it increases the diameter to accom-
modate its own growth.
When fully grown and ready for
transformation the larva partially closes
the ends, spins across them net-like
barriers of silk to keep out intruders
while admitting a fresh water supply.
Then it molts its last larval skin
and transforms into a pupa, of the
form shown in the accompanying figure,
having large compound eyes, long anten-
nas, broad external wing-cases and FIG. 107. ^ Pupa of
copious external gills.
Then ensues a quiescent period of a fortnight or more
during which great changes of form, both external and
internal, take place. The stuffs that the larva accumu-
lated and built into its body during its days of foraging,
and that now lie inert in the soft white body of the pupa
are being rapidly made over into the form in which
they will shortly appear in the body of the dainty aerial
caddis-fly. However, the pupa is not wholly inactive.
By gentle undulations of its body it keeps the water
flowing about its gills; and when, at the approach of
final transformation, its new muscles
have grown strong enough, it is seized
with a sudden fit of activity. It
breaks through the barred door of
the case, pushes out, swims away,
and then walks on the surface of the
water, seeking some emergent plant
stem, up which to climb to a suitable
place for its final transformation.
There the caddis-fly emerges, at first
limp and pale, but soon becoming
daintily tinted with yellow and brown,
full-fledged and capable of meeting the
exigencies of life in a new and wholly
It is a marvelous change of form
and habits that insects undergo in
metamorphosis especially in com-
plete metamorphosis. Such trans-
formations as occur in other groups are hardly com-
parable with it. The change from a tadpole to a frog,
or from a nauplius to an adult copepod, is slight by
comparison; for there is no cessation of activity, and no
considerable part of the body is even temporarily put
out of use. But in all the higher insects an extra-
ordinary reversal of development occurs at the close of
FIG. 1 08. Pupal skins
of Limnophilus, left
at final molting at-
tached to a reed
above the surface of
Nymph and Larva
active larval life. The larval tissues and organs disin-
tegrate, and return to a sort of embryonic condition,
to be rebuilt in new form in the adult insect.
With incomplete metamorphosis development is
more direct, there is no pupal stage, and the form of the
body is less altered during transformation. Metamor-
phosis is incomplete in the stoneflies, the mayflies, the
dragonflies and dam-
selflies and in the
water bugs. The im-
mature stage we shall
speak of as a nymph.
All nymphs agree in
having the wings de-
upon the sides of the
phosis is complete in
all the other orders
FIG. 109. Water boatmen (Conoco) t two o^V, '
adults and a nymph of the same species. 1 n e 1 r 1 m m a t U r e
stage we shall call
a larva. All larvas agree in having the wings devel-
oped internally: they are invisible from the outside
until the pupal stage is assumed. It should be
noted in passing that "complete" and "incomplete"
as applied to metamorphosis are purely relative terms.
There is in the insect series a progressive divergence
in form between immature and adult stages, and the
pupal stage comes in to bridge the widening gap
There is less change of form in the water bugs than
in any other group of aquatic insects. The nymph of
the water boatman (fig. 109) differs chiefly from the
adult in the undeveloped condition of its wings and
2O2 Aquatic Organisms
The groups of aquatic insects that are most com-
pletely given over to aquatic habits are the more
generalized orders that were long included in the single
Linnaean order Neuroptera (stoneflies, mayflies, dragon-
flies, caddis-flies, etc.)* Our knowledge of the immature
stages of aquatic insects was begun by the early micro-
scopists to whom reference has already been made in
these pages: Swammerdam, Rcesel, Reaumur, and
their contemporaries.! They delighted to observe
and describe the developmental stages of aquatic
insects, and did so with rare fidelity. After the days
of these pioneers, for a long time little attention was
paid to the immature stages, and descriptions of these
and accounts of their habits are still widely scattered J.
It is during their immature stages that most insects,
both aquatic and terrestrial ones, are of economic im-
portance. It is then they mainly feed and grow. It
is then they are mainly fed upon. The adults of many
groups eat nothing at all: their chief concern is with
mating and egg-laying. Hence the study of the im-
mature stages is worthy of the increased attention
it is receiving in our own time. It will be a very long
time before the life histories and habits of all our
aquatic insects are made known, and there is abundant
opportunity for even the amateur and isolated student
of nature to make additions to our knowledge by work
in this field.
