BACK-SWIMMERS. 15 
be very restless at the same time. They are found in all our 
rivers and lakes, where they feed upon other insects; they de- 
posit their eggs upon water-plants. In some regions in Mexico 
these eggs are so numerous that the aquatic plants are gathered, 
dried, and beaten, so that the eggs can be collected for food. 
Few insects are more sprightly, and the aquarium acquires 
a new interest by the introduction of these easily obtainable 
creatures. They live on the bottom of puddles, ponds, and run- 
ning waters, but it is especially of interest to observe their habits 
upon the bed of a bayed-out part of a stream, where there is no 
current. In such places they may be seen at home, balanced 
with the neatest delicacy by the tips of their intermediate feet, 
stationed in the midst of their young at intervals of a few inches; 
then, by reason of some disturbance, dashing away with rapid 
strokes of the posterior paddles, and arising to the surface, per- 
chance to take in a fresh supply of the external air. There, 
resting horiontally for an instant, with the long middle feet ex- 
tended forward, the sides and the ventral margin become glazed 
with an air-film which shines like translucent silver. 
Water-boatmen, though very common in Minnesota, are of 
no economic importance. 
FAMILY NOTONECTIDAE. 
(Back-swimmers). 
These aquatic bugs have the back shaped like the bottom of 
a boat, and persistently swim on their backs. The illustration, 
Fig. 10, shows Notonecta undulata Say. The eyes are large, 
reniform, twice sinuated on the outer side, and project slightly 
over the front margin of the prothorax. The beak is long, coni- 
cal, acute, and composed of four joints. And they can make 
good use of this instrument, as those handling such insects care- 
lessly may discover to their cost! They are only too ready to 
insert their poisonous beaks, and in lakes where the. water has 
become low during our warm and dry summers, it is not always 
a pleasure to wade about, as the insects are only too apt to cause 
very severe pains followed by inflammation. Back-swimmers are 
