174 Bugs, 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 
