HOMOPTERA. 305 



Whilst, however, the attacks of each species of Aphis 

 are confined to one species of plant (or, when more 

 than one, to allied species), there are sometimes two 

 or more species of Aphis found on one kind of 

 plant. No part of a plant is secure from their attacks. 

 They live not only on the exposed parts, but under the 

 bark of trees, and upon roots buried under ground ; they 

 have even been found within the heart of apparently 

 sound fruit. 



The gait of the Aphis, except when upon the wing, is 

 a slow creep, but some species have (in common with 

 the Psyllidse) another mode of locomotion which makes 

 up to them for the want of wings. This is similar to that 

 practised by the "Aeronautic," or Gossamer Spiders, 

 which throw forth long silken threads, and by this 

 means are enabled to float in the air currents to great 

 heights and distances.* The Aphides mentioned above 

 as extruding tufts of cottony substances, have been 

 observed floating in the same manner, and this is pro- 

 bably one of the causes of the sudden appearance in 

 fresh places of swarms of these insects. The apple-tree 

 blight, known to us as the " American blight," and which 

 sometimes nearly ruins the orchard, is one of these. The 

 name has been given under the belief that it has been 

 imported from America, but the Americans retort the 

 accusation upon Europe, and Mr. Harris, an American 

 writer on "Insects Injurious to Vegetation," states that 

 it is rare in his own country, and is supposed to have 



* The Gossamer Spiders and Aeronauts are not of any particular genus 

 or species, the young of many kinds floating in the air by means of these 

 threads, and any Spider which throws out long floating threads of " gossa- 

 mer" being a '* Gossamer" Spider. 



X 



