FROM WATER TO AIR
Insects as a whole are preeminently creatures of the land and the air.
This is shown not only by the possession of wings by a vast majority of
the class, but by the mode of breathing to which reference has already
been made (p. 2), a system of branching air-tubes carrying atmospheric
air with its combustion-supporting oxygen to all the insect's tissues.
The air gains access to these tubes through a number of paired air-holes
or spiracles, arranged segmentally in series.
It is of great interest to find that, nevertheless, a number of insects
spend much of their time under water. This is true of not a few in the
perfect winged state, as for example aquatic beetles and water-bugs
('boatmen' and 'scorpions') which have some way of protecting their
spiracles when submerged, and, possessing usually the power of flight,
can pass on occasion from pond or stream to upper air. But it is
advisable in connection with our present subject to dwell especially on
some insects that remain continually under water till they are ready to
undergo their final moult and attain the winged state, which they pass
entirely in the air. The preparatory instars of such insects are
aquatic; the adult instar is aerial. All may-flies, dragon-flies, and
caddis-flies, many beetles and two-winged flies, and a few moths thus
divide their life-story between the water and the air. For the present
we confine attention to the Stone-flies, the May-flies, and the
Dragon-flies, three well-known orders of insects respectively called by
systematists the Plecoptera, the Ephemeroptera and the Odonata.
In the case of many insects that have aquatic larvae, the latter are
provided with some arrangement for enabling them to reach atmospheric
air through the surface-film of the water. But the larva of a stone-fly,
a dragon-fly, or a may-fly is adapted more completely than these for
aquatic life; it can, by means of gills of some kind, breathe the air
dissolved in water.
The aquatic young of a stone-fly does not differ sufficiently in form
from its parent to warrant us in calling it a larva; the life-history is
like that of a cockroach, all the instars however except the final
one--the winged adult or _imago_--live in the water. The young of one of
our large species, a Perla for example, has well-chitinised cuticle,
broad head, powerful legs, long feelers and cerci like those of the
imago; its wings arise from external rudiments, which are conspicuous in
the later aquatic stages. But it lives completely submerged, usually
clinging or walking beneath the stones that lie in the bed of a clear
stream, and examination of the ventral aspect of the thorax reveals six
pairs of tufted gills, by means of which it is able to breathe the air
dissolved in the water wherein it lives. At the base of the tail-feelers
or cerci also, there are little tufts of thread-like gills as J.A.
Palmén (1877) has shown. An insect that is continually submerged and has
no contact with the upper air cannot breathe through a series of paired
spiracles, and during the aquatic life-period of the stone-fly these
remain closed. Nevertheless, breathing is carried on by means of the
ordinary system of branching air-tubes, the trunks of which are in
connection with the tufted hollow gill-filaments, through whose delicate
cuticle gaseous exchange can take place, though the method of this
exchange is as yet very imperfectly understood. When the stone-fly nymph
is fully grown, it comes out of the water and climbs to some convenient
eminence. The cuticle splits open along the back, and the imago, clothed
in its new cuticle, as yet soft and flexible, creeps out. The spiracles
are now open, and the stone-fly breathes atmospheric air like other
flying insects. But throughout its winged life, the stone-fly bears
memorials of its aquatic past in the little withered vestiges of gills
that can still be distinguished beneath the thorax.
The adult dragon-fly (fig. 8 _d_) is specialised in such a way that it
captures its prey--flies and other small insects--on the wing, swooping
through the air like a hawk and feeding voraciously. The head is
remarkable for its large globular compound eyes, its short bristle-like
feelers, and its very strong mandibles which bite up the bodies of the
victims. The thorax bears the two pairs of ample wings, firm and almost
glassy in texture, and its segments are projected forward ventrally, so
that all six legs, which are armed with rows of sharp, slender spines,
can be held in front of the mouth, where they form an effective
fly-trap. The abdomen is very long and usually narrow.