*Under this name (we still call them Neuropteroids} the American forms
were first described and catalogued by Dr. H. A. Hagen in his classic "Synopsis
of the Neuroptera of North America." (Washington, 1861). Bugs, beetles,
moths and flies have received corresponding treatment in systematic synopses
of their respective orders, only the adult forms being considered.
f Much of the best of the work of these pioneers has been gathered from
their ancient ponderous and rather inaccessible tomes, and translated by
Professor L. C. Miall, and reprinted in convenient form in his "Natural History
of Aquatic Insects" (London, 1895).
| The completest available accounts of the life histories and habits of North
American aquatic insects have been published by the senior author and his
collaborators in the Bulletins 47, 68, 86 and 124 of the New York State Museum.
The stoneflies (order Plecoptera) are all aquatic.
They live in rapid streams, and on the wave- washed
rocky shores of lakes. They are among the most
generalized of winged insects. The adults are flat-
bodied inconspicuous creatures of secretive habits.
Little is seen of
them by day,
and less by
when some bril-
liant light by the
tracts them to
flutter around it .
The colors are
predomin a n 1 1 y
black, brown or
gray; but the
are pale green.
They take wing
fly rather slowly,
and may often
be caught in the
They are readily
picked up with
the fingers when at rest. The wings (sometimes
aborted) are folded flat upon the back. They are
rather irregularly traversed with heavy veins. The
tarsi are three- jointed. This, together with the flat-
tened head, bare skin, and long forwardly-directed
FIG. no. An adult stonefly, Perla immarginata.
antennae, will be sufficient for recognition of members
of this group.
Stonefly nymphs are elongate and flattened, and very
similar to the adults in form of body. They possess
always a pair of tails at the end of the body. Most of
them have filamentous gills
underneath the body, tho a
few that live in well aerated
waters are lacking these.
The colors of the nymphs
are often livelier than those
of the adults, they being
adorned with bright greens
and yellows in ornate pat-
The nymphs are mainly
carnivorous. They feed
upon mayfly nymphs and
midge larvas and many
other small animals occur-
ring in their haunts.
One finds these nymphs
by lifting stones from water
where it runs swiftly, and
quickly inverting them.
The nymphs cling closely
to the under side of the
stones, lying flat with legs
outspread, and holding on
by means of stout paired
claws that are like grappling hooks. Their legs are
flattened and laid down against the stone in such a
way that they offer little resistance to the passing
current. Stonefly nymphs are always found associated
with flat-bodied Mayfly nymphs of similar form, and
with greenish net-spinning caddis-worms.
FIG. in. The nymph of a stone-
fly, Perla immarginata.
(Photo by Lucy Wright Smith.)
The mayflies (order Ephemerida) are all aquatic.
They live in all fresh waters, being adapted to the
greatest diversity of situations.
The adults are fragile insects, hav-
ing long fore legs that are habit-
ually stretched far forward, and
two or three long tails that are
extended from the tip of the body
backward. The wings are corru-
gated and fan like, but not folded,
and are held vertically in repose.
The hind wings are small and incon-
spicuous. The antennae are minute
and setaceous. The head is con-
tracted below and the mouth parts
are rudimentary. Thus, many
characters serve to distinguish the
mayflies from other insects and
make their group one of the easiest
Mayflies are peculiar also, in
their metamorphosis. They
undergo a moult after the assump-
tion of the adult form. They
transform usually at the surface
of the water, and, leaving the
cast-off nymphal skin floating, fly
away to the trees. Body and wings
are then clothed in a thin pellicle
of dull grayish and usually pilose
skin, which is retained during a
short period of quiescence. During
FIG. 112. An adult may- this period (which lasts but a few
ay, siphhnurus aitema- m i nute s in Csenis and its allies,
but which in the larger forms lasts
one or two days) they are known as subimagos or
duns. Then this outer skin is shed, and they come
forth with smooth and shining surfaces and brighter
colors, as imagos, fully adult, and ready for their
mating flight. Lacking mouth parts and feeding not
at all, they then live but a few hours.
