THE LIFE-STORY AND THE SEASONS
A number of interesting questions are associated with the seasonal cycle
of an insect's life-history. In a previous chapter (IV. pp. 30, 34)
reference has been made to the contrast between the long aquatic life of
the larval dragon-fly or may-fly, extending over several years, and the
short aerial existence of the winged adult restricted in the case of the
may-flies to a few hours. Here we see that the feeding activities of the
insect are carried on during the larval stage only; the may-fly in its
winged condition takes no food, pairing and egg-laying form the whole of
its appointed task. A similar though less extreme shortening of the
imaginal life may be noticed in many endopterygote insects. For example,
the bot- and warble-flies have the jaws so far reduced that they are
unable to feed, and the parasitic life of the maggot (see p. 74)
extending over eight or nine months in the body of the horse or ox,
prepares for a winged existence of probably but a few days. Again in
many moths the jaws are reduced or vestigial so that no food can be
taken in the winged state, as for example in the 'Eggars'
(Lasiocampidae) and the 'Tussocks' (Lymantriidae). It is noteworthy
that in these short-lived insects the male is often provided with
elaborate sense-organs which, we may believe, assist him to find a mate
with as little delay as possible; the male may-fly has especially
complex eyes, while the feelers of the male silk-moth or eggar are
comb-like or feathery, the branches bearing thousands of sensory hairs.
A box with a captive living female of one of these moths, if taken into
a wood haunted by the species becomes rapidly surrounded by a swarm of
would-be suitors, attracted by the odour emitted from the prisoner's
scent-glands.
Very exceptionally the imaginal stage may be omitted from the life-story
altogether. Nearly fifty years ago N. Wagner (1865) made the remarkable
discovery that in the larvae of certain gall-midges (Cecidomyidae) the
ovaries might become precociously mature and unfertilised eggs might be
developed into small larvae observable within the body of the
mother-larva; ultimately these abnormally reared young break their way
out. In this case therefore there may be a series of larval generations,
neither pupa nor imago being formed. Extended observations on the
precocious reproductive processes of these midges have lately been
published by W. Kahle (1908). A less extreme instance of an abbreviated
life-story was made known by O. Grimm (1870) who saw pupae of
Harlequin-midges (Chironomus) lay unfertilised eggs, which developed
into larvae. Here the imaginal stage only is omitted from the
life-history. Not always however is it the imaginal stage of the
life-history which is shortened. Reference (p. 18) has already been made
to the case of the virgin female aphids, whose eggs develop within the
mother's body, so that active, formed young are brought forth. Among the
Diptera it is not unusual to find similar cases, the female fly giving
birth to young maggots instead of laying eggs. Such is the habit of the
great flesh-fly (Sarcophaga), of some allied genera (Tachina, etc.)
whose larvae live as parasites on other insects, and occasionally of the
Sheep Bot-fly (Oestrus). In such cases we recognise the beginning of a
shortened larval period, and Brace's investigations in 1895, summarised
by E.E. Austen (1911), have shown that females of the dreaded African
Tsetse flies (Glossinia) bring forth nearly mature larvae, which pupate
soon after birth. In another group of Diptera, the blood-sucking
parasites of the Hippoboscidae and allied families, the whole larval
development is passed through within the mother's body, and a full-grown
larva is born the cuticle of which hardens and darkens immediately to
form a puparium; hence these flies are often called, though incorrectly,
Pupipara. Still more astonishing is the mode of reproduction in the
allied family of the Termitoxeniidae, curious, degraded, wingless
'guests' of the termites, or 'white ants,' lately made known through the
researches of E. Wasmann (1901). Here the individual is hermaphrodite--a
most exceptional condition among insects--and lays a large egg, whence
is usually hatched a fully-developed adult! Here then we find that all
the early stages, usual in the higher insects, are omitted from the
life-story.
Interesting comparison may be made between the total duration of various
insect life-stories. To some extent at least, the length of an insect's
life is correlated with its size, its food, the season of the year when
it breeds. Small insects have, as a rule, shorter lives than large ones;
those whose larvae devour highly nutritive food generally develop more
quickly than those which have to live on dry, poor, substances;
life-cycles follow one another most rapidly in summer weather when
temperature is high and food plentiful.
In early chapters we have already noticed the long aquatic life of the
larva and nymph of a dragon-fly, relatively a large insect, and the
rapid multiplication of the repeated summer broods of virgin aphids (p.
