1921] The Age of Insects 41 



ties and tiger beetles. Others are quiescent and construct traps for 

 their prey like the ant lions. These curious fellows usually called 

 doodle bugs construct funnel hke traps in loose sandy soil and then 

 take up their position at the neck of the funnel and wait for a luckless 

 ant to tumble in. The ant thus starts a miniature land slide which 

 the ant lion aids by throwing loose sand above the ant. Some are pre- 

 dacious only in the larval stage like the aphis lion or the aphis maggot, 

 while others are predacious both in larval and adult condition like the 

 lady beetles. The amount of food that these predacious insects will 

 consume is often beyond belief. 



"In order to determine something of the capacity of these insects 

 for devouring plant lice and hence their degree of economic impor- 

 tance, I tried feeding the larvae of Syrphus americanus on cabbage 

 aphids {Aphis hrassicae Linn.) The aphids were touched to the mouth 

 of a larva which had not been kept from food. A four-day-old larva 

 devoured the first aphid in 4.5 minutes, a second, third, fourth and 

 fifth, smaller than the first, in 2, 1, 1, and 0.5 minutes, respectively. 

 The sixth, a larger one, was retained for 3.25 minutes. These were 

 very thoroughly eaten, all the viscera and body fluids being picked 

 and sucked out. After this the lice tendered were not eaten so closely, 

 but killed, a seventh in 2 minutes, an eight in 1.75 minutes, and a 

 ninth in 1.5 minutes." An average of less than 2 minutes for each 

 plant louse or 240 per eight hour union day. 



''It is, of course, not probable that any larva would ever normally 

 devour aphids so rapidly. Yet when plenty are at hand the number 

 eaten by a larva during its life of eight days to two weeks or more 

 must be very considerable. It should be kept in mind also that it is 

 not the actual individuals eaten, alone, that determine the amount of 

 benefit from these insects; but the fact that in this way the production 

 of enormous number of aphids is prevented. If, as Reaumour has 

 calculated, and others have substantiated, one aphid may be the pro- 

 genitor of over 5,000,000,000 individuals during her existence of a 

 month or six weeks, we can see at once the important benefit that must 

 arise from the destruction of one or two of these aphids early in the 

 establishment of the colony. It is a fact that the eggs of Syrphidae 

 are often deposited on the host-plant very early or even in anticipa- 

 tion of the arrival of the aphids." (C. L. Metcalf.) 



On the other hand the small amount of food on wdiich these insects 

 seem to be able to survive is almost beyond belief. I shall never for- 

 get a deserted wagon road below Carolina Beech where both tracks 



