AQUATIC INSECTS IN NEW YORK STATE Sick 
for, no matter at what level sections are taken, the dorsal sur- 
face :‘s always found to be convex, as is represented on plate 28, 
figure 4. 
In order to explain how the larva of Donacia obtains its supply 
of air from the intercellular spaces of plants, I do not think it 
is necessary to assume any extraordinary structures for the 
caudal spines. The caudal spines are nothing more than pro- 
jections of the body wall for rupturing the tissues of the plant; 
and, when this is accomplished, the air, being so much lighter 
than the surrounding water and having a strong tendency to fol- 
low along anything that will carry it to a higher level, simply fol- 
lows along the outer surface of the caudal spines to their base, 
where it is taken up by the spiracles, while the two large longi- 
tudinal trachea connecting with the spines take up the supply 
of air and act as reservoirs for storing it between the air-taking 
periods. 
When the larvae are ready to transform to pupae, they spin 
a tough, brownish cocoon, which is attached to the scars on the 
upper surface of the rhizome from which the leaf stalks have 
been shed. The silk is spun from glands opening in the 
mouth. The cocoons are not only water-tight but air-tight 
and are of a homogeneous consistency throughout without any 
indication of a thread-like structure. The bottom of the cocoon 
where it is attached to the plant is much thinner and lighter in 
color and is firmly glued to the surface of the plant [pl.220]. 
In the case of palmata the cocoons are much smaller than 
the rhizome and always placed on its upper surface, distant 
from each other; but in those species that pupate on the roots 
of Sagittaria, Sparganium and Potamogeton, the cocoons are 
much broader than the roots and are placed singly, usually near 
the point of attachment of the roots to the base of the plant. 
Such a condition is shown well by plate 28, figure 6; while in 
the case of Donacia cincticornis, which fastens its 
cocoons to the larger roots of Nymphaea advena, they 
are arranged close together in groups of five or six around the 
apices of the roots. 
