426 EEPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 



2 or 3 inches, and of course much higher than the air temperature 

 at an elevation of a few feet. According to Hennessey, Prestel, &c., a 

 black-bulb thermometer laid upon the soil probably gives one limit, 

 and the ordinary air temperature the other limit, between which the 

 temperature of the surface soil must be included. An approximate esti- 

 mate of the relative temperatures for slight depths at midday may be 

 formed from the accompanying table for dry, light, sandy soils, bare of 

 grass . 



On surface lOOOR SQCF. 60° F. 40° F. 



Just below - 90 72 55 41 



One-half inch below 80 67 52 42 



Oneinch below 75 63 51 41 



Two inches below 70 60 50 40 



THERMAL CONSTANT FOR LOCUST-EGGS. 



The next step in our study requires that we should determine, if pos- 

 sible, for how long locust-eggs must be exposed to a given temperature 

 in order to hatch, or what total amount of heat must be furnished to 

 them, which amount may be entitled the thermal constant in analogy 

 with the similar determination for the seeds of plants. We of course have 

 to assume that the large majority of the eggs require a certain normal 

 amount of heat. The effect of temperature, and incidentally of rain- 

 fall and dryness of air, upon the eggs is shown by Mr. Eiley^s experi- 

 ments, recorded in Chapter XIII. We add the following observations 

 which have come to hand from other sources: 



The young that are hatched out in February, if caged or in a warm place, may be 

 kept lively for a long time, but those that appear in the open air at this season soon 

 succumb to cold nights and want of food. It is certain that those that appeared in 

 February will do no harm next spring. It is doubtful whether in the year most 

 favorable to their development more than 50 per cent, of the eggs in this climate ever 

 survive the winter in a healthy state. While it is possible for these eggs to survive one 

 or even two freezings after the insect has formed in the egg, they are nearly destroyed 

 by frequent severe changes of weather. — [Prof. S. Aughey in the Nebraska larmeVy 

 March, 1877. 



It seems to be true that the vitality of the egg cannot be destroyed by freezing, 

 but it is more than likely that a rapid thawing out would spoil them. I have sub- 

 jected grasshopper eggs to temperatures 20 and 30 degrees below freezing, and then, 

 upon examination with the microscope, found the contents fluid, but thick, and with 

 ice crystals penetrating the mass. I have also tried freezing and thawing and then 

 hatching. Some 'hoppers appeared from the eggs that were thawed slowly, but none 

 where the temperature was raised with any rapidity. — [G. E. Bailey in Nebraska 

 Farmer, March, 1877. 



Eggs exposed to 78° F. hatch in from 7 to 14 days ; exposed to 20° F. they were 

 frozen, but revived on being warmed ; exposed to a temperature of 5° F. they were 

 killed. A hard frost does not kill many grasshoppers, but a cold of from 15*^ to 0° 

 will kill with hardly any exception. The eggs are not destroyed by a cold winter 

 unless they have already been partly hatched. — [Smith &;, Trask, of the Farmers' Union, 

 Saint Paul, Minn. 



An anonymous correspondent of the Bural New Yorker, March 31, 1877, says: A tem- 

 perature of 100° F. had no affect on the eggs, which hatched out in a few hours. A 



