CH. X.] 



THE PLANT-LOUSE. 



177 



the second or upper layer walk freely over the first, 

 and not being able to suck, are diligently employed 

 in bringing forth young. 



These immense societies of suckers must of course 

 drain the leaf, and exhaust the juices of the plant to 

 which they are attached : this is the fact, and, by 

 Leeuwenhoek, they have been truly termed the pests 

 of the garden. However, the effect is occasionally 

 curious enough ; for instead of withering, the parts 

 to which the aphides are attached enlarge or twist, 

 and by so doing furnish shelter to their enemies. 

 The insect chooses the concavity of a shoot, for ex- 

 ample, and this, through loss of juice, being diverted 

 from its straight direction, assumes the shape of a 

 corkscrew ; in the concave folds of this diverted shoot 

 it is that the aphis shelters itself from the weather. 



Reaumur says that this curve takes place on the 

 side from which the insects suck the juices, for the 

 same reason that a piece of wood soaked in water, 



