12 
BULLETIN OF THE BUREAU OF FISHERIES. 
roof. Last of all she finished the mast, and while doing this she kept her wing covers 
separated enough to supply herself continuously with fresh air. The triangular 
plate and the mast were of the same bright yellow as the eggs and remained this 
color for several hours. 
The whole process lasted only 1 hour and 20 minutes, and the actual time 
consumed in spinning and egg laying was exactly 1 hour. The completed egg case 
is ellipsoidal in shape, the roof and floor somewhat flattened, and the side walls 
strongly convex. One end is tightly closed and the other has a lunate opening 
beneath the triangular plate, which gives free access to the space beneath the egg 
mass. The triangular plate is narrowed above into the mast, which rises vertically 
about 10 mm. above the roof of the case and tapers to a rounded point. On cases 
found floating in the pond both the triangular plate and the mast are dark brown 
in color, almost black, due to the action of the sunlight upon the silk. 
The mast has been represented as a small tube with dense walls, whose sug- 
gested function is the admission of air into the egg chamber. Miger and Lancret 
(1809, p. 442) said: 
It is a mistake to suppose that the turned-up point of the cocoon serves as a mast. It is not unlikely 
that the drawn-out point serves for the supply of air to the cocoon. 
Lyonet (1832) confessed: 
I do not know the use of this little mast. Perhaps it enables the insect to get rid of an excess of 
silky matter. 
Laker (1881, p. 82) wrote: 
The spike consists of a substance somewhat thicker and stronger than the rest of the cocoon and is 
hollow throughout the greater part of its length, except that it is crossed and recrossed inside with a 
dark, threadlike substance, thus somewhat resembling a horn stuffed with tow. 
The apex of the spike does not, however, appear to terminate in an orifice, 
but is closed. It does not seem to me that this spike can serve as a balance 
to the cocoon, because the nests are usually attached to some kind of support. I 
may, however, mention that I cut off the spike from two of the cocoons, and in 
both these cases the eggs did not hatch. It is, however, possible that this may 
have arisen from some other cause, although these particular cocoons ap- 
peared to be similar in every respect to others of which the eggs matured in 
due course. The cocoons from which the spikes were removed subsequently 
sank. These nests are so constructed that when floating loose the spike retains 
its proper position, and even if the cocoon be held so that the spike is par- 
allel with the water, and then suddenly released, it immediately rights itself. If, however, the spike be 
partially submerged and then released, the cocoon turns bottom upward. 
Fig. 4.— Cross section of 
mast ofeggcase, showing 
how the edges are rolled 
back to form an open 
tube. 
In the six egg cases examined by the present author the mast or spike is not a 
tube at all, but a thick flat layer of closely woven silk, whose edges curl backward 
and almost meet along the side next to the body of the case. In this way a partially 
closed tube is formed, but there is a narrow slit running its entire length which 
manifestly disqualifies it from serving as an air tube (fig. 4). Furthermore, as 
Laker says, it is closed at the apex, and we may now add that it is also closed at 
the base, affording no connection whatever with the inside of the case. This fact 
does not seem to have been noted by any of these observers, but it disposes once 
for all of the supposition that the mast has anything to do with supplying air to 
the eggs. 
