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The Natural History of Aquatic Insects

1895; American Association for the Advancement of Science; Volume: 2; Issue: 29 Linguagem: Inglês

10.1126/science.2.29.81

1095-9203

S. H. S.,

Diptera species taxonomy and behavior

INSECTSrear, and rolling forward to the front.All the very simplest animals and plants are aquatic, and a fair degree of complexity of structure is implied in the mere fact of residence out of water.Shallow waters are easily penetrated by the sun's rays, and are therefore often occupied by green plants, whose nutrition depends upon the decom- position of carbonic acid by sunlight.Upon these green plants plant-feeding animals establish themselves, and many predatory animals come in turn to devour the vegetable-feeders.The shallow waters are probably richer in living things of all kinds than any other part of the earth's surface.But the organic products of the plants and animals of the shallow waters may be transported by moving water to great distances.. The great depths of the ocean and the bottom of our large lakes have been explored by the dredge, and are found to support a popula- tion of their own, which subsists upon a nutritive sediment originally formed at or near the surface.The conditions of life in fresh waters are materi- ally different from those afforded by the sea.The chemical difference between fresh and salt waters is of small importance in itself so small that many animals have been able to adapt themselves gradually to a change from one to the other.A degree of saltness far greater than that of sea-water is perfectly compatible with animal life, as is shown by the fact that Crustacea and Insects are often found quite at home in brine-pits and vats.There are even a few animals, like the Salmon, which can pass sud- denly from fresh water to salt, or from salt to fresh. NATURAL HISTORY OF AQUATIC INSECTSform an order or a family, but comprise a miscel- laneous selection of genera and species from many families and several orders.Let us consider what may be the significance of this remark.A family or an order of Insects is a group which the naturalist supposes to have descended from a common stock.An order may include many families, and the common stock of the order is therefore presumed to be much more remote than that of any of its component families.Now if all Insects were at one time terrestrial (and of this we have no sort of doubt, for reasons which will shortly be given), and if we find aquatic Insects in two different orders, which cannot be supposed to be derived one from the other, we believe that the adaptation to aquatic life must have been effected independently.For the common origin of both orders is a stock unknown to us by observation, but A which we have good reason to suppose was not aquatic.When we have a smaller group, such as an aquatic family, to consider, we must ask whether it is more likely that the common stock from which it is descended was aquatic or not.In a few cases there is reason to believe that two or more aquatic families have descended from a common stock which was aquatic also, but this is unusual.The aquatic families of Insects are seldom wholly aquatic, they generally include some terrestrial forms, and these axe, primafacie the more primitive, or less altered in the matter of habitat.\Ye do find some cases of families all of which are aquatic, and so related to allied families which are in the same case, that it is most likely that the common stock in which all originated was aquatic NATURAL HISTORY OF AQUATIC INSECTS proof that Insects were primarily air-breathing animals, but the distribution throughout the different orders and families of the species which breathe gas- eous air and of those which breathe only air dissolved in water, leaves no doubt that the last-named species are those which deviate from the general and primitive rule.I think that every entomologist would agree that the Insects which exhibit the most generalised and presumably the most primitive structure are terrestrial rather than aquatic.Insects are just the sort of animals that might have been supposed likely to change from one medium to another with great ease and advantage.They are active, hardy and ingenious.The plan of their bodily structure has lent itself to a multitude of special adaptations of other kinds.Notice the various shapes of Insects.We find among Beetles, for instance, some so round and fat that they can hardly creep, some long-legged and nimble, one as flat as a wafer for convenience of creeping under the adherent bark of fallen trees, some with long wings, others with short wings, others with no wings at all.Then among caterpillars what a range of shapes !They may be long and narrow, or egg-shaped, hard or soft, with many feet or with none, hairy or smooth, and in special cases with the oddest horns, warts, brushes, tentacles and hooks.Notice too, the extraordinary variety in their food.Insects can be found which live upon wood, upon earth and the things contained in earth, upon fungi, upon decaying seaweed, upon paper, upon fur and wool, upon leather, upon argol (crude potassium tartrate), in addition to the animal and Entomology, Vol.II., p. 231 (1826).What are the qualities which confer dominance upon Insects or any other group of animals ?The question is too hard for us.It may be one quality here, and another there.But though we must not speak with confidence on so difficult a question, there is some likelihood in the supposition that adaptability to new conditions would be one ground of success.If NATURAL HISTORY OF AQUATIC INSECTS venture a little way below the level of the very highest spring-tides ; others are found a little below the ordinary high-water mark, and these are immersed for a few minutes twice a day.But others withstand prolonged immersion, and cannot be drowned in salt water.There is vegetable and animal food in the water, and some creatures will find it and devour it.Insects, nimble, enterprising, and ingenious, have ven- tured in and have succeeded in wresting a share from the slow Mollusks to whom it might seem more properly to belong.The life of some aquatic animals is greatly com- plicated by the peculiar properties of the surface-film of water.Like other external conditions, the surface- film may be either a hindrance or an advantage, according to the way in which it is treated.Those who have had experience of the extraordinary versa- tility of living things, and of their power to adapt themselves to the most various conditions, will be prepared to find that plants and animals have been able in various ways to turn to account the properties of the surface-film.THE SURFACE-FILM OF WATER. 1 I fear that it is not quite safe to presume that the properties of the surface-film are familiar to every reader.Let me, at the risk of superfluous explanation, run over some of the elementary facts.Take a tumbler and pour water into it until it is 1 A fuller but quite elementary account of this subject will be found in Prof. Boys' charming little book on Soap Bubbles.INTRODUCTION 15 with the behaviour of Insects living at the surface of water.The particles close to the surface are in a peculiar condition of aggregation, and temporarily cohere to form a film.The film, as we have seen, offers resistance to the passage of solid bodies', and can therefore support a weight or keep a buoyant object from rising through it.It also exerts a pull, and we shall find that advantage is taken of this pull by certain aquatic Insects.The extreme tenuity of the film, which is thinner than our imagination can realise, helps us to understand how it is that only small objects are affected by it.Ships, boats, swim- ming quadrupeds, and all objects whose weight is large in comparison with their contour, are practically uninfluenced, but objects whose dimensions are given in fractions of an inch may be largely controlled by the peculiar properties of the surface-film.The larva of the Gnat turns these properties to account in a peculiarly interesting way, as do many other aquatic Insects, Crustacea and floating plants.On the other hand, minute aquatic animals, approaching the surface of water without precaution, may find it a death-trap, as we learn by observation of certain small Crustacea, 1EQUILIBRIUM OF AQUATIC INSECTS.The density of water is so much greater than that of air as to require peculiar adjustments in the equilibrium of aquatic animals.A simple case is pre- sented by the air-breathing aquatic Insects.These, 1