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Note 2. The Duration of Life among Mammals
The statements upon this subject in the text are taken from many sources; from Giebel’s ‘Säugethiere,’ from Oken’s ‘Naturgeschichte,’ from Brehm’s ‘Illustrirtem Thierleben,’ and from an essay of Knauer in the ‘Naturhistoriker,’ Vienna, 1880.
Note 3. The Duration of Life among Mature Insects
A short statement of the best established facts which I have been able to find is given below. I have omitted the lengthening of imaginal life which is due to hybernation in certain species. In almost all orders of insects there are certain species which emerge from the pupa in the autumn, but which first reproduce in the following spring. The time spent in the torpid condition during winter cannot of course be reckoned with the active life of the species, for its vital activity is either entirely suspended for a time by freezing (Anabiosis: Preyer8), or it is at any rate never more than a vita minima, with a reduction of assimilation to its lowest point.
The following account does not make any claim to contain all or even most of the facts scattered through the enormous mass of entomological literature, and much less all that is privately known by individual entomologists. It must therefore be looked upon as merely a first attempt, a nucleus, around which the principal facts can be gradually collected. It is unnecessary to give any special information as to the duration of larval life, for numerous and exact observations upon this part of the subject are contained in all entomological works.
I. Orthoptera
Gryllotalpa. The eggs are laid in June or July, and the young are hatched in from two to three weeks; they live through the winter, and become sexually mature in the following May or June. ‘When the female has deposited her eggs, her body collapses, and afterwards she does not survive much longer than a month.’ ‘According as the females are younger or older, they live a longer or shorter life, and hence some females are even found in the autumn’ (Rösel, ‘Insektenbelustigungen,’ Bd. II. p. 92). Rösel believes that the female watches the eggs until they are hatched, and this explains the fact that she outlives the process of oviposition by about a month. It is not stated whether the males die at an earlier period.
Gryllus campestris becomes sexually mature in May, and sings from June till October, ‘when they all die’ (Oken, ‘Naturgeschichte,’ Bd. II. Abth. iii. p. 1527). It is hardly probable that any single individual lives for the whole summer; probably, as in the case of Gryllotalpa, the end of the life of those individuals which first become mature, overlaps the beginning of the life of others which reach maturity at a later date.
Locusta viridissima and L. verrucivora are mature at the end of August; they lay their eggs in the earth during the first half of September and then die. It is probable that the females do not live for more than four weeks in the mature state. It is not known whether the males of this or other species of locusts live for a shorter period.
I have found Locusta cantans in plenty, from the beginning of September to the end of the month. In captivity they die after depositing their eggs: the males are probably more short-lived, for towards the middle and end of September they are much less plentiful than the females.
Acridium migratorium ‘dies after the eggs are laid’ (Oken, ‘Naturgeschichte’).
The male Termes probably live for a short time only, although exact observations upon the point are wanting. The females ‘seem sometimes to live four or five years,’ as I gather from a letter from Dr. Hagen, of Cambridge, Mass., U.S.A.
Ephemeridae. Rösel, speaking of Ephemera vulgata (‘Insektenbelustigungen,’ Bd. II. der Wasserinsekten, 2te Klasse, p. 60 et seq.), says:—‘Their flight commences at sunset, and comes to an end before midnight, when the dew begins to fall.’ ‘The pairing generally takes place at night and lasts but a short time. As soon as the insects have shed their last skin, in the afternoon or evening, they fly about in thousands, and pair almost immediately; but by the next day they are all dead. They continue to emerge for many days, so that when yesterday’s swarm is dead, to-day a new swarm is seen emerging from the water towards the evening.’ ‘They not only drop their eggs in the water, but wherever they may happen to be,—on trees, bushes, or the earth. Birds, trout and other fish lie in wait for them.’
Dr. Hagen writes to me—‘It is only in certain species that life is so short. The female Palingenia does not live long enough to complete the last moult of the sub-imago. I believe that a female imago has never been seen. The male imago, often half in its sub-imago skin, fertilizes the female sub-imago and immediately the contents of both ovaries are extruded, and the insect dies. It is quite possible that the eggs pass out by rupturing the abdominal segments.’
Libellula. All dragon-flies live in the imago condition for some weeks; at first they are not capable of reproduction, but after a few days they pair.
Lepisma saccharina. An individual lived for two years in a pill-box, without any food except perhaps a little Lycopodium dust9.
II. Neuroptera
Phryganids ‘live in the imago stage for at least a week and probably longer, apparently without taking food’ (letter from Dr. Hagen).
According to the latest researches Phrygane grandis10 never contains food in its alimentary canal, but only air, although it contains the latter in such quantities that the anterior end of the chylific ventricle is dilated by it.
III. Strepsiptera
The larva requires for its development a rather shorter time than that which is necessary for the grub of the bee into the body of which it has bored. The pupa stage lasts eight to ten days. The male, which flies about in a most impetuous manner, lives only two to three hours, while the female lives for some days. Possibly the pairing does not take place until the female is two to three days old. The viviparous female seems to produce young only once in a lifetime, and then dies: it is at present uncertain whether she also produces young parthenogenetically (cf. Siebold, ‘Ueber Paedogenesis der Strepsipteren,’ Zeitschr. f. Wissensch. Zool., Band. XX, 1870).
IV. Hemiptera
Aphis. Bonnet (‘Observations sur les Pucerons,’ Paris, 1745) had a parthenogenetic female of Aphis euonymi in his possession for thirty-one days, from its birth, during which time it brought forth ninety-five larvae. Gleichen kept a parthenogenetic female of Aphis mali fifteen to twenty-three days.
Aphis foliorum ulmi. The mother of a colony which leaves the egg in May is 2‴ long at the end of July: it therefore lives for at least two and a half months (De Geer, ‘Abhandlungen zur Geschichte der Insekten,’ 1783, III. p. 53).
Phylloxera vastatrix. The males are merely ephemeral sexual organisms, they have no proboscis and no alimentary canal, and die immediately after fertilizing the female.
Pemphigus terebinthi. The male as well as the female sexual individuals are wingless and without a proboscis; they cannot take food and consequently live but a short time,—far shorter than the parthenogenetic females of the same species (Derbès, ‘Note sur les aphides du pistachier térébinthe,’ Ann. des sci. nat., Tom. XVII, 1872).
Cicada. In spite of the numerous and laborious descriptions of the Cicadas which have appeared during the last two centuries, I can only find precise statements as to the duration of life in the mature insect in a single species. P. Kalm, writing upon the North American Cicada septemdecim, which sometimes appears in countless numbers, states that ‘six weeks after (such a swarm had been first seen) they had all disappeared.’ Hildreth puts the life of the female at from twenty to twenty-five days. This agrees with the fact that the Cicada lays many hundred eggs (Hildreth states a thousand); sixteen to twenty at a time being inserted into a hole which is bored in wood, so that the female takes some time to lay her eggs (Oken, ‘Naturgeschichte,’ 2ter Bd. 3te Abth. p. 1588 et seq.).
Acanthia lectularia. No observations have been made upon the bed bug 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,’ 2ter Bd. 3te 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. Edwards11, 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 possession12:—
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 generation13. 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 year14.
Saperda carcharias. An individual lived from the 5th of July until the 24th of July of the next year15.
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 made16.
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.’