There are few phenomena
of the insect world more strik-
ing than the mating flight of
mayflies. The adult males fly
in companies, each species
maneuvering according to its
habit, and the females come
out to meet them in the air.
Certain large species that are
concerted in their season of
appearance gather in vast"
swarms about the shores of all
our larger bodies of fresh water
at their appointed time. By
day we see them sitting
motionless on every solid sup-
port, of ten bending the stream-
side willows with their weight ;
and when twilight falls we see
all that have passed their final
molt swarming in untold num-
bers over the surface of the
water along shore.
The nymphs of mayflies are all recognizable by the
gills upon the back of the abdomen. These are
arranged in pairs at the sides of some or all of the first
seven segments. The body terminates occasionally in
two but usually in three long tails. The mouth parts
are furnished with many specialties for raking diatoms
and for rasping decayed stems. Mayfly nymphs are
among the most important herbivores in all fresh waters.
FIG. 113. The nymph of the
mayfly, Siphlonurus alternatus.
(Photo by Anna Haven Morgan.)
The dragonflies and damselflies (order Odonata) are
all aquatic. The adults are carnivorous insects that go
hawking about over the surfaces of ponds and meadows,
capturing and eating a great variety of lesser insects.
The larger dragonflies eat the smaller ones.
FIG 114. An adult damselfly, Ischnura verticalis, perching on the stem of a
low galingale, Cyperus diandrus.
The form of body in the dragonflies is peculiar and
distinctive. The head, which is nearly overspread by
the huge eyes, is loosely poised on the apex of a narrow
prothorax. The remainder of the thorax is enlarged and
the wings are shifted backward upon it, and the legs
forward, adapting them for perching on vertical stems.
2O8 Aquatic Organisms
The abdomen is long and slender. On the ventral side
of its second and third segments, far removed from the
openings of the sperm ducts, there is developed in the
male a remarkable copulatory apparatus, that has no
counterpart in any other insects. The venation of the
wings, also, is peculiar, nothing like it being found in
any other order.
The dragonflies hold their wings horizontally in
repose. The damselflies are slender forms that hold
their wings vertically (or, in Lestes, obliquely outward)
in repose. Fore and hind wings are similar in form in
the damselflies; dissimilar, in the dragonflies.
FIG. 115. A nymph of the damsel-
fly, Ischnura verticalis.
The nymphs of the entire order are recognizable by
the possession of an enormous grasping labium, hinged
beneath the head. This is armed with raptorial hooks
and spines, and may be extended forward to a distance
several times the length of the head. It is thrust out
and withdrawn with a speed that the eye cannot follow.
It is a very formidable weapon for the capturing of
living prey. It is altogether unique among the many
modifications of insect mouth parts.
Damselfly nymphs are distinguished by the posses-
sion of three flat lanceolate gill-plates that are carried
like tails at the end of the abdomen. The edges of
these plates are set vertically, and they are swung from
side to side with a sculling motion to aid the nymphs in
Dragonfly Nymphs 209
Dragonfly nymphs have their gills developed upon
the inner walls of a rectal respiratory chamber, and not
visible externally. Hence, the abdomen is much wider
than in the damselflies. Water drawn slowly into the
gill chamber through an anal orifice, that is guarded by
elaborate strainers, may be suddenly expelled by the
strong contraction of the abdominal muscles. Thus
this breathing apparatus, also, is used to aid in locomo-
tion. The body is driven forward by the expulsion of
the water backward.
Damselfly nymphs live for the most part clambering
about among submerged plants in still waters; a few
FIG. 1 1 6. The burrowing nymph of a Gomphine dragonfly,
with an elongate terminal segment for reaching up
through the bottom mud to the water.
cling to plants in the edges of the current, and a very
few cling to rocks in flowing water. Dragonfly nymphs
are more diversified in their habits. Many of them
also clamber among plants, but more of them sprawl
in the mud of the bottom, where they lie in ambush to
await their prey. One considerable group (the Gom-
phines) is finely adapted for burrowing in the silt and
sand of the bottom.
All are very voracious, eating living prey in great
variety. All appear to prefer the largest game they
are able to overpower. Many species are arrant canni-
bals, eating their own kind even when not starved to it.
As a group they are among the most important carni-
vores in shoal fresh waters.
