Copyright
James G. (James George) Needham.

The life of inland waters; an elementary text book of fresh-water biology for students online

. (page 16 of 26)
Online LibraryJames G. (James George) NeedhamThe life of inland waters; an elementary text book of fresh-water biology for students → online text (page 16 of 26)
Font size
QR-code for this ebook


tions already noted in mammals; (i) approximation to
stream-line form, with (2) consolidation of the forward
parts of the body for greater rigidity; (3) lowering of
the eyes and smoothing of all contours; (4) loss of hair



274 Adjustment to Conditions of Aquatic Life

and sculpturing, and (5) shortening of basal segments of
swimming legs, with lengthening of their oar-like tips,
flattening and flexing of them into the horizontal plane,
and limiting their range of motion to horizontal strokes
in line with the axis of gravity of the body. Caudal
propulsion does not occur with adult insects; none of
them has a flexible tail. Oar-like hind feet are the
organs of propulsion. The best swimmers among them
are a few of the larger beetles : Cy bister, which swims
like a frog with synchronous strokes of its powerful
hind legs, and Hydrophilus, with equally good swimming
legs, which, like the whale, has developed a keel for
keeping its body to rights.

Adult insects, like the mammals, lack gills, and rise
to the surface of the water for air; but they take the
air not through single pairs of nostrils, but a number of
pairs of spiracles, and they receive it, not into lungs,
but into tracheal tubes that ramify throughout the
body. The spiracles are located at the sides of the
thorax and abdomen, in general a pair to each seg-
ment.

In diving beetles the more important of these are
the ones located on the abdomen beneath the wings.
Access to these is between the wing tips. The beetles
when taking air hang at the surface head downward.
The horny, highly arched, fore wings are fitted closely
to the body to inclose a capacious air chamber. They
are opened a little at their tips for taking in a fresh air
supply at the surface. Then they are closed, and the
beetle, swimming down below, carries a store of air with
him.

In other beetles there are different methods of gather-
ing and carrying the air. The little yellow-necked
beetles of the family Haliplidae, gather the air with the
fringed hind feet, pass it forward underneath the huge
ventral plates which, in these beetles cover the bases



Aquatic Adaptations of Insects 275

of the hind legs, and thence it goes through a transverse
groove-like passage (fig. 168) to a chamber underneath
the wing bases, where there are two enlarged spiracles
on each side. The beetles of the family Hydrophilidae
have their ventral surface covered with a layer of fine
water-repellant pubescence, to which the air readily
adheres. Thus the air is carried exposed upon the
surface, where it shines like a breastplate of silver.

In the waterbugs, the air is usually carried on the
back under the wings, but the inverted back-swimmers
conduct air to their spiracles through longitudinal




FIG. 168. Diagram of the air-taking apparatus of the beetle,
Haliplus. The arrow indicates the transverse groove that
leads to the air chamber. (From Matheson.)

grooves that are covered by water-repellant hairs, and
that extend forward from the tip of the abdomen upon
the ventral side. The water walking-stick, Ranatra,
and some of its allies have developed a long respiratory
tube out of a pair of approximated grooved caudal
stylets. This long tail-like tube reaches the surface
while the bug stays down below, breathing like a man
in a diving bell.

The immature stages of aquatic insects are far more
completely adapted to life in the water than are the
adults. Some members of nearly all the orders, and all



276 Adjustment to Conditions of Aquatic Life

the members of a few of the smaller orders live and grow
up in the water. These facts have been noted, group
by group, in Chapter IV. Here we may explain that
the reason for this probably lies in the greater plasticity
of the immature stages. All are thin-skinned on hatch-
ing from the egg, and a supply of oxygen may be taken
from the water by direct absorption thro the general
surface of the body. With growth gills develop; but
these have no relation to the structure or life of the
adult and are lost at the final transformation.




FIG. 169. Adult aquatic insects: a, the
back swimmer (Notonecta) ; b, the water-
boatman (Corixa); c, a diving beetle
(Dytiscus) ; d, a giant water-bug (Benacus).

Here again we find all degrees of adaptation. The
larvae of the long-horned leaf beetles (Donacia, etc.)
that live wholly submerged have solved the problem of
getting air by attaching themselves to plants and per-
forating the walls of their internal air spaces, thus
tapping an adequate and dependable air supply that is
rich in oxygen. This method is followed also by the
larvae of several flies and at least one mosquito. There
are many aquatic larvae that breathe air at the surface
as do adult bugs and beetles. Some of these, such as
the swaleflies and craneflies, (fig. 215) differ little from
their terrestrial relatives. Others like the mosquito
are specialized for swimming and breathe thro respira-
tory trumpets. A few like the rat-tailed maggot



Aquatic Adaptations of Insect Larva 277

parallel the method of Ranatra mentioned above in
that they have developed a long respiratory tube,
capable of reaching the surface of the water while they
remain far below.