A female dragon-fly after a remarkable mode of pairing, the details of
which are beside our present subject, drops her eggs in the water, or
lays them on water-weeds, perhaps cutting an incision where they can be
the more safely lodged, or even goes down below the surface and deposits
them in the mud at the bottom of a pond. From the eggs are hatched the
aquatic larvae which differ in many respects from the imago. The
dragon-fly larva has the same predaceous mode of life as its parent, but
it is sluggish in habit, lurking for its prey at the bottom of the pond,
among the mud or vegetation, which it resembles in colour. The thoracic
segments have not the specialisation that they show in the imago; the
abdomen is relatively shorter and broader. The larval head has, like
that of the imago, short feelers, and the eyes are somewhat large,
though far from attaining the size of the great globular eyes of the
dragon-fly. But the third pair of jaws, forming the labium, are most
remarkably modified into a 'mask,' the distal central portion (mentum)
being hinged to the basal piece (sub-mentum) which is itself jointed
below the head. The mentum carries at its extremity a pair of lobes with
sharp fangs. Thus the mask can be folded under the head when the larva
lurks in its hiding place, or be suddenly darted out so as to secure any
unwary small insect that may pass close enough for capture. Dragon-fly
larvae walk, and also swim by movements of the abdomen or by expelling a
jet of water from the hind-gut. The walls of this terminal region of the
intestine have areas lined with delicate cuticle and traversed by
numerous air-tubes, so that gaseous exchange can take place between the
air in the tubes and that dissolved in the water. The larvae of the
larger and heavier dragon-flies (Libellulidae and Aeschnidae) breathe
mostly in this way. Those of the slender and delicate 'Demoiselles'
(Agrionidae) are provided with three leaf-like gill-plates at the tail,
between whose delicate surfaces numerous air-tubes ramify. These
gill-plates are at times used for propulsion. Thus air supply is ensured
during aquatic life. But occasionally, when the water in which the
larva lives is foul and poor in oxygen, the tail is thrust out of the
water so that air can be admitted directly into the intestinal chamber.
The aquatic life of these insects lasts for more than a year, and F.
Balfour-Browne (1909) has observed from ten to fourteen moults in
Agrion. Outward wing-rudiments are early visible on the thoracic
segments; when these have become conspicuous the insect, beginning in
some respects to approach the adult condition, is often called a nymph.
In an advanced dragon-fly nymph, H. Dewitz (1891) has shown that the
thoracic spiracles are open, and, as the time for its final moult draws
near, the insect may thrust the front part of its body out of the water,
and breathe atmospheric air through these. Thus before the great change
takes place the nymph has foretastes of the aerial mode of breathing
which it will practise when the perfect stage shall have been attained.
The emergence of the dragon-fly from its nymph-cuticle has been
described by many naturalists from de Réaumur (1740) to L.C. Miall
(1895) and O.H. Latter (1904). The nymph climbs out of the water by
ascending some aquatic plant, and awaits the change so graphically
sketched by Tennyson:
A hidden impulse rent the veil,
Of his old husk, from head to tail,
Came out clear plates of sapphire mail.
'From head to tail,' for the nymph-cuticle splits lengthwise down the
back, and the head and thorax of the imago are freed from it (fig. 8
_a_), then the legs clasp the empty cuticle, and the abdomen is drawn
out (fig. 8 _b, c_). After a short rest, the newly-emerged fly climbs
yet higher up the water-weed, and remains for some hours with the
abdomen bent concave dorsalwards (fig. 8 _d_), to allow space for the
expansion and hardening of the wings. For some days after emergence the
cuticle of the dragon-fly has a dull pale hue, as compared with the dark
or brightly metallic aspect that characterises it when fully mature. The
life of the imago endures but a short time compared with the long
aquatic larval and nymphal stages. After some weeks, or at most a few
months, the dragon-flies, having paired and laid their eggs, die before
the approach of winter.
