from which the normal length of its life can be ascertained, but many
statements tend to show that it is exceedingly long-lived, and this is
advantageous for a parasite of which the food (and consequently growth
and reproduction) is extremely precarious. They can endure starvation
for an astonishingly long period, and can survive the most intense
cold. Leunis (‘Zoologie,’ p. 659) mentions the case of a female which
was shut up in a box and forgotten: after six months’ starvation it was
found not only alive but surrounded by a circle of lively young ones.
Göze found bugs in the hangings of an old bed which had not been used
for six years: ‘they appeared white like paper.’ I have myself observed
a similar case, in which the starving animals were quite transparent.
De Geer placed some bugs in an unheated room in the cold winter of
1772, when the thermometer fell to -33°C: they passed the whole winter
in a state of torpidity, but revived in the following May. (De Geer,
Bd. III. p. 165, and Oken, ‘Naturgeschichte,’ 2^{ter} Bd. 3^{te} Abth. p.
1613.)
V. Diptera.
_Pulex irritans._ Oken says of the flea (‘Naturgeschichte,’ Bd. II.
Abth. 2, p. 759) that ‘death follows the deposition of the eggs in the
course of two or three days, even if the opportunity of sucking blood
is given them.’ The length of time which intervenes between the
emergence from the cocoon and fertilization or the deposition of eggs
is not stated.
_Sarcophaga carnaria._ The female fly dies ten to twelve hours after
the birth of the viviparous larvae; the time intervening between the
exit from the cocoon and the birth of the young is not given (Oken,
quoting Réaumur, ‘Mém. p. s. à l’hist. Insectes,’ Paris, 1740-48, IV).
_Musca domestica._ In the summer the common house-fly begins to lay
eggs eight days after leaving the cocoon: she then lays several times.
(See Gleichen, ‘Geschichte der gemeinen Stubenfliege,’ Nuremberg, 1764.)
_Eristalis tenax._ The larva of this large fly lives in liquid manure,
and has been described and figured by Réaumur as the rat-tailed larva.
I kept a female which had just emerged from the cocoon, from August
30th till October 4th, in a large gauze-covered glass vessel. The
insect soon learnt to move freely about in its prison, without
attempting to escape; it flew round in circles, with a characteristic
buzzing sound, and obtained abundant nourishment from a solution of
sugar, provided for it. From September 12th it ceased to fly about,
except when frightened, when it would fly a little way off. I thought
that it was about to die, but matters took an unexpected turn, and on
the 26th of September it laid a large packet of eggs, and again on the
29th of the same month another packet of similar size. The flight of
the animal had been probably impeded by the weight of the mass of ripe
eggs in its body. The deposition of eggs was probably considerably
retarded in this case, because fertilization had not taken place. The
fly died on the 4th of October, having thus lived for thirty-five days.
Unfortunately, I have been unable to make any experiments as to the
duration of life in the female when males are also present.
VI. Lepidoptera.
I am especially indebted to Mr. W. H. Edwards[11], of Coalburgh, W.
Virginia, and to Dr. Speyer, of Rhoden, for valuable letters relating
to this order.
The latter writes, speaking of the duration of life in imagos
generally:—‘It is, to my mind, improbable that any butterfly can live
as an imago for a twelvemonth. Specimens which have lived through the
winter are only rarely seen in August, even when the summer is late. A
worn specimen of _Vanessa cardui_ has, for instance, been found at this
time’ (‘Entomolog. Nachrichten,’ 1881, p. 146).
In answer to my question as to whether the fact that certain
Lepidoptera take no solid or liquid food, and are, in fact, without a
functional mouth, may be considered as evidence for an adaptation of
the length of life to the rapid deposition of eggs, Dr. Speyer
replies:—‘The wingless females of the _Psychidae_ do not seem to
possess a mouth, at any rate I cannot find one in _Psyche unicolor_
(_graminella_). They do not leave the case during life, and certainly
do not drink water. The same is true of the wingless female of
_Heterogynis_, and of _Orgyia ericae_, and probably of all the females
of the genus _Orgyia_; and as far as I can judge from cabinet
specimens, it is probably true of the males of _Heterogynis_ and
_Psyche_. I have never seen the day-flying _Saturnidae_, _Bombycidae_,
and other Lepidoptera with a rudimentary proboscis, settle in damp
places, or suck any moist substance, and I doubt if they would ever do
this. The sucking apparatus is probably deficient.’
