6 BULLETIN 1016, U. S. DEPARTMENT OF AGRICULTURE. 
of 664 eggs was obtained during the spring and summer of 1919 and 
1920. These data are given in Table 2. The longest period obtained 
for the egg stage was 31 days. This egg hatched on June 8 from a lot 
of 103 deposited on May 8. The shortest egg period was 9 days, 
during July. There is a .gradual decrease in the length of the egg 
stage from early May to July, and during August it again increases. 
Not all of the eggs of one day's deposition hatch on the same day, 
although the reason for this is not definitely known. It may be 
that some eggs are not sufficiently supplied with moisture, or with 
the same amount that the others receive, and consequently develop 
more slowly. There may be a difference of nine days in the time of 
hatching in the same lot of eggs, as will be noted in Table 2 under 
entry of May 8. The greater number of this lot of 103 eggs, how- 
ever, hatched within 3 days after hatching began. The first eggs in 
this lot hatched on May 31, the last on June 8. There is a greater 
range in the time of hatching in a single lot of eggs in the early 
spring months than in the summer months. In the latter period a lot 
of eggs laid on the same day generally hatch within at most 3 days 
after hatching has begun, and quite often all of them hatch on the 
same day. 
The weighted average for the egg period of the 664 eggs recorded 
in Table 2 is 21.96+ days. 
The type of cage used for observation purposes on the incubation 
period consisted of a very small tin ointment box with a plaster of 
paris bottom. The porous qualities of this substance permitted intro- 
duction of just the right amount of moisture. No great difficulty 
was experienced with fungus, which often develops in closed boxes 
where moisture is present, especially when used continuously for as 
long a time as in these experiments. This type of egg cage has been 
found exceedingly satisfactory for making observations on the egg 
stages of various insects, especially those which, lay their eggs in 
the soil or other damp places, and therefore have to be kept fairly 
moist if good results are to be obtained. It was devised in connec- 
tion with investigations of the southern corn rootworm {Diabrotica 
12-p.unctata Oliv.). 
