HATCHIXG OF GIPSY-MOTH EGGS. 5 



might have and at the same time expose them fully to all actions of 

 the elements. The second ^vas 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 open 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 Avith —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 thpn 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- 

 perature and relative humidity. As soon as these records for the 



