The Agents concerned in the Production of Galls 37 



VI. The Hemiptera-Homoptera, or Aphides. 



The ravages of several species of Aphides, commonly termed 

 Green-fly and Plant-lice, are only too well known by floriculturists, 

 professional and amateur. There are, however, a very large 

 number of species of aphides, all of which live on plants, and 

 very few genera are exempt from their attacks. 



The life-history of an aphis is exceedingly interesting, but 

 it is too lengthy to be given here. 



The body is divided into three sections — head, thorax, and 

 abdomen. 



The most noteworthy organ of the head is the rostrum. 

 It is a long, hollow bag, and contains three exceedingly fine 

 lancets. With them the insect punctures the plant, and 

 sucks the sap through the rostrum. In some species it is 

 short, but in the majority it is long. It projects from the 

 head, and when not in use is recurved beneath the body. 

 The long rostra project beyond the body and produce the 

 appearance of a tail. 



Some species of aphides have neither eyes nor cornicles, but 

 these features are almost entirely confined to subterranean species. 



The wings are very large in comparison with the body. 

 When in flight a compound hamulus on the costal margin of 

 the hind wing fastens in a fold of the posterior margin of the 

 fore wing, keeping both in the same plane. The legs are long 

 and thin ; in some species smooth, in others hairy. 



The chief external feature of the abdomen is a pair of 

 cornicles ; they are of a horny nature, diverse in form and size, 

 upright and mobile, tubular in form, and they act as excretory 

 ducts. The fluid which exudes from them is known as honey- 

 dew. Within the abdomen a substance is secreted in certain 

 glands, from which it exudes in the form of a flocculent sub- 

 stance, often entirely covering the insect. 



One remarkable feature about aphides is that manyspecies have, 

 in addition to mature winged c?c? and winged ? ?,an alateform. 



VII. The Heterocera, or Moths. 

 Very few indeed are the species of moths the larv^ of 

 which are known to produce galls. Not half a dozen can be 

 enumerated. At present five only are known. None occur 



