The Zoologist/4th series, vol 6 (1902)/Issue 734/The Temperature of Insects

by Geoffrey Watkins Smith

 

THE TEMPERATURE OF INSECTS.

By Geoffrey Smith.

 

Naturalists whose pleasure it is to try and enter sympathetically into the conditions and capacities of all living things will be greatly interested in an account which Prof. Bachmetjew, of Sophia, has published of his experiments on the temperature of insects.^[1] This account tells us in a clear and masterly manner of an excursion into the field of invertebrate physiology— a field too little cultivated by professed biologists, owing, it must be supposed, to the great difficulties encountered, and not to the innate barrenness of the land; indeed, it seems that the problems of biology, which have been so long attacked from an almost purely morphological standpoint, can at this stage of enquiry only be further elucidated by a wider and more searching scrutiny of organs and organisms from the point of view of function. This wider view of Biology is one which is likely to find favour with readers of 'The Zoologist'; and since the researches under consideration are directed towards the advancement of knowledge in this direction, and since they may not be readily accessible to all naturalists, I have ventured to think that a short abstract of Prof. Bachmetjew's work, with a discussion of its bearings on certain problems of insect coloration, might be acceptable.

We need not occupy ourselves for long in considering the Professor's method of research; it is essentially simple and accurate. The fact is well known to physicists that when two suitable metals are placed in contact an electric current is generated, and this current is accurately proportionate in strength to the temperature of the two metallic poles. In the researches which we are going to describe the metals employed were steel and manganese; the insect whose temperature was to be taken was pierced by a fine needle of this composition, and the strength of the current induced by the contact of the two metals inside the insect's body was measured by means of a galvanometer; the changes in strength of the current indicated the changes of heat in the insect's body.

The book begins with an historical review of the work of naturalists on this subject since the time of Réaumur in 1734; among other names we notice that of the English naturalist Newport. The conclusion to be drawn from this earlier work is that very different results may be obtained from working at the same material; that the temperature may vary within wide bounds without prejudice to life, and that this variation of temperature is largely dependent on the temperature of the surrounding medium. But the temperature of the surrounding medium is not the only factor in determining the temperature of insects, and it is the first merit of Prof. Bachmetjew's work to have fixed and defined the other important factors which cooperate with it. He separates these factors under four heads—1, the influence of the temperature of the surrounding air; 2, influence of moisture; 3, influence of exercise; and 4, the influence of food and respiration.

The first experiments described were made with the Hawk-Moth (Deilephila euphorbiæ). It was found that at temperatures higher than 37° C. the temperature of the moth was always lower than that of the air, the greatest difference being 2·5°, when the moth was at 45·1° C. Above 48·1° the insect ceased to flutter, at 48·6° its wings sank, and at 51·4° it died. At death the temperatures of the air and of the moth were equal. These experiments were conducted in air of normal moisture, but when the air was supplied with additional vapour a different result was observed, for then the insect had a higher temperature than that of the air, and its wings did not sink until a body-temperature of 53° was reached, the air being at 49°. This effect is probably brought about by the moisture in the air preventing evaporation of the insect's juices, and so preventing cooling; while the normal metabolism of the insect naturally tends to raise the temperature. At low temperatures the temperature of the insect was always higher than that of the air. It is interesting to note, in relation to the effect of evaporation, that hairy insects tend to have a higher temperature than smooth, and this fact may be well explained by the prevention of evaporation from the former.

With regard to the effects of exercise, it was shown by Newport that the temperature of an insect at rest is always lower than when it is in motion; while Lecoq found that a species of Sphinx, during active motion, reached the normal temperature of birds, which is peculiarly high. Bachmetjew has considered the influence of exercise at ordinary room temperatures, at heightened temperatures, and under the application of cold. He found that at ordinary room temperatures (18·5°) Sphinx pinastri raised its temperature by rapid wing-vibration up to 36°. At this point the vibration ceased, owing to a partial paralysis of the wing-muscles; the temperature then dropped, and the paralysis passed away. On repeating the rapid vibration immediately paralysis set in again more rapidly, but not until the temperature reached 36°; furthermore, if the surrounding temperature was increased, less humming is required to bring on partial paralysis. There is therefore considerable ground for assuming that it is the heightened temperature which causes the partial paralysis. Just as there is a maximum temperature which brings on paralysis, so there is a minimum; thus D. euphorbiæ ceased rapid vibration when its temperature was at 17·6°, and all movement stopped at 0·5°. Putting these observations together, we see that for the Sphingids observed normal flight is only possible roughly between the temperatures of 18° and 36° C.

The influence of food and of respiration is only touched upon, but we may gather that everything that tends to increase metabolism tends also to raise the temperature.

In the second part of the volume the vital temperature extremes of Lepidoptera are discussed, especial attention being paid to the minimal temperature; and at the outset a very curious phenomenon is offered for consideration. If a butterfly or moth be cooled by being kept in an iced chamber, a certain point of under-cooling is reached (called the critical point, or K); at this point the temperature suddenly rises through more or fewer degrees, and freezing takes place at a temperature above the critical point (called the normal freezing-point, or N). This behaviour of the juices of insects shows a striking analogy to the under-cooling of water under certain conditions. This process of under-cooling and freezing does not cause death on the first occasion, but, if the process be repeated for a second time, the insect dies when the critical point is reached for a second time. This is indicated by the following experiments on Aporia cratœgi, the black-veined white:—

1st lot.—K= –10°, N = –1·2°. When N was reached the animals were removed from the ice-chamber, and lived.

2nd lot.—K= –8°, N = –0·8°. On under-cooling again to –6·5°, and removing the animals, they still lived.

3rd lot.—K= –6·8°, N = –1·1°. On under-cooling again to –10·0, death occurred.