In answer to my question as to whether the males of any species of
butterfly or moth are known to pass a life of different length from
that of the female, Dr. Speyer stated that he knew of no observations
on this point.
The following are the only instances of well-established direct
observations upon single individuals, in my possession[12]:—
_Pieris napi_, var. _bryoniae_ [male] and [female], captured on the
wing: lived in confinement ten days, and were then killed.
_Vanessa prorsa_ lived at most ten days in confinement.
_Vanessa urticae_ lived ten to thirteen days in confinement.
_Papilio ajax._ According to a letter from Mr. W. H. Edwards, the
female, when she leaves the pupa, contains unripe eggs in her body, and
lives for about six weeks—calculating from the first appearance of this
butterfly to the disappearance of the same generation[13]. The males
live longer, and continue to fly when very worn and exhausted. A worn
female is very seldom seen;—‘I believe the female does not live long
after laying her eggs, but this takes some days, and probably two
weeks.’
_Lycaena violacea._ According to Mr. Edwards, the first brood of this
species lives three to four weeks at the most.
_Smerinthus tiliae._ A female, which had just emerged from the pupa,
was caught on June 24th; on the 29th pairing took place; on the 1st of
July she laid about eighty eggs, and died the following day. She lived
nine days, taking no food during this period, and she only survived the
deposition of eggs by a single day.
_Macroglossa stellatarum._ A female, captured on the wing and already
fertilized, lived in confinement from June 28th to July 4th. During
this time she laid about eighty eggs, at intervals and singly; she then
disappeared, and must have died, although the body could not be found
among the grass at the bottom of the cage in which she was confined.
_Saturnia pyri._ A pair which quitted the cocoons on the 24th or 25th
of April, remained in coitu from the 26th until May 2nd—six or seven
days; the female then laid a number of eggs, and died.
_Psyche graminella._ The fertilized female lives some days, and the
unfertilized female over a week (Speyer).
_Solenobia triquetrella._ ‘The parthenogenetic form (I refer to the one
which I have shown to be parthenogenetic in Oken’s ‘Isis,’ 1846, p. 30)
lays a mass of eggs in the abandoned case, soon after emergence. The
oviposition causes her body to shrivel up, and some hours afterwards
she dies. The non-parthenogenetic female of the same species remains
for many days, waiting to be fertilized; if this does not occur, she
lives over a week.’ ‘The parthenogenetic female lives for hardly a day,
and the same is true of the parthenogenetic females of another species
of _Solenobia_’ (_S. inconspicuella_?). Letter from Dr. Speyer.
_Psyche calcella, O._ The males live a very short time; ‘those which
leave the cocoon in the evening are found dead on the following
morning, with their wings fallen off, at the bottom of their cage.’ Dr.
Speyer.
_Eupithecia_, sp. (_Geometridae_), ‘when well-fed, live for three to
four weeks in confinement; the males fertilize the females frequently,
and the latter continue to lay eggs when they are very feeble, and are
incapable of creeping or flying.’ Dr. Speyer.
The conclusions and speculations in the text seem to be sufficiently
supported from this short series of observations. There remains, as we
see, much to be done in this field, and it would well repay a
lepidopterist to undertake some exact observations upon the length of
life in different butterflies and moths, with reference to the
conditions of life—the mode of egg-laying, the degeneracy of the wings,
and of the external mouth-parts or the closure of the mouth itself. It
would be well to ascertain whether such closure does really take place,
as it undoubtedly does in certain plant-lice.
VII. Coleoptera.
_Melolontha vulgaris._ Cockchafers, which I kept in an airy cage with
fresh food and abundant moisture, did not in any case live longer than
thirty-nine days. One female only, out of a total number of forty-nine,
lived for this period; a second lived thirty-six days, a third
thirty-five, and a fourth and fifth twenty-four days; all the rest died
earlier. Of the males, only one lived as long as twenty-nine days.