The true bugs (order Hemiptera) are mainly terres-
trial, and have undergone on land their greatest differ-
entiation. The aquatic ones are usually found in still
waters and in the shelter of submerged vegetation.
Tho comparatively few in species, they are important
members of the predatory population of ponds and
FIG. 117. A giant water bug (Benacus griseus)
clinging to a vertical surface under water,
pools. They are often present in great numbers, if
not in great variety. The giant water bugs (fig. 117)
are among the largest of aquatic insects. These are
widely known from their habit of flying to arc lights,
falling beneath them, and floundering about in the dust
of village streets.
The eggs of the giant water-bugs are attached to
vertical stems of reeds just above the surface of the
water. They are among the largest of insect eggs.
Those of Benacus (fig. 1 18) are curiously striped. The
eggs of a smaller, related water-bug, ZaithaorBelostoma,
are attached by the female to the broad back of the
FIG. 118. Eggs of Benacus, enlarged; the lower-
most are in process of hatching.
male, and are carried by him during their incubation.
The nymphs of this family, on escaping from the egg
suddenly unroll and expand their flat bodies, and attain
at once proportions that would seem impossible on
looking at the egg (fig. 119).
Most finely adapted to life in the water are the water
boatmen (fig. 109 on p. 201) and the back-swimmers,
which swim with great agility and are able to remain for
a considerable time beneath the surface of the water.
The eggs of these are attached beneath the water to any
solid support. Most
grotesque in form
are the water-scor-
pions (Nepidae) , that
breathe through a
long caudal respira-
tory tube. The
eggs of these are in-
serted into soft plant
tissues, with a pair
of long processes on
the end of each egg
At the shore-line
we find the creep-
FIG. 119. A new-hatched Benacus, and am n S matted roots
a detached egg. in the edge of the
water, with shore
bugs and toad bugs just out on land.
Nymphs and adults alike are distinguished from the
members of all other orders by the possession of a
jointed puncturing and sucking proboscis beneath the
head, directed backward between the fore legs.
Nymphs and adults are found in the water together
and are alike carnivorous. Being similar in form they
are readily recognized as the same animal in different
The net-winged insects (Neuroptera) are mainly
terrestrial or arboreal. Two families only have aquatic
representatives, the Sialididae and the Hemerobiidae,
and these are so different, they are better considered
i. Sialididee These are the dobsons, the fish flies
and the orl flies. The largest is Corydalis, the common
dobson (fig. 120), whose larva is the well known "hell-
grammite", that is widely
used as bait for bass. It
lives under stones in
rapids. It is a "crawler"
of forbidding appearance,
two or three inches long
when grown, having a
stout, greenish black body,
sprawling, hairy legs, and
paired fleshy lateral pro-
cesses at the sides of the
abdomen. There is a
minute tuft of soft white
gills under the base of each
lateral process. There is
a pair of stout fleshy pro-
legs at the end of the ab-
domen, each one armed
with a pair of grappling
hooks. The larvae of the
fish -flies (CJiauliodes) are
similar in form, but smaller
and lack the gill tufts under
the lateral filaments. The larva of the orl-fly differs
conspicuously in having no prolegs or hooks at the end
of the body, but instead, a long tapering slender
tail. Fish-fly larvae are most commonly found clinging
to submerged logs and timbers. Orl-fly larvae burrow
in the sandy beds of pools in streams and in lake shores.
All appear to be carnivorous, but little is known of
the feeding habits of either larvae or adults. Tho large
and conspicuous insects they are rather secretive and
are rarely abundant, and they have been little observed.
FIG. 1 20. An adult female dob-
son, Corydalis cornuta, natural
2. Hemerobiidce Of this large family of lace- wing?
but two small genera (in our fauna) of spongilla flies,
Climacia and Sisyra, have aquatic larvae. The adults
are delicate little insects that are so secretive in habits
and so infrequently
seen that they are
rare in collections.
Their larvae are com-
monly found in the
cavities of fresh water
sponges. They feed
upon the fluids in the
body of the sponge.
They are distin-
guished by the posses-
sion of long slender
longer than the head
SS^$ and thorax together,
and by paired ab-
dominal respiratory filaments, that are angled at the
base and bent underneath the abdomen. These larvae
are minute in size (6 mm. long when grown) and are
quite unique among aquatic insect larvae in form of
mouthparts and in manner of life.