18). Within the one order of the Coleoptera it is instructive to compare
the small jumping leaf-beetles, the 'turnip-flies' of the farmer, whose
larvae mine in the green tissues, and complete their transformations so
rapidly that several successive broods appear in the spring and early
summer, with the larger click-beetles whose larvae, the equally
notorious 'wireworms,' feed on roots for three or four years before they
become fully grown. Among the Diptera, the 'leather-jacket' grub of the
crane-fly, feeding like the wireworm on roots, has a larval life
extending through the greater part of a year, while the maggot of the
bluebottle, feeding on a rich meat diet, becomes mature in a few days.
As examples of excessively long life-cycles the 'thirteen-year' and
'seventeen-year' cicads of North America, described by C.L. Marlatt
(1895), are noteworthy. Certain specially populous 'broods' of these
insects are known and localised, so that the appearance of the imagos in
future years can be accurately predicted. Here again we have to do with
bulky insects whose subterranean larvae and nymphs feed on comparatively
innutritious roots.
In our own climate, it is of interest to notice the variation among
insects as to the stage which carries the race over the winter. The
click-beetles, mentioned just above, emerge from their buried pupae in
summer, hibernate under stones or clods, and lay eggs among the herbage
next spring. At the same time of course, owing to the extended term of
the larval life, many more individuals of the species are wintering
underground as 'wireworms' of various ages, and these, except in very
severe frosts, can continue their occupation of feeding on roots. But in
the case of the 'turnip-flies' the food-supply is cut off in winter, and
all those beetles of the latest summer brood that survive hibernate in
some sheltered spot, waiting for the return of spring, that they may lay
their eggs, and start the life-cycle once again. Among the Diptera, most
species pass the winter as pupae, the sheltering puparium being a good
protection against most adverse conditions, or as flies. But where there
is a prolonged parasitic larval life, as with the bot- and warble-flies,
the maggot, warm and well-fed within the body of its mammalian host,
affords an appropriate wintering stage.
Among the Hymenoptera an especially interesting seasonal life-cycle is
afforded by the alternation of summer and winter generations in many
Gall-flies (Cynipidae) as H. Adler (1881, 1896) demonstrated for most of
our common species. The well-known 'oak-apples' are tenanted in summer
by grubs, which after pupation develop into winged males and wingless
females. The latter, after pairing, burrow underground and lay their
eggs in the roots, the larvae causing the presence there of globular
swellings or root-galls within which they live, pass through their
transformations and develop into wingless virgin females. These shelter
until February or March in their underground chambers, then climb up
the tree and lay on the shoots eggs, from which will be hatched the
grubs destined to grow within the oak-apples into the summer sexual
brood of flies.
The Lepidoptera afford examples of hibernation in all stages of the
life-history. In this order a few large moths with wood-boring
caterpillars, the 'Goat' (Cossus) for example, undergo a development
extending over several years, while at the other extreme a few small
species may have three or more complete cycles within the twelve months.
But in the vast majority of Lepidoptera we find either one or two
generations, definitely seasonal, within the year; the insect is either
'single-brooded' or 'double-brooded.'
Almost every winter one or more letters may be read in some newspaper
recording the writer's surprise at seeing on a sunny day during the cold
season, one of our common gaily-coloured butterflies of the Vanessa
group, a 'Tortoiseshell' or 'Red Admiral,' flitting about. Surprise
might be greater did the observers realise that the imaginal is the
normal hibernating stage for these species. Emerging from the pupa in
late summer or autumn, they shelter during winter in hollow trees, under
thatched eaves, in outbuildings or in similar situations, coming out in
spring to lay their eggs on the leaves of their caterpillars'
food-plants. The larvae feed and grow through the early summer months,
in the case of the Small Tortoiseshell (_Vanessa urticae_) pupating
before midsummer and developing into a July brood of butterflies whose
offspring after a late summer life-cycle, hibernate; while for the
larger species of the group there is, in our islands, only one complete
life-cycle in the year, though the same insects in warmer countries may
be double-brooded. C.G. Barrett records (1893, vol. I. pp. 153-4) how in
the August of 1879 hundreds and thousands of 'Painted Ladies' (_Pyrameis
cardui_) migrated into the south of England from the European continent
where in many places great swarms had been observed early in the summer.
'These August butterflies, the progeny of the June swarms, coming from a
warmer climate, had no intention of hibernating, but paired and laid
eggs. Some of the larvae were collected and reared indoors [butterflies]
emerging in November and December, but out of doors all must have been
destroyed by damp or frost, in either the larva or pupa state, for no
freshly emerged specimens were noticed in the spring, and no trace of
the great migration remained.'
In September and October the pedestrian, even in a suburban square, may
see moths with pretty brown, white-spotted wings flying around trees.
These are males of the common 'Vapourer' (_Orgyia antiqua_), in search
of the females which, wingless and helpless, rest on the cocoons
surrounding the pupae whence they have just emerged, the cocoons being
attached to the branches of the trees where the caterpillars have fed.
