102 The Galls of Essex ; a Contribution to a 



that surrounds it, becoming considerably larger before it is 

 hatched than it was when first deposited. When chemically 

 analysed, galls are found to contain only the same principles 

 as the plant from which they spring, but in a more con- 

 centrated state." 



Hartig, in his specially scientific memoirs on the cynip- 

 ideous gall-makers, treats somewhat lengthily on the 

 physiology of the gall insects themselves, but on the growth 

 of their productions he says but little, limiting himself 

 strictly to the facts observed. He merely states, " The gall- 

 flies are divisible into three groups, according to then- mode 

 of life. To the first of these belong those species which live 

 some on the plant-juices and some on the cellular tissue and 

 cork (starchy portions). These alone produce, through their 

 oviposition in certain plants and parts of plants, an hyper- 

 trophy of the part from the usual form, and shape the plant- 

 substance into a larva-house, the inner structure and outer 

 form of which is dependent on the producing species of gall- 

 fly. Every true gall-producing gall-fly has its own special 

 form of gall, which is constant within certain limits ; and we 

 never find two similar species of true gall-fly inhabiting 

 different formed galls, nor do we breed from similarly formed 

 galls different species of gall-flies of the first group, which I 

 therefore call Psenides."^ 



To come to the more feasible theory, as expounded by 

 Dr. Holiis, we find that Van der Hoeven started on what may 

 be considered the right road, when, after speaking of the 

 deposition of the egg by the female Cynips, he says, " The 

 irritation thus produced causes the sap to flow in greater 

 abundance to the wounded part."^° Dr. Johnston, of 

 Berwick, marked a still clearer line when he tells us, " We 

 observe that the u*ritation caused by the deposition and 

 evolution of the egg will produce growth of the most curious 

 kind, and differences in the irritation, too slight to be traced, 

 will occasion very remarkable differences in the appearance of 



9 Translated from Germar's ' Zeitschrift fur die Entomologie,' vol. iii. 

 p. 322. Leipzig, 1841. 



^° 'Handbook of Zoology,' translated by ClarK, 1856, i. 384. 



