The natural history of aquatic insects

Preface

I HAVE here attempted to help those naturalists,, especially those young naturalists, who take delight in observing the structure and habits of living animals. It has also been my hope that I might do something to revive an interest in the writings of certain old zoologists — Swammerdam, Reaumur, Lyonnet, and De Geer — who are at present unjustly neglected; I have tried to carry on as well as to popularise their work. It would be much if I could persuade some few working naturalists to lay aside their technical lists and records of parish distribution, and study the works of Nature with open eyes, seeking above all things to know more of life in its infinitely varied forms.

Some passages in this book, if taken alone and read hastily, may appear to disparage systematic Zoology. This is far from my intention. No one can study the great naturalists of the seventeenth and eighteenth centuries without feeling how seriously their work is impaired by the defective systems of the time. It is

VI

PREFACE not systematic but aimless work that I deprecate — work which springs from no real curiosity about Nature, and attempts to answer no scientific questions.

I have great pleasure in acknowledging the advice and contributions of several friends, and especially of Messrs. W. F. Baker, T. H. Taylor, and J. J. Wilkinson. The drawings made by Mr. Hammond, in most cases direct from the Insects, are a valuable feature of the book, and I could say more in their praise if they did not speak for themselves.

L. C. M.

Leeds, February 1895.

Table Of Contents

Water as a Sphere of Life, p. i. The Invasion of the Waters by Insects, 3. The Dominance of Insects, 8. Degrees of Adaptation to Aquatic Conditions, xi. The Surface-film of Water, 12. Equilibrium of Aquatic Insects, 15. The Wintering of Aquatic Insects, 18. Some Cases of Abridged Life-histories, 20. Live Natural History, 24. The Groups to which Aquatic Insects belong, 28.

Chapter I

The Whirligig Beetle (Gyrinus), p. 30. Respiration by Tracheal Gills, 36. Dytiscus, 39. The Great Water-beetle (Hydrophilus), 61. Life of Lyonnet, 62. ITydrobius, 87. Donacia, 93.

Chapter II Fites With Aquatic I.arwe

The Gnat (Culex), p. 97. Experiment on Surface-tension, 100. Corethra, 1 1 3. Collecting Tackle, 1 14. Mochlonyx, 121. Chironomus, 122. Microscopic Study of Salivary Glands, 127. Haemoglobin in Blood, 130. Development of the Winged Insect within the Larva, 134. Animals which coat their Eggs with Jelly, 148. Tanypus, 152. Ceratopogon, 155. Dixa, 157. Dicranota, 164. Pupal Armature for enabling the Insect to creep to the Surface, 169. Ptychoptera, 170. Simulium, 175. Stratiomys, 189. The Rat-tailed Larva (Eristalis), 198. J. J. Wilkinson on the Tail of Eristalis, 201 ; on the Pharynx of ditto, 206. Baron Osten Sacken on the Oxen-born Bees of the Ancients, 215.

TABLE OF CONTENTS viii

Chapter XIII The Contrivances Of Aquatic Insects

Modes of Locomotion, p. 382. Modes of Feeding, 385. Modes of Respiration, 385. Attack and Defence, 386. Egg-laying, 387.

THE NATURAL HISTORY

OF

AQUATIC INSECTS

THE NATURAL HISTORY

OF

AQUATIC INSECTS

Water as a Sphere of Life

Water offers many advantages to the simpler animals and plants. The vast extent of the seas, lakes, and rivers is in itself a sufficient motive for their occupation by living things, considering the crowded state of the land. But more special inducements are not wanting. The density of water supports the weight of the most fragile organisms, and enables them to move from place to place by such easily contrived means as cilia, which may be mere drawn-out threads of protoplasm, or fins, which may be shaped out of a fold of skin or a row of bristles. Some of the very simplest aquatic animals, such as Amoeba, move along in the water by an act which can only be described as flowing, the stream of living protoplasm gathering itself up continually in the

£ R rear, and rolling forward to the front. All the very simplest animals and plants arc aquatic, and a fair degree of complexity of structure is implied in the mere fact of residence out of water.

Shallow waters arc easily penetrated by the sun’s rays, and are therefore often occupied by green plants, whose nutrition depends upon the decomposition 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 population of their own, which subsists upon a nutritive sediment originally formed at or near the surface.

The conditions of life in fresh waters arc materially 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 suddenly from fresh water to salt, or from salt to fresh.

