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786 a On the Origin of the Lac. [November, 
drop of resin on the bark, where a small insect had covered its 
dome-shaped body with a layer of resin, was as thin-walled as 
the shell of a mustard seed. In such a case we are called upon 
to suppose that a flow of resinous juices starts from below the in- 
sect, passes up over its body and nowhere else, and covers it 
with an even layer of resin. This is to me a difficult conception. 
On the other hand, it is easy to conceive how the insect simply 
feeding on the juices from below, and secreting this resinous sub- 
stance from its body, could build such a'shell of resin. 
4. By careful examination of bark and wood, no puncture or 
abrasion could be detected at all adequate to account for such a 
Spontaneous flow of the sap of the plant, as would produce the 
amount of resin present. This examination was repeated with the 
assistance of Prof. Joseph Le Conte, who concurred fully in the 
conclusion arrived at. 
All these facts, so inexplicable on the exudation theory, appear 
to me to be readily explained on the basis of the insect origin of 
the gum. The insect fixes itself to a spot on the bark where it 
lives and dies. For its sustenance it is dependent on the sap of 
the plant. Certain plants are adapted to this purpose, others are 
not. e juices sucked up and absorbed by the insect serve as 
its food, and at the same time as material, from which is elabo- 
rated the resinous envelope, destined to serve as a protection 
for the eggs and larve. This resinous substance may be exuded 
from the entire surface of the insect, or from particular organs OF — 
glands; I am in no position to pursue this point, interesting as it 
is. This elaboration thickens as the insect grows older, and as- 
the insects live in close proximity they become crowded and 
distorted, and the spaces between them compactly filled with their 
_ united elaborations, so that the result is as we see it, a resinous 
mass of coarse, irregular, cellular structure, with the egg-sac fill- 
ing the cell, or, after the specimen is dried or the young escaped, 
| _ with the shrunken remains of sac and eggs in the cell. 
This explains the occurrence of practically the same resin on 
various plants—the form and structure of the resin—that 1t sur- 
rounds the sacs on all sides perfectly, but does not run off along 
the bark of the twig nor collect into solid drops or masses—a- 
fact difficult to explain on the simple exudation theory. It atso 
gives a definite meaning to the “alteration” of the juice by s 
insect referred to in “ Chambers’ Encyclopædia.” 
