The Agents concerned in the Production of Galls 35 



there are more, which will become known as the knowledge 

 in this department of natural history increases. 



The beetles deposit their ova in various parts of the plants, 

 including the roots, stem, flower-head, and seed-cases. They 

 are mostly unilocular galls, and are not of any very great 

 size. 



The family of the Weevils embraces almost all the gall- 

 makers, and in point of size the imagines themselves are not 

 large. Saperda poptdiiea is the largest and the principal species. 



IV. The Diptera, or Flies (with Two Wings). 



The order of Diptera is represented amongst the gall-making 

 creatures by about 105 species. They are usually spoken of as 

 Gall-gnats and Gall-midgcs. The most renowned of them all 

 is the formidable and dreaded yet diminutive insect, the Hessian 

 fly {Cecidoinyia destructor Say.). So much has been written 

 about this destructive fly that we forbear to add any further 

 remarks concerning it. 



The sub-genus of Cecidomyia Lw. embraces the largest 

 number and the smallest in size of the British Diptera, and 

 also several species whose natural life is numbered by a few 

 hours only. The sub-genus Diplosis Lw. contains about twelve 

 gall-making gnats, the most interesting species being Diplosis 

 tritici Kirby (the wheat-niidge). The sub-genus Hormomyia 

 Lw. contains about ten species. The larvae of Hormomyia 

 piligera Lw. produce one of the prettiest of the small galls. 

 The remaining sub-genera and families (for which see Tables 

 at end of volume) embrace fifteen other species. The largest 

 of the gall-making Diptera is Lasioptera riibi. 



The gall-gnats have very small bodies, with relatively long and 

 broad wings, which are narrowed at the point of attachment to 

 the thorax and rounded at the distal end ; they are generally 

 covered on both surfaces with exceedingly fine hairs. The 

 anterior margin of the fore wing is edged with hairs ; the 

 posterior margin having attached to it a number of small scales, 

 similar in shape and appearance to lepidopterous scales. Owing 

 to the presence of hairs, the wings are highly iridescent A 

 large number of the hairs, and almost all the scales, are lost 

 after the fly has been on the wing for some time. 



