August, ’24] 
467 
BLACKMAN: HICKORY BARK BEETLE 
backward. When the egg is placed in the niche she creeps out of the 
burrow, turns around and enters it head first, packs the egg with frass 
and resumes her burrowing. 
Such being the habits of the adult beetles it is, I think, very evident 
that a deficiency in rainfall during the time when they are on the wing, 
feeding on the leaf petioles, starting their brood galleries and laying the 
eggs, would be very much to their advantage. Conversely, an excess in 
precipitation at these times would result in the destruction of many of 
them. A deficiency in rainfall during late June and throughout July 
would work very much to the advantage of the beetles and an excess 
during this time would have the reverse effect. Departures from the 
normal during July—especially during the first half of the month, would 
be of greatest importance in their direct effect upon the adults. Bv 
consulting the accompanying table it will be seen that from 1908 to 
1913 inclusive there was each year a deficiency in precipitation during 
the month of June. In July-—the most important month in its effect 
upon the adult beetles—the deficiency in rainfall extended from 1908 
to 1914 inclusive. 
But it is the opinion of the writer that rainfall not only exerts this 
direct influence upon the adult beetles, but also, through the effect of 
excess or deficiency upon the physiological processes of the tree, exerts 
a perhaps greater effect upon both adults and larvae. A deficiency in 
rainfall would mean a lessened amount of moisture in the soil available 
for absorption by the root hairs. Such a deficiency also would be ac¬ 
companied as a rule by a greater amount of sunshine and a decreased 
amount of humidity in the atmosphere. Thus at such times there 
would not only be a lessened amount of moisture available to the tree, 
but there would be an increased loss of moisture due to more rapid 
evaporation from the leaf surface. This would result in a lessening of 
the moisture content of the tissues of the tree and a decrease in the sap- 
pressure. It has been established by recent investigations of McDougal 
by means of the dendrograph, that during periods of sunshine when the 
loss of evaporation through the leaf surface is greatest, there is an actual 
measurable decrease in the diameter of a living tree, and conversely 
during periods of cloudiness an actual increase in the diameter. This 
can only mean that the effect of the cloudy weather is an excess in the 
moisture content of the tree, resulting in a swelling or turgidity of its 
component living tissues. Thus an insect, such as either the adult or 
larva of the hickory bark-beetle, boring through the living tissues of a 
tree would in wet cloudy weather release into its burrow a much greater 