More important from many points of view is the fact that the seas arc of vast size and, for the most part, continuous ; while the fresh-water areas are relatively inconsiderable in extent, and broken up into countless small areas. Every lake and river-system is an isolated territory, and most of the plants or animals found therein can only make their way into another territory by help of a rare opportunity. This particular circumstance has governed the course of life to a material degree, and has brought about some of the most marked differences between the fluviatile and the marine populations. It is also worth while to notice that in the large fresh-water areas, which in nearly every part of the world are the rivers, the water is in motion, and flowing steadily towards the sea. Immersion in the sea is quickly fatal to most fresh-water animals and plants, so that here is a special danger to which the smaller and more helpless fluviatile animals are exposed. We have reason to believe that this special danger has helped to bring about one of the most noteworthy peculiarities in the life-history of fluviatile animals, viz., the suppression of free-swimming and ciliated larval stages.1

The Invasion of the Waters by Insects.

The waters, both fresh and salt, have been successfully invaded by Insects, and this not in one or two cases only, but many times over, and by the most diverse kinds of Insects. The aquatic Insects do not

1 1 his has been pointed out by Professor Sollas (Proc. Roy. Dublin Society, 1884).

B 2 form an order or a family, but comprise a miscellaneous 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 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 are primA facie the more primitive, or less altered in the matter of habitat. We 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 too. But this is not very frequent, and the naturalist, exercised in cases of affinity, who studies a table of aquatic Insects, is likely to conclude that different families, and different genera within one family, and different species within one genus have betaken themselves independently to an aquatic life. I think we can say with a considerable degree of probability that this change of habitat from terrestrial to aquatic has taken place in the class of Insects at least a hundred times quite independently, and the number may be very much higher than a hundred.

I have spoken with confidence of the original terrestrial habitat of Insects, and this requires some further discussion. Writers have been found to maintain the very high antiquity of the aquatic habitat, and to suppose that many, if not all, of existing Insects had aquatic ancestors. What can be found out on this point ? It is of no use to appeal to the geological history of the earth, for the fossil remains of Insects are too few and scattered to justify any conclusion as to the mode of life of the first Insects which appeared on the globe. But we are not altogether without the means of forming an opinion. One of the most universal features of Insects in all stages of growth is the aeration of the tissues by air-tubes or tracheae, which ordinarily open on the outside of the body by special valvular inlets, the spiracles. Even in aquatic Insects the spiracles are commonly present and functional, and we can hardly name an Insect which has not tracheal tubes, though they may in rare instances be greatly reduced. This is pretty good proof that Insects were primarily air-breathing animals, but the distribution throughout the different orders and families of the species which breathe gaseous 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 arc 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. 1 hen among caterpillars what a range of shapes! I hey 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 vegetable products which are the ordinary food of the higher animals.

Equally remarkable is the facility with which Insects adapt themselves to various conditions. The most primitive Insects are terrestrial, running about on the surface of the earth to seek their food. But many have learnt to burrow into the earth, and this in a great variety of ways. A large proportion have learnt to fly. Some run their galleries in the wood of trees ; others eat out slender mines in the thickness of a leaf ; others live in the bodies of dead animals ; and there are not a few which burrow in the bodies of living animals — a painful example of the cruelty of Nature! The adaptations of Insects to aquatic life, varied and ingenious as we shall find them to be, are only fresh examples of the versatility which characterises the whole class.

Even if we confine our attention to the Insects which pass part or the whole of their lives in water, we shall find a great diversity in their habitats. Still water abounding in vegetation is suitable to many ; others like running water, and some few prefer torrents or cascades, though in this case they are seldom exposed to the full force of the current. The most rapid parts of an English mountain brook will, however, often be found to harbour certain kinds of Caddis-worm and the larvae of Simulium. Water loaded with dead organic matter is congenial to other Insects, and the larva of Eristalis tenax prefers water which is putrid and stinking. Salt water is selected by others. The sea nourishes a considerable number, and brine-pits, notwithstanding the bitterness and density of the water with which they are filled, have an Insect population of their own. Hot springs are preferred by some, arctic snows by others ; 1 some few Dipterous larvae live immersed in the juices which flow from wounded trees and many in the fluids of living animals. Some attach themselves to the roots of submerged plants, and several bury themselves in the substance of the freshwater sponge.

Tiie Dominance of Insects.

The countless adaptations of Insects to the most various conditions not only illustrate the extraordinary elasticity of their plan of structure, but also their power in competition. The profuse variations of the Insect type would never have been called forth, had not the type been eminently successful in the struggle for existence.