FIG. 170. Trachea! gill of the mayfly nymph, Heptagenia, show-
ing loops of tracheoles toward the tip.

Of those that breathe the air that is dissolved in the
water a few lack gills even when grown to full size; but
these for the most part live in well aerated waters, and
possess a copious development of tracheae in the thinner
portions of their integument. Such are the pale
nymphs of the stonefly, Chloroperla, that live in the



278 Adjustment to Conditions of Aquatic Life

rapids of streams and the slender larvae of the punkie
Ceratopogon, that live where algae abound.

The gills of insect larvae are of two principal sorts:
blood-gills and tracheal gills. Blood-gills are cylindric
outgrowths of the integument into which the blood
flows. Exchange of gases is between the blood
inside the gills and the water outside. Such gills
are most commonly appended to the rear end of the
alimentary canal, a tuft of four retractile anal gills
being common to many dipterous larvae. Bloodworms
have also two pairs developed upon the outside wall of
the penultimate segment of the body (see fig. 236 on
P- 393)- Such gills are most like those of vertebrates.

Tracheal gills are more common among insect larvae.
These are similar outgrowths of the skin, traversed by
fine tracheal air-tubes. In these the exchange of gases
is between the water and the air contained within the
tubes, and distribution of it is thro the complex system
of tracheae that ramify throughout the body. The
tracheae where they enter such a gill usually split up
into long fine multitudinous tracheoles that form
recurrent loops, rejoining the tracheal branches (fig.
170).

Tracheal gills differ remarkably in form, position and
arrangement. In form they are usually either slender
cylindric filaments, or small flat plates. Filamentous
gills are more common, only this sort occurring on stone-
fly nymphs (fig. 1 1 1 on p. 204), and on caddis-worms.
Lamelliformor plate-like gills occur on the back of may-
flies (fig. 113), and on the tail of damselflies (fig. 115).
Either kind may grow singly or in clusters. Filament-
ous gills are often branched. In the stonefly,Taeniop-
teryx, they are unbranchedbut composed of three some-
what telescopic segments. Both filamentous and
lamelliform gills occur on many mayflies.



Aquatic Adaptations of Insect Larvcz 279



There is another form of tracheal gills, sometimes
called "tube gills" developed upon the thorax of many
dipterous pupae. Whatever their form
they are merely hollow bare chitinous
prolongations from the mouth of the
prothoracic spiracle. They are ex-
panded ' 'respiratory trumpets" in
mosquito pupae, branching horns in
black-fly pupae, and fine brushes of
silvery luster in bloodworm pupae.
No pupae, save those of the caddis-
flies, have tracheal gills of the ordi-
nary sort.

Gills are developed rarely on the
head, more often on the thorax, and
very frequently on the abdomen.
They grow about the base of the maxil-
lae in a few stonefly and mayfly
nymphs, about the bases of the legs
in most stonefly nymphs and almost
anywhere about the sides or end of
the abdomen in all the groups. They
are ventral in the spongilla flies, dorsal
in the mayflies, lateral in the orl-fly
and beetle larvae, caudal in the damsel-
flies, anal in most dipterous larvae,
and they cover the inner walls of a
rectal respiratory chamber in dragon -
flies. Such extraordinary diversity in
structures that are so clearly adaptive
is perhaps the strongest evidence of
the independent adaptation of many
insect larvae to aquatic life.
Propulsion by means of fringed swimming legs
occurs in a few insect larvae, such as the caddis- worm,
Triaenodes, and the "water-tiger" Dytiscus. The gill




FIG. 171. Tube-gills
of Dipterous pupae :
a, of a mosquito,
Culex ; b, of a black-
fly, Simulium ; c,
of a midge, Chiro-
nomus. (a and b
detached).



280 Adjustment to Conditions of Aquatic Life

plates of many mayflies and damselflies are provided
with muscles, and these are used for swimming.
Caudal propulsion is also the rule in these same groups.
Among beetle and fly larvse locomotion is mainly
effected by wrigglings of the body, that are highly
individualized but only moderately efficient, if judged
by speed.