After pairing, the female lays her eggs among the silk of the cocoon,
partly covering them with hairs shed from her body, and then dies. The
eggs thus protected remain through the winter, the larvae not being
hatched till springtide, when the young leaves begin to sprout forth.
The caterpillars, adorned and probably protected by their 'tussocks' of
black or coloured bristles, feed vigorously. Their activity and habit of
occasional migration from one tree to another, compensates, to some
extent, as Miall (1908) has pointed out, for the females' enforced
passivity; only in the larval state can moths with such wingless females
extend their range. The caterpillars spin their cocoons towards the end
of summer, and then pupate, the moths emerging in the autumn and the
eggs, as we have seen, furnishing the winter stage.
After midsummer, the conspicuous cream, black and yellow-spotted
'Magpie' moth (_Abraxas grossulariata_) is common in gardens. The female
lays her eggs on a variety of shrubby plants; gooseberry and currant
bushes are often chosen. From the eggs caterpillars are hatched in
autumn, but these, instead of beginning to feed, seek almost at once for
rolled-up leaves, cracks in walls, crannies of bark, or similar places,
which may afford winter shelters. Here they remain until the spring,
when they come out to feed on the young foliage and grow rapidly into
the conspicuous cream, yellow and black 'looper' caterpillars mentioned
in a previous chapter (p. 60). These, when fully-grown, spin among the
twigs of the food-plant a light cocoon, in which the black and
yellow-banded wasp-like pupa spends its short summer term before the
emergence of the moth.
An equally familiar garden insect, the common 'Tiger' moth (_Arctia
caia_) with its 'woolly bear' caterpillar, affords a life-cycle slightly
differing from that of the 'Magpie.' The gaudy winged insects are seen
in July and August, and lay their eggs on a great variety of plants. The
larvae hatched from these eggs begin to feed at once, and having moulted
once or twice and attained about half their full size, they rest through
the winter, the dense hairy covering wherewith they are provided forming
an effective protection against the cold. At the approach of spring they
begin to feed again, and the fully-grown 'woolly bear' is a common
object on garden paths in May and June. Before midsummer it has usually
spun its yellow cocoon under some shelter on the ground and changed into
a pupa.
Another modification with respect to seasonal change is shown by the
Turnip moth (_Agrotis segetum_) and other allied Noctuidae (Owl-moths).
These are insects with brown-coloured wings, flying after dark in June.
The dull greyish larvae feed on many kinds of low-growing plants,
usually hiding in the earth by day and wandering along the surface of
the ground by night, biting off the farmer's ripening corn, or burrowing
into his turnips or potatoes. On account of the burrowing habits of this
insect it can feed throughout the winter, except when a hard frost puts
a temporary stop to its activity. By April it has become fully grown and
pupates in an earthen chamber a few inches below the surface. The Turnip
moth in our countries is partially double-brooded, a minority of the
autumn caterpillars growing more rapidly than their comrades so that
they pupate, and a second brood of moths appear in September. These pair
and lay eggs, the resulting caterpillars going as Barrett suggests
(1896, vol. III. p. 291) 'to reinforce the great army of wintering
larvae.'
Such underground caterpillars, to a great extent protected from cold,
can continue to feed through the winter. With other species we find that
the larva becomes fully grown in autumn, yet lives through the winter
without further change. This is the case with the Codling moth
(_Carpocapsa pomonella_), a well-known orchard pest, which in our
countries is usually single-brooded. The moth is flying in May and lays
her eggs on the shoots or leaves of apple-trees, more rarely on the
fruitlets, into which however the caterpillar always bores by the upper
(calyx) end. Here it feeds, growing with the growth of the fruit,
feeding on the tissue around the cores, ultimately eating its way out
through a lateral hole, and crawling upwards if its apple-habitation has
fallen, downwards if it still remains on the bough, to shelter under a
loose piece of bark where it spins its cocoon about midsummer and
hibernates still in the larval condition. Not until spring is the pupal
form assumed, and then it quickly passes into the imaginal state. In the
south of England, as F.V. Theobald (1909) has lately shown, and also in
southwestern Ireland, this species may be double-brooded, the usual
condition on the European continent and in the United States of America.
There the midsummer larvae pupate at once and the moths of an August
brood lay eggs on the hanging or stored fruit; in this case, again,
however, the full-grown larva, quickly fed-up within the developed
apples, is the wintering stage.
