Xiv INTRODUCTION 
variety of forms on numerous plants. One form is 
illustrated on Plate LXITI, and others are referred to 
elsewhere. 
Of the Cynipide there are-many species. They 
attack various plants, but they are found principally 
on the oak. They have four wings, and to distinguish 
them from other gall producers may correctly be 
known as gall-wasps, and henceforth they will be 
referred to as such. They are briefly described in 
Chapter IV. 
While the Diptera are comparatively free from the 
attacks of other insects, the gall-wasps are not; they 
are extensively preyed upon by parasites, and hyper- 
parasites, and the gall tissue around the larva is often 
the welcome abode of a host of other creatures which 
play the parts of imquilines, commensal tenants, or 
casual occupants. They seldom cause the death of 
the rightful inhabitant except in the case of inquilines 
living gregariously around the larval cell of Cynips 
Kollari. The number of lodgers in oak galls is very 
great (more than 180), a good many occurring in 
several kinds of galls. Hurytoma rose occurs in four- 
teen different galls, Megastigmus dorsalis in eleven, 
Synergus facialis in ten, S. vulgaris in nine, and five 
other parasites each in eight galls. There are many 
more of similar habits. 
It will be found that thirteen species are without 
alternate generations, viz.: Andricus exstivalis, 
amenti, = clementine, = glandium, = glandulez, = 
lucidus, = solitarius, Aphilothrian albopunctata, = 
marginalis, = quadrilineatus, = seminationis, Dryo- 
phanta agama, and = disticha. At present these are 
unknown; and of two other species there is yet some 
doubt. In this, however, our British species are not 
alone. According to Beyerinck the sexual generations 
of twenty-six continental forms are unknown (p. 152). 
A very large number of oak galls occur in Mid- 
European countries. The various species and varieties 
of oak are also numerous, consequently a great many 
