112 The Galls of Essex; a Contribution to a
Reaumur's 12th Memoir22 might be consulted with advantage,
and Malpighi23 should also be referred to.
The natural history and affinities of the gall-producers
come more within the field of my own entomological studies;
but before entering upon that branch of my subject, I should
like to call attention to the "individuality," so to speak, of a
gall. Dr. Mayr has already recorded the fact (and I have
myself frequently noticed the same thing) that when such
galls as those of Aphilothrix radicis, Andricus terminalis, or
Dryophanta folii are detached from their root, bud, or leaf,
and supplied with moisture, or kept on damp earth or sand,
they grow considerably. This is probably owing to the
swelling of their cells and the attenuation of the cell-walls.
The oak spangle galls (Neuroteri) nearly double in size, and
almost the whole growth of the inner larva-cell takes place
when the gall is on the ground, after its fall from the leaf;
this must greatly alter or modify the individual cellular
structure.
Two animal sub-kingdoms include species which are
vegetable gall makers, although these structures are mostly
the work of insects. Two or three species of Anguillulidae are
known as gall-producers; these belong to the Annuloida.
Among the Arthropoda nearly all the species of the somewhat
abnormal but large genus Phytoptus, belonging to the Acaridea
(or Mites), are gall-makers, and many of them produce some
of our commonest plant-deformities (e. g., on the birch, lime,
maple, sycamore, alder, sallow, blackthorn, &c.).24 For some
time these were believed to be but true vegetable growths,
and were described as Fungi. Of the nine orders belonging
to the class Insecta, few contain gall-makers—Collembola,
Thysanura, Orthoptera, and Neuroptera being the exceptions.
22 'Memoires pour servir a l'histoire des Insectes,' par M. de Reaumur.
12th Mem. 'Des Galles des plantes et des arbres.' Vol. iii., pp. 413—
532; pl. 31—47 (Paris, 1737); vol. iii., pt. ii., pp. 176—333; pl. 34—47
(Amsterdam, 1738).
23 'Anatome plantarum,' Marcelli Malpighii. Part ii. 'De Gallis';
pp. 22—50 ; pls. vi.—xxi., figs. 7—74 (London, 1679).
21 See 'Economic Entomology.' By Andrew Murray, F.L.S. Aptera,
pp. 331—374. (London, n. d. [1877]).