Several of the insects mentioned in this survey, like the last-named
codling moth, are occasionally double-brooded. As an example of the many
Lepidoptera, which in our islands have normally two complete life-cycles
in the year, we may take the very familiar White butterflies (Pieris) of
which three species are common everywhere. The appearance of the first
brood of these butterflies on the wing in late April or May is hailed as
a sign of advanced spring-time. They pair and lay their eggs on
cabbages and other plants, and the green hairy caterpillars feed in June
and July, after which the spotted pupae may be found on fences and
walls, attached by the silken tail-pad and supported by the
waist-girdle. In August and September butterflies of the second brood
have emerged from these and are on the wing; their offspring are the
autumn caterpillars which feed in some seasons as late as November,
doing often serious damage to the late cruciferous crops before they
pupate. The pupae may be seen during the winter months, waiting for the
spring sunshine to call out the butterflies whose structures are being
formed beneath the hard cuticle.
Reviewing the small selection of life-stories of various Lepidoptera
just sketched, we notice an interesting and suggestive variety in the
wintering stage. The vanessid butterflies hibernate as imagos; the
'vapourer' winters in the egg, the magpie as a young ungrown larva, the
'tiger' as a half-size larva; the Agrotis caterpillar feeds through the
winter, growing all the time; the codling caterpillar completes its
growth in the autumn, and winters as a full-size resting larva; lastly,
the 'whites' hibernate in the pupal state. And in every case it is
noteworthy that the form or habit of the wintering stage is well adapted
for enduring cold.
Our native 'whites' afford illustration of another interesting feature
often to be noticed in the life-story of double-brooded Lepidoptera. The
butterflies of the spring brood differ slightly but constantly from
their summer offspring, affording examples of what is called _seasonal
dimorphism_. All three species have whitish wings marked with black
spots, larger and more numerous in the female than in the male. In the
spring butterflies these spots tend towards reduction or replacement by
grey, while in the summer insects they are more strongly defined, and
the ground colour of the wings varies towards yellowish. In the
'Green-veined' white (_Pieris napi_) the characteristic greenish-grey
lines of scaling beneath the wings along the nervures, are much broader
and more strongly marked in the spring than in the summer generation,
whose members are distinguished by systematic entomologists under the
varietal name _napaeae_. The two forms of this insect were discussed by
A. Weismann in his classical work on the Seasonal Dimorphism of
butterflies (1876). He tried the effect of artificially induced cold
conditions on the summer pupae of _Pieris napi_, and by keeping a batch
for three months at the temperature of freezing water, he succeeded in
completely changing every individual of the summer generation into the
winter form. The reverse of this experiment also was attempted by
Weismann. He took a female of _bryoniae_, an alpine and arctic variety
of _Pieris napi_, showing in an intensive degree the characters of the
spring brood. This female laid eggs the caterpillars from which fed and
pupated. The pupae although kept through the summer in a hothouse all
produced typical _bryoniae_, and none of these with one exception
appeared until the next year, for in the alpine and arctic regions this
species is only single-brooded. Weismann experimented also with a small
vanessid butterfly, _Araschnia levana_, common on the European
continent, though unknown in our islands, which is double (or at times
treble) brooded, its spring form (_levana_) alternating with a larger
and more brightly coloured summer form (_prorsa_). Here again by
refrigerating the summer pupae, butterflies were reared most of which
approached the winter pattern, but it was impossible by heating the
winter pupae to change _levana_ into _prorsa_. Experiments with North
American dimorphic species have given similar results. Weismann argued
from these experiments that the winter form of these seasonally
dimorphic species is in all cases the older, and that the butterflies
developing within the summer pupae can be made to revert to the
ancestral condition by repeating the low-temperature stimulus which
always prevailed during the geologically recent Ice Age. On the other
hand, a high temperature stimulus applied to one generation of the
winter pupae cannot induce the change into the summer pattern, which has
been evolved still more recently by slow stages, as the continental
climate has become more genial. In tropical countries where instead of
an alternation of winter and summer, alternate dry and rainy seasons
prevail, somewhat similar seasonal dimorphism has been observed among
many butterflies. Not a few forms of Precis, an African and Indian genus
allied to our Vanessa, that had long been considered distinct species
are now known, thanks to the researches of G.A.K. Marshall (1898), to be
alternating seasonal forms of the same insect. The offspring when adult
does not closely resemble the parent; its appearance is modified by the
climatic environment of the pupa. The experiments of Weismann just
sketched in outline show at least that the same principle holds for our
northern butterflies.
We are thus led to see from the life-story of such insects, that the
course of the story is not rigidly fixed; the creature in its various
stages is plastic, open to influence from its surroundings, capable of
marked change in the course of generations. And so the seasonal changes
in the history of the individual from egg to imago point us to changes
in the age-long history of the race.