HYMENOPTERA OAK GALLS. 131 



considered, are as follows: "Antennae 13- to 15-jointed; wings few 

 nervures ; palpi short ; head small and transverse ; thorax thick, 

 oval ; abdomen mucli compressed ; peduncle short ; antennte inserted 

 middle of face." 



Hartig was the first to restore order and improve the classification. 

 He separated the true gall-makers from the various species of parasites, 

 and then subdivided the parasites into true parasites, inquilines or 

 guest-flies, and commensals. 



Thus a true parasite preys directly on the larva of the gall, and after 

 undergoing its own metamorphosis finally kills its host. Inquilines or 

 guest-flies prey on the substance of the gall, and, as a rule, indirectly 

 kill the gall-forming insects. Commensals are generally found in large 

 galls, living as lodgers in the substance of the gall, l)ut do not kill the 

 insect. 



These statements are corroborated by Eatzeburg in his ' Forst- 

 insekten,' published in 1844. Afterwards we find several workers 

 in the field : Bassett, a Canadian, 1865 ; Riley, an American, 1873 ; 

 Adler,! a German, 1875; and Cameron of Mancliester, 1892. The 

 work of the last-mentioned writer was published by the Eay Society 

 in 1892. 



Before dealing with galls from a purely entomological standpoint, it 

 may be interesting, in view of the scientific importance of the subject, 

 to consider galls botanically, in so far as their morphological structure 

 and origin are concerned. With regard to morphology, it must be 

 borne in mind that a gall is not a structureless tumour, but a dis- 

 tinctly differentiated structure, and, as a subject, the histology of gall 

 structure would prove an interesting study in itself. 



Firstly, we have galls of cellular tissue, but with no differentiation of 

 separate layers, as, for example, the currant gall or Spathegaster hacca- 

 Tum. Secondly, galls similar to these, but containing an inner gall, 

 as, for instance, the leaf-twisting gall caused by Andrlcus curvator. 

 Thirdly, galls developing vascular bundles throughout the cellular 

 tissue, with distinct epidermis «s well as diflerentiated inner galls, as, 

 for example, the large root-gall caused by Aphilothrix noduU. Fourthly, 

 galls, as, for instance, the marble gall, having a complicated structure. 



1 This work was translated by Dr Stratton, and published by the Oxford 

 Clarendon Press in 1894 ; and I beg to acknowledge my indebtedness to this 

 imi^ortaut work for much of the foregoing, and also for many of the statements 

 which follow. 



