HORNETS, WASPS, AND FLIES SUCKING THE SAP OF TREES. 11
strewn thickly with dead and dying wasps. Perhaps, however,
the substance (whatever it is) which the wasps gather in the
bottoms of the cracks has some poisonous or intoxicating effect
upon them; for, whilst engaged in obtaining it, they generally
appear drowsy and semi-stupified. I have never before seen
so many hornets in one year as I have seen this autumn; when,
on almost any day, half-a-dozen at least might be seen at a
time on the trunk of this one tree. I have never seen any species
of lepidoptera round the tree-trunk.
One is bound, I think, to conclude that these hornets and
wasps are busy imbibing the sap of the tree, which, naturally,
they can get at more easily at the bottoms of the cracks in
the bark than elsewhere. But why do they seek the sap of
this one particular tree only, and not that of any of the others?
I have never seen the same assembly on any of the other trees.
Again, why do they restrict their attentions to the stem only?
for I have not seen them at work at a greater height than about
ten or twelve feet, at which height the large lower branches
branch out. The tree is of about the same size and height
as the others, being about 85 feet high and 4 feet in diameter
at 3 feet from the ground. Moreover, it differs from them
in no obvious respect, except that it inclines over to the south-
ward at an angle of 15 or 20 degrees, whilst the others are quite
upright.
Various friends have suggested to me that the phenomenon
is connected with the boring of the larvae of the Goat-Moth
(Cossus ligniperda), which causes trees to exude sap (a phe-
nomenon generally known as "bleeding"). On this exuded
sap, many kinds of insect feed very greedily, especially, no doubt,
when it is in a more-or-less fermented condition, owing to its
exposure to the air. I am satisfied, however, that this is not
the case in connection with this particular elm tree; for I
can detect about it no trace of the well-known and unmis-
takeable smell of the goat-moth larva ; nor can I find, in this
or in any other of the trees, any trace of its borings.
Further, I cannot see about the tree any evidence of bleeding
due to this or any other cause. I think it very probable, how-
ever, that the inclination at which the tree grows gives a strain
to the wood of the trunk, or the bark, or to both, which causes
splitting, thus giving rise fo what one may term "internal