Such success may be shortly named dominance , and Insects are an excellent example of a dominant class. They are able to drive out their rivals, able to maintain and even to enlarge their territory in spite of all competition. YVc cannot ascertain such qualities by direct observation of the struggles which are going on everywhere around us. Human experience is too short and too blind for direct measurement of these secular changes. But -though we cannot see for ourselves the turns of the slow and infinitely complex struggles of the races of animals and plants, we have before us the results of ages of such strife. The com-

1 Examples are given by Kirby and Spence, Introduction to Entomology , Vol. II., p. 231 (1826).

monest animals of to-day are those which have so far proved victorious, and numerical strength is one mark of dominance, a sure one, when it can be clearly ascertained.

Any group of animals or plants which is numerically very strong will be found to include a large number of species, and these will be finely graded, one species being with difficulty distinguished from those which most nearly approach it. Dominant species are usually combined into what we may call continuous groups. It is only when a group is dominant, or so to speak in easy circumstances, that many nearly allied forms can maintain themselves side by side within it. For declining groups there is a rigorous selection. The principle applies to other things beside natural species. It has become a proverb that even bad workmen find employment when trade is good. Growing industries, such as lithography or paper-making, turn out an immense variety of forms and qualities ; but articles which are going out of use, say tallow candles or sand-glasses, can only be bought in one or two patterns. Sealing-wax thirty years ago was sold in an endless variety of colours, but the competition of adhesive envelopes has now reduced the number to one or two. I am told that while you can get ample choice of cloth-covered buttons, you can hardly buy more than one kind of polished steel buttons, which have now gone out of fashion.

All prosperous species, families, orders and classes at least hold their ground. If they can be successfully invaded, they arc no longer dominant. Dominant groups arc therefore geographically continuous ; they

IO NATURAL HISTORY OF AQUATIC INSECTS

lie within a ring-fence, and the facts of distribution will soon tell us whether a group is dominant or not. It is especially characteristic of the most dominant groups that they can hold their own in the wider areas, where the struggle for existence is most severe. Groups which can maintain themselves in the wide continents of the northern hemisphere, will easily prevail over the populations of the southern islands ; groups which can maintain themselves in the vast fresh-water lakes of North America, will easily occupy the small fresh-water lakes and ponds of England, if ever they are brought into competition with the indigenous population.

All the various marks of dominance are united in Insects, and arc there exhibited in a striking degree- Numerical abundance, zoological continuity, or the co-existence of many allied forms, geographical continuity, and predominance in the widest areas of competition distinguish the whole class. There are, of course, degrees of dominance among them. The Diptera are eminently dominant among Insects, the Muscidae dominant among Diptera, and so on. The extraordinary variety in form, size, and habitat which Insects present is in keeping with all that we know of their range and frequency.

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

1 1

two groups of animals, both terrestrial, were to have a supply of food opened up to them in the water, it seems probable that the group which had proved itself the stronger by land would be more likely to appropriate it. The past success of the European in competition with the Hottentot gives us some reason to believe that Europeans will occupy any country at present uninhabited before the Hottentots will. But we soon get out of our depth in such discussions as this.

Degrees of Adaptation to Aquatic Conditions.

How did Insects ever come to seek the water, seeing that their mode of respiration is primarily adapted to another element ? We can see almost all the steps of the adaptation on the shores of our rivers, lakes and seas. We can see Dipterous larvae which, like the “leather jacket” (the larva of the Daddy-longlegs), burrow in the ground for their vegetable food, and devour the roots of grasses. Other larvae of the same family (Tipulidae) prefer moist earth in the neighbourhood of streams. Others again live immersed in water, or mud saturated with water, though they come to the surface at times and push their tails, which carry the spiracles, into the air. Some few have become so completely aquatic that they seldom, if ever, come to the surface, and all their supply of oxygen is obtained from the water. Again we find on any sandy sea-shore little Beetles, which hide in the decaying seaweed. Some of the species only 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 ventured 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 complicated 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 versatility 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.

Tiie Surface-film of Water.1

I fear that it is not quite safe to presume that the properties of the surface-film arc 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.

full to the brim, i.e. until the water-level is as high as the edge of the tumbler. Then we say that the tumbler is full of water. But it will still hold a good deal more without spilling. Add water by teaspoonfuls, or drop coins in, one by one. The level rises well above the edge, and still if due care is taken, no water is spilt. The tumbler can be made over-full of water. It is plain that something restrains the natural tendency of water to seek the lowest level, and to escape by flowing over the edge.