These facts may be graphically represented thus:—

Both the critical point and the normal freezing-point vary not only in different species, but in different individuals of the same species, and at different life-stages of the same individual. Indeed, many factors play a part in determining the nature and relations of these points, such as the rapidity with which the cooling takes place, the sex of the insect, the quantity of food it has eaten, and the amount of time it is kept at any particular temperature.

The number of degrees lying between the critical point and the normal freezing-point is complicatedly dependent on the rapidity of cooling, but the alternatives are so various that it is impossible at present to draw any concise conclusion with regard to them. It is an extremely interesting discovery that males have normally a greater difference between their critical and normal freezing-points than females; but this difference is equalized by prolonged hunger. This points to one of those curious relations between sex and alimentation, which are so striking and yet so difficult to fix exactly.

In this short review of Prof. Bachmetjew's results, it is hoped that enough has been said to show that a considerable foundation has been laid down for further researches; but reference should be made to the book itself, which is full of carefully tabulated experiments, and most clearly expressed deductions from them.

It is clear that in dealing with the temperature of insects we have to do with a complex phenomenon dependent on a variety of interacting factors, some of which we have already touched upon. In the remainder of this paper I intend to consider one factor which I believe will have to be taken into account if we wish to gain a complete idea of the temperature relations of the so-called poikilothermic animals, i.e. animals whose temperatures vary with the surrounding medium. This factor is colour. The radiant powers of differently coloured surfaces are notably different; those surfaces which absorb the long-waved colours are better radiators than those which absorb chiefly those colours which lie at the other end of the spectrum. The emissive and absorptive radiant powers of a substance are directly proportional—a good radiator is a good absorber; it must also be remembered that a good reflector of radiant heat is a bad absorber and radiator, and vice versâ. It has for long been pointed out that a dark coloured animal would be able to take advantage of sunshine more readily than a light coloured one, and Lord Walsingham used this fact in explaining certain phenomena of melanism in Lepidoptera. In the controversy which arose on this head nothing conclusive was reached, but a certain amount of evidence was brought forward to show that on the whole, in regions where the sunshine was intermittent, a melanic tendency in the Lepidoptera became the rule rather than the exception. My chief collecting-ground for Lepidoptera abroad has been Haute Savoie, in the neighbourhood of Mont Blanc, and I have been struck there with the fact that the two kinds of butterflies which frequented the highest mountain regions were, on the one hand, the dark brown Erebias, and, on the other, the white Pierids and pale Coliads. This contrast struck me for some time as inexplicable on the theory that the colouration bore any relation to the temperature, but a little consideration showed that the white wings of the Pierids might act as reflectors of heat, glancing off the sun's rays on to the black body of the insect, which would thus absorb a greater quantity of heat. I have since tried many experiments in order to test this hypothesis; the bulb of a sensitive thermometer is tied round with black cloth, and hung up in bright sunshine. This morning a thermometer so prepared registered a temperature which varied between 30° and 31° C. I then backed the thermometer with a sheet of white paper folded so as to imitate the position of a butterfly's wings when expanded upon a flower. In three minutes the temperature had risen to 35° C, and was still rising when I removed the paper; the temperature immediately dropped. I repeated the experiment, substituting the cups of variously coloured flowers—such as poppies, Canterbury-bells, and so forth—to take the place of the white paper, and I obtained rises of temperature through two or three degrees, according to the reflecting powers of the various colours; the worst reflector being a dark purple larkspur, and the best a bright red poppy, which increased the temperature from 30·3° to 33·5° C. in a few minutes.

I surmise therefore that the influence of colour on the temperature of Lepidoptera is not so simple as it is usually assumed to be; on the one hand, the wings may absorb heat directly; on the other, they may be used as reflectors. It has been urged that the absorption of heat into the wings is a useless proceeding, since they are largely composed of dead structures; but it must be remembered that hæmolymph is present between the lamellæ of the wings, and I conceive that a circulation of this hæmolymph occurs from the body to the wings, and vice versâ, owing to the movements of the wings and body.

Butterflies and flowering plants afford us, both in variety and brilliance, the greater part of the great boon of colour in animated Nature; both of these orders of beings are in general dependent for the fulfilment of their vital functions on warmth and sunshine. The dark centre of the poppy, where the sexual products are matured, is encircled by a broad open tent of crimson, which flashes from its walls the most potent of the sun's rays. If we are to compute the circumstances favourable to the certain and speedy occurrence of the chemical action which brings about the maturation of pollen and ova, we cannot neglect this factor of colour. The dusky mountain Argus, gathering radiant energy in its wings and body on a sunny slope, at the onset of an alpine storm creeps into the cover of the thick grass, where radiation from its body-surface is not so rapid as in the open air; it folds its wings above its body, and this again prevents rapid radiation from the vital regions so covered. If it be one of those Erebias—such as E. lappona or tyndarus—which are confined to the highest regions, it offers to the expanse of the outside air not the dark brown of its upper surface, but the lighter grey of the under side of its hind wings, which thus have a lower emissive power. Again, we cannot neglect the factor of colour in determining the vital capacities and functions in so far as they are influenced by temperature.

My object in these remarks has been to draw attention to some experiments by Prof. Bachmetjew on the temperature of insects; these experiments confirm the opinion that the effects of Nature are seldom brought about by causes acting singly, but by a complex interaction of many simple causes.

The surface colour of organisms must certainly be taken into account when considering their temperature relations; there appear to be suggestions in Nature that certain colours have been selected as being advantageous to the animals possessing them, owing to their absorptive, emissive, and reflecting powers; and this factor may have acted in common with many others, known and unknown, in producing the varied effects which we see and admire.

 

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1. 'Experimentelle entomologische Studien,' von P. Bachmetjew. Leipzig, 1901. Erster Band.