These periods are less by some days than the true maximum duration of
life, for the beetles were captured in the field, and had lived for at
least a day; but the difference cannot be great, when we remember that
out of forty-nine beetles, only three females lived thirty-five to
thirty-nine days, and only one male twenty-nine days. Those that died
earlier had probably lived for some considerable time before being
caught.
Exact experiments with pupae which have survived the winter would show
whether the female really lives for ten days more than the male, or
whether the results of my experiment were merely accidental. I may add
that coitus frequently took place during the period of captivity. One
pair, observed in this condition on the 17th, separated in the evening;
they paired again on the morning of the 18th, and separated in the
middle of the day. Coitus took place between another pair on the 22nd,
and again on the 26th.
I watched the gradual approach of death in many individuals: some days
before it ensued, the insects became sluggish, ceased to fly and to
eat, and only crept a little way off when disturbed: they then fell to
the ground and remained motionless, apparently dead, but moved their
legs when irritated, and sometimes automatically. Death came on
gradually and imperceptibly; from time to time there was a slow
movement of the legs, and at last, after some hours, all signs of life
ceased.
In one case only I found bacteria present in great numbers in the blood
and tissues; in the other individuals which had recently died, the only
noticeable change was the unusual dryness of the tissues.
_Carabus auratus._ An experiment with an individual, caught on May
27th, gave the length of life at fourteen days; this is probably below
the average, since the beetles are found, in the wild state, from the
end of May until the beginning of July.
_Lucanus cervus._ Captured individuals, kept in confinement, and fed on
a solution of sugar, never lived longer than fourteen days, and as a
rule not so long. The beetles appear in June and July, and certainly
cannot live much over a month. As is the case with many beetles
appearing during certain months, the length of the individual life is
shorter than the period over which they are found. Accurate
information, especially as to any difference between the lengths of
life in the sexes, is not obtainable.
* * * * *
Isolated accounts of remarkably long lives among beetles are to be
found scattered throughout the literature of the subject. Dr. Hagen, of
Cambridge, Mass., has been kind enough to draw my attention to these,
and to send me some observations of his own.
_Cerambyx heros._ One individual lived in confinement from August until
the following year[14].
_Saperda carcharias._ An individual lived from the 5th of July until
the 24th of July of the next year[15].
_Buprestis splendens._ A living individual was removed from a desk
which had stood in a London counting-house for thirty years; from the
condition of the wood it was evident that the larva had been in it
before the desk was made[16].
_Blaps mortisaga._ One individual lived three months, and two others
three years.
_Blaps fatidica._ One individual which was left in a box and forgotten,
was found alive when the box was opened six years afterwards.
_Blaps obtusa._ One lived a year and a half in confinement.
_Eleodes grandis_ and _E. dentipes_. Eight of these beetles from
California were kept in confinement and without food for two years by
Dr. Gissler, of Brooklyn; they were then sent to Dr. Hagen who kept
them another year.
_Goliathus cacicus._ One individual lived in a hot-house for five
months.
In addition to these cases, Dr. Hagen writes to me: ‘Among the beetles
which live for more than a year,—_Blaps_, _Pasimachus_,
(_Carabidae_)—and among ants, almost thirty per cent. are found with
the cuticle worn out and cracked, and the powerful mandibles so greatly
worn down that species were formerly founded upon this point. The
mandibles are sometimes worn down to the hypodermis.’
From the data before me I am inclined to believe that in certain
beetles the normal length of life extends over some years, and this is
especially the case with the _Blapidae_. It seems probable that in
these cases another factor is present,—a _vita minima_, or apparent
death, a sinking of the vital processes to a minimum in consequence of
starvation, which we might call the hunger sleep, after the analogy of
winter sleep. The winter sleep is usually ascribed to cold alone, and
some insects certainly become so torpid that they appear to be dead
when the temperature is low. But cold does not affect all insects in
this way. Among bees, for example, the activity of the insects
diminishes to a marked extent at the beginning of winter, but if the
temperature continues to fall, they become active again, run about, and
as the bee-keepers say, ‘try to warm themselves by exercise’; by this
means they keep some life in them. If the frost is very severe, they
die. In the tropics the period of hibernation for many animals
coincides with the time of maximum heat and drought. This shows that
the organism can be brought into the condition of a _vita minima_ in
various ways, and it would not be at all remarkable if such a state
were induced in certain insects by hunger. Exact experiments however
are the only means by which such a suggestion can be tested, and I have
already commenced a series of experiments. The fact that certain
beetles live without food for many years (even six) can hardly be
explained on any other supposition, for these insects consume a fair
amount of food under normal conditions, and it is inconceivable that
they could live for years without food, if the metabolism were carried
on with its usual energy.
