STEUCTIjEE axd biology of schizoneuea lanigeea. 657 
S. lanigera some poisonous substance is secreted into the 
wound. Riley (1879) is inclined to the view that the salivary 
g’lands in Phylloxera vastatrix may produce the necessary 
irritating substance ; but the same argument used above may 
be applied in this case, for well-developed salivary glands 
are present in aphids which do not produce galls. Glrove 
(1909) has described salivary glands in Siphonophora 
r os arum which closely correspond to those of Schizoneura 
lanigera, yet the former species does not produce galls on 
the host plant. It may be however, that the histology of 
the salivary glands of those species which do not produce 
galls differs from the species which cause galls, and that 
in the latter case a special ferment is produced which is 
not present in the non-gall-producing species. 
Blomfield (1906) has described in some detail the origin 
and structure of the cankerous growths produced by woolly 
aphis. From sections through diseased galls he has shown 
that the effect is due to some undue influence acting on the 
cambium cells. He considers that the factor of mechanical 
irritation is not the important one, and suggests that a 
ferment substance is possibly produced by the salivary 
gland.'!, but he failed to establish proof of this suggestion. 
Kiinckel (1867, p. 45), who inoculated plants with the 
extract of salivary glands of some Hemiptera, found it was 
innocuous. 
The great damage to infected trees is caused by the fact 
that the soft, spongy tissue comprising the gall-like swellings 
gradually hardens and then cracks. These cracks enlarge 
owing to changing -weather conditions, and thus allow the 
entrance of spores of the canker fungus (Xectri a ditissi m a), 
as pointed out by Blomfield (1905) and Theobald (1909), 
and observed by the author. The question as to whether 
the product of the salivary glands exerts the influence on 
the wounds suggested by Riley and Blomfield is not at present 
established. 
The photogiaphs (Text-figs. A, B, C, and D) reproduced in 
this paper show parts of a young apple tree taken from a tree 
