April, 1904.] Galls and Insects Producing Them. 
131 
her of specific external characters ? Let us first review the struc- 
tural characters of the leaf galls, since these galls show the most 
uniform line of development. Considering Neuroterous irregu- 
laris the gall of greatest simplicit}-, w’ecan formulate the following 
diagram : 
X. irregularis — C. tumifica, 
C. papillatus. 
A. confluentus. 
I — H. centricola. 
A. inanis. 
D. palustris. 
A. petiolicola. 
In X. irregularis the zones are not so well developed as in 
C. tumifica. In C. tumifica the zones are perfect, but in contact. 
In C. papillatus the protective and parenchyma zones are sepa- 
rated, but connected b}' long parenchyma cells. In H. centricola 
and A. inanis the protective and parencli 3 ’ma zones are connected 
by fibro-vascular bundles. In A. confluentus the}* are connected 
both b}’ fibro-vascular bundles and b}' parenchjmia cells (Fig. 
1 21 ). In D. palustris the parenchyma and protective zones are 
not connected. In A. petiolicola the zones are in contact, but 
the tissues are verj* dense, due to location in the petiole of mid- 
rib of the leaf. 
If galls become independent structures the}’ are undoubtedly 
subject to the same laws of natural selection as au}- other group 
of organisms, or if the}’ be considered as parts of the plant they 
must also be subject to the same laws of natural selection as any 
other part of the plant on which they live. How, then, have 
these laws affected the gall? It may be a protective coloration 
against birds and rodents, and other insects, but this cannot be 
very important since many species of galls are very conspicuous. 
Furthermore, animals make but ver}’ little use of galls for food. 
So far I have observed other animals using galls for food but 
once and then birds were tearing open the large galls of Pemphi- 
gus vagabundus and eating the insects. The tannin which devel- 
ops in such abundance in all galls as they approach maturity is 
probably a great protection against insectivorous animals. 
The greatest insect enemy with which the gall insect has to 
contend is the great number of parasites. The size, shape and 
character of the epidermal covering of the gall may be a protec- 
tion against this numerous enemy. The thickness of the gall and 
the density of the tissues, especially the protective zone, is an 