A very striking example, showing that longevity may be induced by the
lengthening of the period of reproductive activity, is communicated to
me by Dr. Adler in the following note: ‘Three years ago I accidentally
noticed that ovoviviparous development takes place in _Chrysomela
varians_,—a fact which I afterwards discovered had been already
described by another entomologist.
‘The egg passes through all the developmental stages in the ovary; when
these are completed the egg is laid, and a minute or two afterwards the
larva breaks through the egg-shell. In each division of the ovary the
eggs undergo development one at a time; it therefore follows that they
are laid at considerable intervals, so that a long life becomes
necessary in order to ensure the development of a sufficiently long
series of eggs. Hence it comes about that the females live a full year.
Among other species of _Chrysomela_ two generations succeed each other
in a year, and the duration of life in the individual varies from a few
months to half a year.’
VIII. Hymenoptera.
_Cynipidae._ I have been unable to find any accurate accounts of the
duration of life in the imagos of saw-flies or ichneumons; but on the
other hand I owe to the kindness of Dr. Adler, an excellent observer of
the _Cynipidae_, the precise accounts of that family which are in my
possession. I asked Dr. Adler the general question as to whether there
was any variation in the duration of life among the _Cynipidae_
corresponding to the conditions under which the deposition of eggs took
place; whether those species which lay many eggs, or of which the
oviposition is laborious and protracted, lived longer than those
species which lay relatively few eggs, or easily and quickly find the
suitable places in which to deposit them.
Dr. Adler fully confirmed my suppositions and supported them by the
following statements:—
‘The summer generation of _Neuroterus_ (_Spathegaster_) has the
shortest life of all _Cynipidae_. Whether captured or reared from the
galls I have only kept them alive on an average for three to four days.
In this generation the work of oviposition requires the shortest time
and the least expenditure of energy, for the eggs are simply laid on
the surface of a leaf. The number of eggs in the ovary is also smaller
than that of other species, averaging about 200. This form of _Cynips_
can easily lay 100 eggs a day.
‘The summer generation of _Dryophanta_ (_Spathegaster Taschenbergi_,
_verrucosus_, etc.) lives somewhat longer; I have kept them in
confinement for six to eight days. The oviposition requires a
considerable expenditure of time and strength, for the ovipositor has
to pierce the rather tough mid-rib or vein of a leaf. The number of
eggs in the ovary averages 300 to 400.
‘The summer generation of _Andricus_, which belongs to the extensive
genus _Aphilotrix_, have also a long life. I have kept the smaller
_Andricus_ (such as _A. nudus_, _A. cirratus_, _A. noduli_) alive for a
week, and the larger (_A. inflator_, _A. curvator_, _A. ramuli_) for
two weeks. The smaller species pierce the young buds when quite soft,
but the larger ones bore through the fully grown buds protected by
tough scales. The ovary of the former contains 400 to 500 eggs, that of
the latter over 600.
‘The agamic winter generations live much longer. The species of
_Neuroterus_ have the shortest life; they live for two weeks at the
outside; on the other hand, species of _Aphilotrix_ live quite four
weeks, and _Dryophanta_ and _Biorhiza_ even longer. I have kept
_Dryophanta scutellaris_ alive for three months. The number of eggs in
these agamic _Cynipidae_ is much larger: _Dryophanta_ and _Aphilotrix_
contain 1200 and _Neuroterus_ about 1000.’
It is evidently, therefore, a general rule that the duration of life is
directly proportional to the number of eggs and to the time and energy
expended in oviposition. It must of course be understood that, here as
in all other instances, these are not the only factors which determine
the duration of life, but many other factors, at present unknown, may
be in combination with them and assist in producing the result. For
example, it is very probable that the time of year at which the imagos
appear exerts some indirect influence. The long-lived _Biorhiza_
emerges from the gall in the middle of winter, and at once begins to
deposit eggs in the oak buds. Although the insect is not sensitive to
low temperature, for I have myself seen oviposition proceeding when the
thermometer stood at 5° R., yet very severe frost would certainly lead
to interruption and would cause the insect to shelter itself among dead
leaves on the ground. Such interruptions may be of long duration and
frequently repeated, so that the remarkably long life of this species
may perhaps be looked upon as an adaptation to its winter life.
