extermination by enemies and parasites. As the 

 Syrphus larvae cannot follow the winged insects, they must make the 

 best of their limited opportunities and feed quickly or perish of starva- 

 tion. It is curious to mark how nature in the case of these insects has 

 responded to the necessities of the situation and given their Iarva3 rest- 

 less 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 perfectly organized rivals in the food struggle which takes 

 place over every. Aphis colony. 



Broods, &c. The Iarva3 of these Syrphus flies feed only upon Aphis 

 and depend upon them for their existence. They therefore appear and 

 disappear with the colonies of the latter, and the broods may be sup- 

 posed to follow rapidly one upon another during the seasons 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 foliage 

 in the early spring, depend in a great measure upon the prevalence of 

 rains and vary from year to year, but are usually three or four in num- 

 ber during the year. The colonies of Aphis and likewise their Syrphus 

 enemies are most abundant in June and September. 



Three representatives of the family Syrphida? are found among Aphis 

 on the Orange. They belong to the genus Baccha. 



