56 
Psyche 
[June 
absorption of water alone could account for the increase 
in the size of Diploptera eggs, we have studied the changes, 
with age, in wet weight and in dry matter and water con- 
tent of the developing eggs. 
Female Diploptera were removed from the cultures, 
anesthetized with C0 2 , and their oothecae were expelled by 
finger pressure against the base of the abdomen. Because 
ootheca formation was rarely observed, the exact age of 
the eggs was unknown. Therefore, the length of an end 
egg in each ootheca was measured with an ocular micro- 
meter and its size was used in lieu of age. All weights were 
determined on torsion balances sensitive to 0.01 and 0.05 
mg. Oothecae were air-oven dried at about 100°C. to con- 
stant weight. All determinations were made on eggs en- 
closed in oothecae (rather than on individual eggs), but cal- 
culations are presented on a per egg basis. The ootheca 
itself (fig. 5, arrow) is so reduced in this species that its 
inclusion in the calculations is a negligible factor, particu- 
larly with older eggs which have greatly increased in size. 
Sometimes an ootheca contained one or two eggs which 
failed to develop ; these were easily removed without dam- 
age to the remaining eggs and were not included in the 
weight determinations. The usual number of eggs in an 
ootheca is 12 (Hagan, 1941) ; of 51 oothecae examined by 
us, the number of eggs ranged from 9 to 13 (including 
undeveloped eggs), with a mean of 11.4 per ootheca. Hagan 
(1954) thinks that this average may be due to altered 
environment, food, and captivity. In Hawaii he seldom 
found less than 12 eggs per ootheca and usually more than 
that number, although he did not record the data. 
Explanation of Plate 4 
Figures 2-4. Diploptera dytiscoides. Fig. 2. Female with an everted 
uterus (as a result of once having had her abdomen squeezed to remove 
an ootheca) and a recently formed ootheca (arrow) which she extruded 
because it could not be retracted into the brood sac (X3.8). Fig. 3. 
Female, after accidentally being exposed to high temperatures, prema- 
turely aborting an ootheca containing well-developed embryos (X3.8). 
Fig. 4. End view of abdomen of female; genital segments separated to 
show 4 embryos visible in the brood sac. Note eye (arrow) of embryo 
( X7.5). 
