186 NEUROPTERA. 
contrivance sink itself to the bottom of 
tlie water, in search of its prey. For this 
purpose it ghies to the outside of the case, 
grains of sand, gravel, or small shells, 
which increases its specific gr^avity, or 
weight, and enables it to dive ; if it hap- 
pens to have glued on so many as to 
make its house too heavy to float, then it 
sticks on, in addition, bits of straw, or 
rotten wood, or grass ; thus endeavouring 
to keep itself exactly suspended in the 
water ; by which means it can both run, 
and swim, and sink with equal ease. 
When preparing to change to a chry- 
salis, it w^eaves a slight net across each 
end of the tube, sufficiently open to ad- 
mit the water, but strong enough to ex- 
clude other voracious insects*. 
Lucy, As it preys on smaller insects, 
it is very fair that it should be devoured 
in turn by the larvae of others. 
Mother, But they are sought after by 
men, as well as by insects. 
* Plate XVI. 
