WATER BEETLES IN RELATION TO PONDFISH CULTURE. 
333 
the aeration of the eggs. Unfortunately in the only instances where this would be 
possible from a physical standpoint, the attachment proves to be solid and covered 
on the outside with a glazing impervious to air. In the great majority of cases the 
eggs are wholly immersed in water and the only method of aeration is through the 
water itself. 
There are several genera (Ochthebius, Hydroscapha, Cymbiodyta, etc.) that 
do not construct any case at all but deposit their eggs either singly or in a mass in 
or near the water. Evidently, therefore, eggs can be developed without the help 
of this cap attachment. Why not infer that the eggs of all the genera can be thus 
developed and seek some other explanation for the attachment ? In order to test 
whether the attachment is essential to egg development in genera whose egg cases 
possess it, 30 egg cases of Tropisternus glaber were brought into the laboratory and 
their attachments were cut off, taking care not to injure the cases themselves. These 
cases were in various stages of development, some just completed, others from one 
to three days old. They were gathered and the attachments removed on July 18, 
the larvae began to hatch on July 22, and the last of them appeared on July 27. 
From these 30 cases were obtained 363 larvae, an average of 12 for each case, and on 
examining the cases at the close of the experiment it was found that only four eggs 
had failed to hatch. This certainly is up to the normal percentage of hatching and 
proves conclusively that the cap attachment is not essential to the development of 
the eggs. What then is the attachment and what reason can be given for its 
existence ? 
In the great majority of cases the attachment is different in color and texture 
from the rest of the case. Miger claimed that in Hydrous (Hydrophilus) there 
were three secretions — one the normal secretion for the construction of the case, 
another for spinning the loose covering which surrounds the eggs, and the third 
for making the cap attachment. There is a similar difference in the egg cases of 
Tropisternus glaber and T. mixtus, but in the case of T. lateralis there are only 
two kinds of silk, that which forms the cap and its attachment ribbon not being 
distinguishable from that which forms the case itself. According to Richmond 
(1920) this last is true of many other hydrophilids. 
These facts seem to indicate that the true explanation of this cap attachment 
is the one originally given by Lyonet, but apparently not appreciated by any 
entomologist since his day. In speaking of the "mast” of the egg case of the 
European species Hydrous piceus, Lyonet said: "I do not know the use of this 
little mast. Perhaps it enables the insect to get rid of an excess of silky matter” 
(1832, p. 135). In the account here given of the spinning of the egg case of 
Tropisternus lateralis it is shown that the case is spun first out of the normal silk 
secretion, then the eggs are laid and fastened in place inside the case with the 
loose silk, and finally the cap and its attachment are made. The normal silk secre- 
tion is white in color and fine in texture, and it forms the body of the case. During 
the laying of the eggs some of the contents of the oviducts may come out with the 
eggs and mingle with the silk secretion, making it thinner and looser in texture, 
thus forming the loose fibers used for covering the individual eggs. Wben the 
egg-laying ceases, the silk secretion returns to its normal texture and is used to 