It is worthy of note that the completest adaptations
to conditions of aquatic life do not occur in those groups
of insects that are aquatic in both adult and larval
stages. Beetle larvag and water-bug nymphs take air
at the surface, and in structure differ but little from
their terrestrial relatives. Fine developments of tra-
cheal gills occur in the nymphs of mayflies and stone-
flies, and in caddis worms; internal gill chambers, in the
dragonfly nymphs; attachment apparatus for with-
standing currents, in some dipterous larvae ; the utmost
adaptability to all sorts of freshwater situations occurs
in the midges; and in adult life these insects are all
aerial.

What then is the explanation of the dominance of
this remarkable insect group in the world to-day a
dominance as noteworthy in all shoal freshwaters as it
is on land? What advantages has this group over
other groups? There is no single thing; but there are
two things that, taken together, may give the key to
the explanation. These are:

1. Metamorphosis, the changes of form usually per-
mitting an entire change of habitat and of habits
between larval and adult life. The breaking up of the
life cycle into distinct periods of growth and reproduc-
tion permits development where food abounds.

2. The power of flight in the adult stage permits easy
getting about for finding scattered sources of food supply
and for laying eggs.



Aquatic Adaptations of Insect Larva 281

In quickly growing animals no larger than insects
these matters are very important ; for even a small and
transient food supply may serve for the nurture of a
brood of larvae. And if the food supply be exhausted
in one place, or if other conditions fail there, the adults
may fly elsewhere to lay their eggs. The facts of
dominance would seem to justify this explanation, since
those groups that most abound in the world to-day are
in general .the ones in which metamorphosis is most
complete and in which the power of flight is best
developed.





II. MUTUAL
ADJUSTMENT



ARIOUS phenomena of
association between non-
competing species are
manifest alike in terres-
trial and aquatic socie-
ties. The occurrence of
producers and consumers
is universal. Carnivores
eat herbivores, and para-
sites and scavengers fol-
low both in every natural
society. Symbiosis is as
well illustrated in green hydra and green ciliates as in
the lichens. The mutually beneficial association be-
tween fungus and the roots of green plants is as well
seen in the bog as in the forest. The larger organisms
everywhere give shelter to the smaller, and many ex-
amples, such as that of the alga, Nostoc, that dwells in
the thallus of Azolla, or the rotifer Notommata parasita
that lives in the hollow internal cavity of Volvox, occur
in the water world.

We shall content ourselves here with a very brief
account of two associations, one of which has to do
mainly with a mode of getting a living, the other with
providing for posterity. The first will be insectivorous
plants; the second the relations between fishes and
fresh-water mussels.



282



Insectivorous Plants



283



Insectivorous plants The plants that capture insects
and other animals for food are a few bog plants such as
sundew and pitcher-plant, and a number of submerged
bladderwort s . These
have turned tables on
the animal world. Liv-
ing where nitrogenous
plant-foods of the or-
dinary sorts are scanty,
they have evolved ways
of availing themselves
of the rich stores of pro-
teins found in the bodies
of animals. The sun-
dew seems to digest its
prey like a carnivore;
the bladderwort ab-
sorbs the dissolved sub-
stance like a scavenger.
Charles Darwin studied
these plants fifty years
ago, and his account
('75) is still the best
we have.

The sundew, Dro-
sera, captures insects
by means of an adhesive
secretion from the tips
of large glandular hairs
that cover the upper surface of its leaves (fig. 172).
The leaves are few in number and spatulate in form, and
are laid down in a rosette about the base of a stem,
flat upon the mud or upon the bed of mosses in the
midst of which Drosera usually grows. They are red
in color, and crowned and fringed with these purple




FIG. 172. A leaf of sundew with a
captured caddis-fly. The glandular
hairs are bent downward, their tips
in contact with the body of the
insect. Other erect hairs show
globules of secretion enveloping their
tips.



284 Adjustment to Conditions of Aquatic Life

hairs, each with a pearly drop of secretion at its tip
sparkling in the light, like dew, they are very attractive
to look upon. The insect that makes the mistake of
settling upon one of these leaves is held fast by the tips
of the hairs it touches : the more it struggles the more
hairs it touches, and the more firmly it is held. Ere it
ceases its struggles all the hairs within reach of it
begin to bend over toward it and to apply their tips to
the surface of its body. Thus it becomes enveloped
with a host of glands, which then pour out a digestive
secretion upon it to dissolve its tissues. When digested
its substance is absorbed into the tissue of the leaf.

