Frank and Tom returned from the horse pond one day with their jars
filled with curious creatures.
“We took the water beetle back to the pond, Uncle George,” said Frank,
“and I almost felt sorry to let such a cruel and greedy monster loose
among the other creatures in the pond.”
“You have here creatures even more cruel and greedy than the diving
beetle,” said his uncle, looking into the glass jars which the boys had
brought.
“Here is the larva of the diving beetle, and a fiercer or greedier
creature is not to be found in the whole world.”
Uncle George refilled the three large tanks. He poured the contents of
one of the jars into one of the tanks. It contained, besides the larvæ
he spoke of, a number of tadpoles, some small stickle-backs, and a great
many water snails or whelks.
In addition to these, there were one or two curious insects, with a pair
of very long legs. They came to the surface every now and then, and
moved their long limbs like oars. Whenever the tank was touched, they
would suddenly dive to the bottom. Here they would remain for a short
time, then, returning to the surface, would “oar” themselves along as
usual.
“What are these queer things, Uncle George?” Tom asked. “They look
exactly like little boats.”
“They are beetles,” Uncle George replied, “and they do resemble little
row-boats—so much so that they are called ‘water boatmen.’ This beetle’s
body is shaped like a boat, and he lies on his back and moves his very
long hind legs exactly like oars. He has a large, sharp beak, which you
can see if you look closely.”
Just as Uncle George was speaking, a small fly alighted on the surface
of the water. It was immediately seized by a water boatman, who dived
with it to the bottom of the tank.
[Illustration]
1. Water Boatman.
2. Water Whelk.
3. Water Hog Louse.
“That is how the creature feeds,” said Uncle George. “When an unlucky
fly comes near, he seizes it with his two fore pairs of short legs,
drives his cruel beak deep into its body, and dives with it to the
bottom of the pool. When he has drowned his victim, he devours it at
leisure.
“But let us look at the creature I first spoke of—the larva of the
diving beetle.”
There were several in the tank. Uncle George lifted one out with a small
gauze net. He placed it in a saucer of water and asked the boys to
sketch it.
“It has six legs,” said Frank. “Its head is large and flat, and there
are two things like horns sticking out of it.”
“These are the creature’s jaws,” said his uncle.
Uncle George touched the creature’s head with the point of his pencil.
Instantly the long, curved fangs moved apart, and the creature sprang at
the pencil and closed its fangs upon the wood.
[Illustration]
1. Larva of Diving Beetle.
2, 3. Head, showing how Fangs work.
4. Larva seizing Tadpole.
“What a fierce creature,” said Tom.
“Yes,” replied his uncle. “We must remove the others from the tank, or
there will be nothing left alive in it. See, one of them has caught a
tadpole already.” And, sure enough, the creature had sunk his cruel
pincers deep into the body of a tadpole. He shook the tadpole as a dog
would shake a rat.
“Those huge fangs are hollow,” Uncle George continued. “With them he
sucks every drop of blood from his victim’s body. In this way he devours
tadpoles, whelks, little fishes, and even his own weaker brethren.
“We shall have to put these larvæ in a tank by themselves, and feed them
upon pieces of fresh raw meat, to keep them from devouring one another.
“Now, boys, I am going to describe this larva. Look closely, and see if
I am right. You can correct your drawings as we go along.
“His body, which is nearly two inches long, is narrow at each end and
broad at the middle. It is divided into twelve segments, and to each of
the first three of these segments a pair of legs are attached. His
large, flat head is almost heart-shaped. At its broad end are a pair of
large, beady eyes, three pairs of organs which look like feelers, and
the two large, curved fangs with which he seizes his prey.
“Notice how beautifully the legs are adapted for swimming. They are
edged with long, stiff hairs, which stand out flat with every down
stroke, but fold in when the legs move upwards. That is how the creature
is such a graceful swimmer.
“The last three segments of his body are thickly covered with long
hairs. His tail is divided into two long plumes. You can see what these
are for, I hope.”
“Oh yes,” said Frank. “Like the parent beetle, the larva comes often to
the surface and hangs downwards with his tail exposed to the air. So
these plumes must be his breathing organs.”
“And do these long, slender creatures actually become broad, stout
beetles like the one we returned to the pond to-day?” Tom asked.
“Yes, Tom, they do. Just as your green caterpillar became a beautiful
white butterfly. These larvæ hatched out of eggs which were laid by the
female beetle in the stem of some water weed. She first cuts a slit in
the water weed, and then places an egg in the slit.
“When the larvæ are full grown, they crawl out of the pond and bury
themselves in ground. Here they pass the winter in the pupa stage. They
come out in spring, fully-formed beetles, and pass the remainder of
their lives in the water.”
“What are these curious creatures that crawl among the water weeds?”
Frank asked. “They are very like the ‘slaters’ which we find among old
wood-work in the garden.”
“That creature is the ‘water hog.’ Its proper name is _Asellus_, and it
is very closely related to the garden ‘slater’ or wood-louse.
“These water hogs, like the whelks, are very useful in an aquarium. They
eat up all waste matter, and so help greatly to keep the water fresh.
“By the by, I want you to watch the whelks from day to day. They usually
lay their eggs in a clear, broad band on the inside of the glass tank;
and with our lenses we can watch the changes in these eggs nicely.”
Exercises on Lesson X.
1. Procure some “water boatmen” from the nearest pond (see Appendix),
feed them upon flies, and watch their habits. Notice
particularly their method of swimming and of feeding.
2. We should not keep all our pond animals in one tank. Can you
explain (giving examples) why this is?
3. Write the life-history of the diving beetle.
4. Make large sketches of the diving beetle and its larva. Compare
them.
[Illustration]
Water Beetles.
Hog Lice.
Water Whelks.
Water Boatman.