_Ants._ _Lasius flavus_ lays its eggs in the autumn, and the young
larvae pass the winter in the nest. The males and females leave the
cocoons in June, and pair during July and August. The males fly out of
the nest with the females, but they do not return to it; ‘they die
shortly after pairing.’ It is also believed that the females do not
return to the nest, but found new colonies; this point is however one
of the most uncertain in the natural history of ants. On the other hand
it is quite certain that the female may live for years within the nest,
continuing to lay fertilized eggs. Old females are sometimes found in
the colony, with their jaws worn down to the hypodermis.
Breeding experiments confirm these statements. P. Huber[17] and Christ
have already put the life of the female at three to four years, and Sir
John Lubbock, who has been lately occupied with the natural history of
ants, was able to keep a female worker of _Formica sanguinea_ alive for
five years; and he has been kind enough to write and inform me that two
females of _Formica fusca_, which he captured in a wood together with
ten workers, in December 1874, are still alive (July 1881), so that
these insects live as imagos for six and a half years or more[18].
On the other hand, Sir John Lubbock never succeeded in keeping the
males ‘alive longer than a few weeks.’ Both the older and more recent
observers agree in stating that female ants, like queen bees, are
always protected as completely as possible from injury and danger. Dr.
A. Forel, whose thorough knowledge of Swiss ants is well known, writes
to me,—‘The female ants are only once fertilized, and are then tended
by the workers, being cleaned and fed in the middle of the nest: one
often finds them with only three legs, and with their chitinous armour
greatly worn. They never leave the centre of the nest, and their only
duty is to lay eggs.’
With regard to the workers, Forel believes that their constitution
would enable them to live as long as the females (as the experiments of
Lubbock also indicate), and the fact that in the wild state they
generally die sooner than the females is ‘certainly connected with the
fact that they are exposed to far greater dangers.’ The same relation
seems also to obtain among bees, but with them it has not been shown
that in confinement the workers live as long as the queens.
_Bees._ According to von Berlepsch[19] the queen may as an exception
live for five years, but as a rule survives only two or three years.
The workers always seem to live for a much shorter period, generally
less than a year. Direct experiments upon isolated or confined bees, or
upon marked individuals in the wild state, do not prove this, but the
statistics obtained by bee-keepers confirm the above. Every winter the
numbers in a hive diminish from 12,000-20,000 to 2000-3000. The queen
lays the largest number of eggs in the spring, and the workers which
die before the winter are replaced by those which emerge in the summer,
autumn or during a mild winter. The queen lays eggs at such a variable
rate throughout the year that the above-mentioned inequality in numbers
is explained. The workers do not often live for more than six to seven
months, and at the time of their greatest labour, (May to July), only
three months. An attempt to calculate the length of life of the workers
and drones by taking stock at the end of summer, gives six months for
the former and four months for the latter[20].
The drones do not as a rule live so long as four months, for they meet
with a violent death before the end of this period. The well-known
slaughter of the drones is not, according to the latest observations,
brought about directly by means of the stings of the workers, but by
these latter driving away the useless drones from the food so that they
perish of starvation.
_Wasps._ It is interesting that among these near relations of the bees,
the life of the female should be much shorter, corresponding to the
much lower degree of specialization found in the colonies. The females
of _Polistes gallica_ and of _Vespa_ not only lay eggs but take part in
building the cells and in collecting food; they are therefore obliged
to use all parts of the body more actively and especially the wings,
and are exposed to greater danger from enemies.
It is well known from Leuckart’s observations, that the so-called
‘workers’ of _Polistes gallica_ and _Bombus_ are not arrested females
like the workers of a bee-hive, but are females which although
certainly smaller, are in every way capable of being fertilized and of
reproduction. Von Siebold has nevertheless proved that they are not
fertilized, but reproduce parthenogenetically.