The pitcher-plant, Sarracenia, captures insects in a
different way. Its leaves are aquatic pitfalls. They
rise usually from the surface of the sphagnum in a bog
(see fig. 207 on p. 350) on stout bases from a deep seated
root stalk. They are veritable pitchers, swollen in the
middle, narrowed at the neck and with flaring lips.
The rains fill them. Insects fall into them and are
unable to get out again ; for all around the inner walls
in the region of the neck there grows a dense barrier of
long sharp spines with points directed downward.
This prevents climbing out. The insects are drowned,
and their decomposed remains are absorbed by the
plant as food.

It is mainly aerial insects that are destroyed, flies,
moths, beetles, etc. ; and we should not omit to note in
passing that there are other insects, habituated to life
in the water of the pitchers, and that normally develop
there. Such are the larvae of the mosquito, Aedes
smithi, and of a few flies and moths.

The bladderworts (Utricularia) are submerged plants
that float just beneath the surface. On their bright
green, finely dissected leaves are innumerable minute
traps (not bladders or floats as the name of the plant
implies) having the appearance shown in the accom-



Insectivorous Plants



panying figure. These capture small aquatic animals,
such as insect larvae, crustaceans, mites, worms, etc.

The mec-
hanism of
the trap is
shown dia-
grammati-
cally in
figure 174.
First of all
there is a
circle of
r adi at-
ing hairs
about the
entrance,
set diagon-
ally out-
ward, like
the leaders
of a fisher-
man's fyke
net, and
well adapt-
ed to turn
the free-
swimming
water - flea
to ward the





FIG. 173. A
spray of the
common
bladderwort,
Utricularia.



proper point of ingress. Then there is a trans-
parent elastic valve stopping the entrance, hinged by



286 Adjustment to Conditions of Aquatic Life



\



one side so that it will readily push inward, but holding
tightly against the rim when pressed outward. This

is the most important
single feature of the trap.
It makes possible getting
in easily and impossible
getting out at all. Dar-
win speaks of a Daphnia
which inserted an anten-
nae into the slit, and was
held fast during a whole
day, being unable to with-
draw it. On the outer
face of the valve near its
margin is a row of gland-
ular hairs. These have
roundly swollen terminal
secreting cells. They may
be alluring in function, tho
this has not been proven.
Directed backward across
the center of the valve are
four stiff bristles, that
may be useful for keeping

FIG. 174. Diagram of the mechanism Out of the passageway ani-

of a trap of one of the common blad- ma l s too j^g to pass

derworts. A, The trap from the ,- ,.. & ., 1 ,

ventral side, showing the outspread through it SUChaS might

leader hairs converging to the entrance, blockade the entrance.
L leaders, r. rim, v, valve. B, A ^ -.. . 1 1
median section of the same r', rim; v, bmall animals When en-
valve; w x, y, s epidermal hairs; trapped swim about for a

w, from the inner side of the rim; x, - rfr , . . . - -

from the free edge of the valve; y, long time inside, but in

from the base of the valve; z, from t he end they die and are

the general inner surface of the trap. ., 1 XT

decomposed. New traps

are of a bright translucent greenish color; old ones are
blackish from the animal remains they contain. The
inner surface of the trap is almost completely covered




The Larva Habits of Fresh-water Mussels 287



with branched hairs. These are erect forked hairs ad-
jacent to the rim, and flat-topped four-rayed hairs over
the remainder of the wall space. These hairs project
into the dissolved fluids, as do roots into the nutriet
solutions in the soil, and their function is doubtless the
absorption of food.

II

The larval habits of fresh-water mussels The early life
of our commonest fresh-water mussels is filled with





FIG. 175. Small minnows bearing larval
mussels (glochidia) on their fins.

shifts for a living that illustrate in a remarkable way
the interdependence of organisms. The adult mussels
burrow shallowly through the mud, sand and gravel of
the bottom (as noted on page 108) or lie in the shelter of
stones. Their eggs are very numerous, and hatch into
minute and very helpless larvae. For them the vicissi-
tudes of life on the bottom are very great. The chief
peril is perhaps that of being buried alive and smoth-
ered in the mud. In avoidance of this and as means



288 Adjustment to Conditions of Aquatic Life



of livelihood during early development the young
of mussels have mostly taken on parasitic habits.
They attach themselves to the fins and gills of fishes
(fig. 1 75) . There they
feed and grow for a
season, and there they
undergo a metamor-
phosis to the adult
form. Then they fall
to the pond bottom
and thereafter lead
independent lives.




FIG. 176. A gravid mussel (Symphynota
complanatd) with left valve of shell and
mantle removed, showing brood pouch
(modified gill) at B. (After Lefevre and
Curtis.)



