174 B u g s > Cicadas, Aphids, and Scale-insects 



Characteristic variations in the general course are described later in 

 connection with the accounts of the few particular aphid species for which 

 we have place, but it should be kept in mind that considerable variations 

 may occur in the case of a single species. Extrinsic influences, such as 

 crowding a host-plant and hence the lessening of food, or an unusual 

 humidity or lack of humidity, an early lowering of temperature in autumn, 

 etc., seem to be very potent in producing or acting as effective stimuli for 

 adaptive variations of the usual course of life. Slingerland reared ninety- 

 four successive generations (in 

 four years) of an aphid species 

 in the insectary at Cornell 

 University under such constant 

 conditions of food-supply and 

 summer temperature that not 

 a single winged aphid nor single 

 sexual generation was pro- 

 duced. Even longer series of 

 identical wingless agamic gen- 

 erations have been obtained by 

 certain European experiment- 

 ers. Clarke, in California, has 

 been able to produce a winged 

 generation at will by simply 

 changing the chemical constitu- 

 tion of the sap of the host- 

 plant on which the aphids were 

 reared in his laboratory. 



In addition to the interest- 

 ing variation as regards wings 

 and reproductive processes 

 among the various individuals 



of a single aphid species, it has been found that of the wingless males some 

 have no mouth, while others are furnished with functional mouth-parts 

 and opening. An interesting physiological variation also occurs in the 

 matter of the food-plant selected. The winged individuals frequently 

 migrate to a plant of different species from that on which they were born. 

 For instance, the apple-aphids, Aphis mali, "spend the summer upon grasses, 

 where they continue breeding until autumn, when they return to the apple and 

 the winged females establish colonies of the wingless egg-laying form upon 

 the leaves. The males fly in from the summer host-plant; the eggs are 

 then laid on the twigs and buds, and the cycle for the year is completed." 

 The common cherry-aphis, Myzus cerasi, has a similar history, described by 



FIG. 247.- 



-Rose-aphids visited by ants, 

 size ; from life.) 



(Natural 



