HATCHING OF GIPSY-MOTH EGGS. 5 
might have and at the same time expose them fully to all actions of 
the elements. The second was an endeavor to protect the clusters 
from wind and rain or snow, but to expose them to fluctuations in 
temperature and atmospheric moisture. This was accomplished by 
placing them in ojDen inverted preserving jars and holding them up 
near the bottom with wire screen. The jars were wired to tree 
trunks. 
Both types of artificial protection were performed with laboratory- 
bred clusters and were put out at a considerable number of chronic 
nonhatch points. The clusters were brought to the laboratory in the 
spring and handled like all the other experimental clusters. 
EXPOSURES OF CLUSTERS TO ONE SEVERE DROP IN 
TEMPERATURE. 
After results from some of the other experiments had pointed 
rather conclusively to low temperatures as the cause of nonhatch, it 
was desirable to gain more information on the degree of cold and the 
extent of exposure necessary to kill. 
It was planned to expose sets of clusters to a single severe drop, 
beginning with —15° F., apparently the temperature at which the 
first killing took place. It was hoped that a complete chain would be 
obtained from —15° downward as far as the thermometer goes at the 
points where the weather instruments were placed. This was not 
found to be possible, as one could not foresee the temperature fluctua- 
tions. A certain number of such sets of clusters, however, were ex- 
posed. 
The sets of laboratory-bred clusters were placed near the weather 
instruments and allowed to remain until they had been exposed to a 
single drop of at least —15°. They were then brought in and 
handled like other clusters. 
TEMPERATURE RESISTANCE. 
Sets of 10 laboratory-bred clusters were exposed during the winter 
in small wire cages along the line marking the northern limits of 
the gipsy-moth area. These were placed in towns from which the 
Weather Bureau office at Boston obtains records, so that a close 
record of the cold might be available, for it was desired to note the 
effect of as extreme cold as possible. 
These remained out all winter until just before hatching time, 
when they were brought to the laboratory. 
SUMMER SURVEY OF HATCHING. 
Each summer for three years, after the hatching season was over, 
an extensive series of observations on hatch was made in the field. 
Practically the entire area of infestation was gone over from the most 
southern limits to the most northern, and a very large number of ob- 
servations were made. Fortunately it was possible to make these ob- 
servations after two very cold winters and one very mild one, the 
latter coming between the two cold ones. 
WEATHER. 
The instruments located at the three points gave records of tem- 
jDerature and relative humidity. As soon as these records for the 