The eggs of the river-
mussels are passed in-
to the watertubes of
the gills where they
are incubated for a
time. Packed into these passageways in enormous
numbers they distend them like cushions, filling them
out in various parts of one or both gills according to
the species, but mostly filling the outer gill. When
one picks up a gravid mussel from the river bed the
difference between the thin normal gill and the gill that
is serving as a brood chamber (fig. 176) is very marked.

Glochidia In the case of a very few river mussels
(Anodonta imbecillis, etc.) development to the adult
form occurs within the brood chamber; but in most
river mussels the eggs develop there into a larval form
that is known as a glochidium. This is already a bivalve
(fig. 177) possessing but a single adductor muscle for
closing the valves and lacking the well developed system
of nutritive organs of the adult. It is very sensitive
to contact on the ventral surface. In this condition
it is cast forth from the brood chamber.



Glochidia 289



If now the soft filament of a fish's gill, or the pro-
jecting ray of a fin by any chance conies in contact with
this sensitive surface the glochidium will close upon it
almost with a snap; and if the fish be the right kind
for the fostering of this particular mollusc, it will
remain attached. It is indeed interesting to see how
manifestly ready for this reaction are these larvae. If a
ripe brood chamber of Anodonta (fig. 88 on p. 180) be
emptied into a watch glass of water, the glochidia
scattered over the bottom will lie gaping widely and
will snap their toothed valves together betimes, whether
touched or not. And they will tightly clasp a hair
drawn across them.

Doubtless gills become infected when water contain-
ing the glochidia is drawn in through the mouth and
passed out over them. Fins by their lashing cause
in the water swirling currents that bring the glochidia
up against their soft rays and thin edges.

Glochidia vary considerably in form and size, in so
much that with careful work species of mussels can
usually be recognized by the glochidia alone. Thus it
is possible on finding them attached to fishes, to name
the species by which the fishes are infected.

In size glochidia range usually between .5 and .05
millimeter in greatest diameter. Some are more or
less triangular in lateral outline and these have usually
a pair of opposed teeth at the ventral angle of the valves.
Others are ax-head shaped and have either two teeth or
none at all on the ventral angles. But the forms that
have the ventral margin broadly rounded and toothless
are more numerous. Whether toothed or not they are
able to cling securely when attached in proper place to
a proper host.

The part taken by the fish in the association is truly
remarkable. The fish is not a mere passive agent of
mussel distribution. Its tissues repond to the stimulus



290 Adjustment to Conditions of Aquatic Life





FIG. 177. Glochidia and their development,
into larval mussels, a, b, c, d, stages in the
encystment of glochidia of the mussel, Ano-
donta, on the fin of a carp; e and /, young
mussels (Lampsilis) a week after liberation from
the fish; g, glochidium of the mussel, Lampsilis,
before attachment. (After Leiavre and Curtis).

h, glochidium of the wash-board mussel, Quadrula
heros, greatly enlarged and stained to show the
larval thread (/ and sensory hair cells (5 h c)
The clear band is the single adductor muscle.

*, a gill filament of a channel cat-fish bearing
an encysted glochidium of the warty-back mussel:
the eyst is set off by incisions of the filament.
The darker areas on the edges of the valves indi-
cate new growth of mussel shell. (After Howard.)

j, Encysted young of Plagiola donaciformis, showing

great growth of adult shell, beyond the margin

of glochidial shell much greater growth than

occurs in most species during encystment. (After

Surber.)



of the glochidia in a
way that parallels the
response of a plant to
the stimulus of a gall
insect. As a plant
develops a gall by new
growth of tissue about
the attacking insect,
and shuts it in and
both shelters and feeds
it, so the fish develops
a cyst about the glo-
chidium and protects
and feeds it. The tis-
sues injured by the
valves of the glochi-
dium produce new
cells by proliferation.
They rise up about the
larva and shut it in
(fig. 177). They sup-
ply food to it until the
metamorphosis is com-
plete, and then, when
it is a complete mussel
in form, equipped with
a foot for burrowing
and with a good sys-
tem of nutritive or-
gans, they break away
from it and allow it
to fall to the bottom.
Since this period lasts
for some weeks, or
even in a few cases,
months, the fishes by



Glochidia 291



wandering from place to place aid the distribution of
the mussels, but they do much more than this.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 18 19 20 21 22 23 24 25 26

Online LibraryJames G. (James George) NeedhamThe life of inland waters; an elementary text book of fresh-water biology for students → online text (page 16 of 26)