The caddis -flies (order Trichoptera) are all aquatic,
save for a few species that live in mosses. They con-
stitute the largest single group of predominantly aquatic
insects. They abound in all fresh waters.
The adults are hairy moth-like insects that fly to
lights at night, and that sit close by day, with their long
antennae extended forward (see fig. 1 03 on p . 1 97) . They
are not showy insects, yet many of them are very dainty
and delicately colored. They are short-lived as adults,
and, like the mayflies, many species swarm at the shore
line on summer evenings in innumerable companies.
FIG. 121. Insect larvae.
a, a diving-beetle larva (Coptotomus interrogates)
after Helen Williamson Lyman) ; b, a hellgrammite,
(Corydalis cornuta, after Lintner); c, an orl-fly
larva (Sialis infumata, after Maude H. Anthony).
The larvae of the caddis-flies mostly live in portable
cases, which they drag about with them as they crawl
or climb; but a few having cases
of lighter construction, swim
freely about in them. Such is
Tri&nodes, whose spirally wound
case made from bits of slender
stems is shown in the accompany-
The cases are wonderful in
their diversity of form , of materials
and of construction. They are
usually cylindric tubes, open at
both ends, but they may be
sharply quadrangular or trian-
gular in cross section, and the
tube may be curved or even coiled
into a close spiral*.
Almost any solid materials that
may be available in the water in
pieces of suitable size may be used in their case build-
ing: sticks, pebbles, sand-grains and shells are the
staple materials. Sticks may be
placed parallel and lengthwise,
either irregularly, or in a con-
tinuous spiral. They may be
placed crosswise with ends over-
FIG. 122. The larva
spongilla fly, Sisyra 'after
Maude H. Anthony) .
FIG. 123. The case of the free- lapping like the elements of a
larvae of Triae- s ti c k chimney, making thick
walls and rather cumbrous cases.
However built, the case is always lined with the secre-
tion from the silk glands of the larva. This substance
is indeed the basis of all case construction. The larva
*As in Helicopsyche, (see fig. 221, on page 370) whose case of finely textured
sand grains was originally described as a new species of snail shell.
builds by adding pieces one by one at the end of the
tube, bedding each one in this secretion, which hardens
on contact with the water and holds fast. Small snails
and mussel shells are
sometimes added to the
exterior with striking
ornamental effect, and
sometimes these are
added while the protes-
ting molluscs are yet
living in them.
Some of the micro-
caddis-flies (family Hy-
" parchment" cases of
the silk secretion alone.
These are brownish in
color and translucent.
They are usually com-
pressed in form and
are carried about on
FIG. 124. Cylindric sand cases of edge. AgTaylea decor-
Leptoceridae ' (en - ates the parchment
with filaments of Spiro-
gyra, arranged concentrically over the sides in a single
Some caddis-worms build no portable cases at all, but
merely barricade themselves in the crevices between
stones, attaching pebbles by means of their silk secre-
tion, and thus building themselves a walled chamber
which they line with silk. In this they live, and out of
the door of the chamber they extend themselves half
their length in foraging. Other caddisworms construct
fixed tubes among the stones, and at the end of the tube
that opens facing the current they spin fine-meshed
funnel-shaped nets of silk. These are open up stream,
and into them the current washes organisms suitable
for food. The caddis -worm lies with ready jaws in wait
at the bottom of the funnel, and cheerfully takes what
heaven bestows, seizing any bit of food that may chance
to fall into its net. These net-spinners belong to the
family Hy dropsy chidae. __ ^
When minute animals
abound in the current the
caddis-worms appear to
eat them by preference:
at other times, they eat
diatoms and other algae
and plant fragments.
The order as a whole tends
to be herbivorous and
many members of it are
strictly so; but most of
them will at least vary
their diet with small may-
fly and midge larvae and
these are to be had.
Caddis-worms are more or less caterpillar-like, but
lack paired fleshy prolegs beneath the body, save for a
single strongly -hooked pair at the posterior end. The
thoracic legs are longer and stronger and better devel-
oped than in caterpillars, and they are closely applicable