THE PRINCIPLES OF OAK GALL FORMATION. 5 
tudinal cylindrical cavity, at the lower end of which 
is a little, partly-attached larval cell. This cell is 
undoubtedly amenable to the laws controlling other 
oak galls, but not so the outer structure. That con- 
tinues its growth, throws out buds from which leayes 
of normal size expand, and not infrequently a shoot 
will grow from the apex, upon which, in the next 
summer, a gall of the same species will develop (see 
Plate XI, div. C.) 
The gall of Andvicus curvator, when upon a twig or 
leaf petiole, exhibits similar peculiarities, but not to so 
great an extent; when a leaf is involved in its growth 
there is no comparison. This gall is also bilocular; 
the larval cell, however, is loose within the external 
gall growth, while that of Inflator is attached. 
In the compound gall of Aphilothria feeundatrix 
the inverse is exemplified. The inner gall, when 
pushed out in the late summer by the dying leaf 
scales, falls to the ground, and continues growth in a 
manner similar to that of the Newroterus galls. The 
leaf scales, although dead, persist for many months, 
and only disappear from the twig when pushed off by 
new growth in the following spring. 
The component parts of galls are variable. 
The simplest forms are those of Spathegaster bac- 
carum and Trigonaspis crustalis, in which the tissue is 
very loose, filled with a copious thin fluid, and the 
exterior merely a thin epidermis with chlorophyll. 
After the emergence of the imago it withers and 
shrivels to a condition sometimes beyond recognition. 
The next form is that of galls such as Andrirus 
nodult, A. nudus, Aphilothrie fecundatriz, A. quadi- 
lineatus, A. solitarius, in which the inner layer is 
moderately thick, and is composed of cells containing 
nutritive materials for the larva, and surrounded by a 
thin layer of hard woody cells filled with colour pig- 
ments. The same is found in Aph. globuli, with the 
addition of an outer sappy green rind. Aph. Siebolda 
partakes of the same nature, but the rind, instead of 
