PLANT-EATING INSECTS 209 



need for protection. The garrison is withdrawn, the ants 

 going away in search of other, younger flower-heads." 



Many bugs, as well as all kinds of aphides and scale 

 insects, subsist upon sap. We have seen that their mouth- 

 parts have been changed into piercing and sucking organs, 

 whereby they are able to puncture the cuticle of leaves 

 and stems, and absorb the sweet juices of the plant. One 

 would imagine that such a method of feeding would in- 

 variably conduce to a sedentary condition of life ; but this 

 is by no means always the case. Many species of aphides 

 migrate periodically from one kind of plant to another 

 without any apparent reason. The life-cycle of a typical 

 aphid may be roughly summarised as follows : Eggs are 

 laid in the autumn. From these, in the following spring, 

 young nymphs hatch, which are all females. They rapidly 

 complete their metamorphosis, and when adult produce 

 living young, which are also all females. Successive 

 generations of viviparous females appear, some being wing- 

 less, while others are winged and capable of flying to 

 other plants. The final generation of the year comprises 

 both males and females — the latter laying the eggs which 

 are destined to start the next year's attack. The appear- 

 ance of winged individuals is often correlated with a 

 remarkable change of habit, these forms migrating to 

 plants which may be of a quite different kind from those 

 on which the wingless broods were reared. Thus the 

 winged females of the hop aphid often fly in autumn to 

 sloe, damson, or plum trees, on the twigs of which they 

 deposit their eggs ; while the nymphs which hatch from 

 these eggs ultimately attain wings and fly back to the 

 hop, where they reproduce living young for ten or twelve 

 generations before fresh winged forms are developed. 

 Similarly, an aphid of the apple, after producing several 

 wingless generations, gives rise to winged individuals that 



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