CHAPTER VI 

 GALLS INDUCED BY PLANT-LICE (HOMOPTERA) 



THE order Hemiptera comprises insects provided with 

 a mouth specially adapted for piercing the tissues and 

 sucking the sap from the plants on which they feed. It 

 contains two suborders, Heteroptera and Homoptera. In 

 the insects classed under the former the anterior wings are 

 of unequal consistency, and the front of the head does not 

 touch the coxae ; in those of the latter the anterior wings 

 are homogeneous, and the front of the head and the coxae 

 are in contact. With the Heteroptera we are not concerned ; 

 two representatives of the family Tingidae — viz., Copium 

 clavicome Linn, and C. teucrii Host. — deform the flowers of 

 Teucrium chamaedvys and T. montanum on the Continent, but 

 I am not aware of their occurrence in Britain. Three 

 families of the Homoptera contain gall-causing insects — ^the 

 Apfiidae, or " green-fly " ; the Psyllidae (springing plant-lice 

 or leaf -fleas) ; and the Coccidae (scale insects and mealy- 

 bugs). 



The majority of homopterous gall-causers are Aphidae. 

 Most people are acquainted with them under the names of 

 "blight" and "green-fly." These insects are remarkable 

 for the enormous production of young by parthenogenetic 

 females, and the rapidity with which the young themselves 

 attain the same function ; within a summer the progeny of 

 a single individual is almost innumerable. Huxley calcu- 

 lated that the produce of a single Aphis would, if all the 

 individuals survived, in the course of only ten generations 

 "contain more ponderable substance than five hundred 



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