[Illustration: Fig. 8 _a, b_. Dragon-fly (_Aeschna cyanea_). Two stages
in emergence of fly from nymph-cuticle. From Latter's _Natural
History_.]
[Illustration: Fig. 8 _c_. Dragon-fly emerged, wings
expanding. From Latter's _Natural History_.]
[Illustration: Fig. 8 _d_. Dragon-fly (_Aeschna cyanea_) with
expanded wings.]
The life-story of a may-fly follows the same general course as that just
described for the dragon-flies, but there are some suggestive
differences. In the first place, we notice a wider divergence between
the imago and the larva. An adult may-fly is one of the most delicate
of insects; the head has elaborate compound eyes, but the feelers are
very short, and the jaws are reduced to such tiny vestiges that the
insect is unable to feed. Its aquatic larva is fairly robust, with a
large head which is provided with well-developed jaws, as the larval and
nymphal stages extend over one or two years, and the insects browse on
water-weeds or devour creatures smaller and weaker than themselves. They
breathe dissolved air by means of thread-like or plate-like gills
traversed by branching air-tubes, somewhat resembling those of the
demoiselle dragon-fly larva. But in the may-fly larva, there is a series
of these gills (fig. 9_b_) arranged laterally in pairs on the abdominal
segments, and C. Börner (1909) has recently given reasons, from the
position and muscular attachments of these organs, for believing that
they show a true correspondence to (in technical phraseology are
homologous with) the thoracic legs. One feature in which the larva often
agrees with the imago is the possession on the terminal abdominal
segment of a pair of long jointed cerci, and in many genera a median
jointed tail-process (see fig. 9) is also present, in some cases both in
the larva and the imago, in others in the larva during its later stages
only. The prolonged larval life in may-flies often involves a large
series of moults; Lubbock (1863) has enumerated twenty-one in the
life-history of Chloeon. In the second year of aquatic life
wing-rudiments (fig. 9 _a_) are visible, and the larva becomes a nymph.
When the time for the winged condition approaches the nymphs leave the
water in large swarms. The vivid accounts of these swarms given by
Swammerdam (1675), de Réaumur (1742) and other old-time observers are
available in summarised form for English readers in Miall's admirable
book (1895). May-flies are eagerly sought as food by trout, and the rise
of the fly on many lakes ushers in a welcome season to the angler.
The nymph-cuticle opens and the winged insect emerges. But this is not
the final instar; may-flies are exceptional among insects in undergoing
yet another moult after they have acquired wings which they can use for
flight. The instar that emerges from the nymph-cuticle is a sub-imago,
dull in hue, with a curious immature aspect about it. A few hours later
the final moult takes place, a very delicate cuticle being shed and
revealing the true imago. Then follow the dancing flight over the calm
waters, the mating and egg-laying, the rapid death. The whole winged
existence prepared for by the long aquatic life may be over in a single
evening; at most it lasts but for a few days.
[Illustration: Fig. 9. Nymph of May-fly (_Chloeon dipterum_) showing on
right side wing-rudiment (_a_), on left tracheal gills (_b_). Magnified
4 times. [Feelers and legs are cut short.] From Miall and Denny after
Vayssière.]
In the development of the may-flies, then, we notice not only a
considerable divergence between larva and imago, both in habitat and
structure; we see also what is to be observed often in more highly
organised insects--a feeding stage prolonged through the years of larval
and nymphal life, while the winged imago takes no food and devotes its
energies through its short existence to the task of reproduction. Such
division of the life-history into a long feeding, and a short breeding
period has, as will be seen later, an important bearing on the question
of insect transformation generally, and the dragon-flies and may-flies
afford examples of two stages in its specialisation. The sub-imaginal
instar of the may-fly furnishes also a noteworthy fact for comparison
with other insect histories. In two points, however, the life-story of
these flies with their aquatic larvae recalls that of the cockroach. All
the larval and nymphal instars are active, and the wing-rudiments are
outwardly visible long before the final moult.