Can it be that the water is sensibly viscous, like glycerine or thick treacle ? This can hardly be the explanation, for the excess of water does not level down gradually, but takes up a position of rest in less time than it costs us to focus the eye upon it. It is not the want of fluidity in the water which prevents it from flowing over the edge. There is a film overspreading the surface, even of the purest water, which prevents it from escaping until the pressure becomes considerable.

Dip your finger into water and lift it out again. The water does not all run off, but flows down to the tip of the finger, and there forms a big drop. What holds up the drop ? It is the surface-film, which does not differ in its composition from the water within, but is for the moment in a peculiar physical state. The film is incredibly thin. It can be renewed in an instant, fresh water-particles taking its place, while the old film loses all its peculiar features, and mingles with the water beneath the surface.

The surface-film can be made evident in another way. Take a clean needle, poise it horizontally on r4 NATURAL HISTORY OF AQUATIC INSECTS

the prongs of a fork, and lower the fork and needle steadily upon the surface of the water in a basin. If the fork is taken away from beneath, the needle will be left floating on the surface. Steel is many times heavier than water, but the resistance of the surface-film prevents the needle from sinking. Once beneath the surface, the needle sinks rapidly to the bottom.

The surface-film is in a state of tension, i.e. it exerts ■a pull. It behaves like a drum-head, or a sheet of stretched india rubber. This may be made evident in various ways. One of the simplest is this. Take two lucifer matches, which are to form opposite sides of a square. Tie their ends together with pieces of sewing-cotton, which will form the remaining sides. Then run a needle into the middle of one of the matches to make a handle. If this frame is dipped into soapy water (a strong solution is required) and then lifted out, a transparent film of soapy water will overspread the square. This film will be in a state of tension, as you will sec by holding the frame upright with the two matches horizontal. The threads will be curved inwards by the pull of the film. Why do we use soapy water in this experiment ? Because the film of pure water will not last long enough for us to examine it. It breaks up instantly into a crowd of small drops. The film of soapy water, lifted off in this way, is not a simple surface-film. It consists of a double film, with an appreciable quantity of water imprisoned between the two layers.

The two or three experiments just described will, if actually performed, establish in the reader’s mind some elementary truths, which have a good deal to do 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, swimming 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.1

Equilibrium 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 presented by the air-breathing aquatic Insects. These,

1 D. J. Scourfield, Linn. Joiirn., Zool., Vol. XXV., pp. 1 — 19, pi. 1, 2, 1894.

with few exceptions, arc lighter than water.1 Hence, if by loss of respirable air, or from any other cause, they arc disabled, they have only to cease to struggle in order to come to the surface, and it will in all cases be found that they float upwards in such a position that the part of the body by which air is taken in comes to the top.

Locomotion in any medium by an animal of complex structure involves some power of orientation, and especially a rapid and instinctive judgment as to the horizontal plane and the upward and downward directions. The pull of gravity may furnish the necessary indication, but an animal which swims in water, or flies in air, may be unable to determine the horizontal plane with facility, especially if its body is nearly of the same density as the surrounding medium, since the pressure will be approximately equal on all sides. The semi-circular canals of Vertebrates and the so-called auditory organs or otoliths of many Invertebrates are probably organs for the perception of the horizontal position. Some animals, however, such as Insects, possess no organs of this kind, although they are excellent fliers or swimmers. How arc they enabled to maintain their course at a fixed distance above the surface of the earth or below the surface of the water ? Sight may help, but that sight does not furnish the sole indication may be inferred from the behaviour of sightless animals, such as the inhabitants of dark caverns, or animals temporarily blinded for the purpose of

1 Among the exceptions are Corixa, and the larva: of the Gnat, Agabus, and Hydrobius.

experiment. These have been found to be capable of maintaining a horizontal course, or of ascending and descending according to need. Their perception of the upward and downward directions is, as experiment shows, falsified by artificially induced changes in the specific gravity of the medium, so that it is probable that they are guided chiefly, if not altogether, by the action of gravity.

We have seen that air-breathing aquatic animals are, as a rule, lighter than water, and that when they come to rest they float upwards in one particular position. The normal attitude is maintained by the distribution within the body of two or more substances of very different density, such as air and flesh or shell. Whatever the density of the medium, and whether the body rises or falls, it will tend to maintain the same position, the lighter tract tending to rise.

Ground-feeders, and swimmers which do not breathe gaseous air, maintain the normal attitude in -a different way. Here the body is of nearly uniform specific gravity, and the normal attitude is secured by the movements of the limbs and the form of the external surface.

In ground-feeding aquatic animals, whose bodies are a little heavier than water, the legs are usually placed ventrally, and the back is rounded from side to side. A Lobster is a familiar example. Here the convex dorsal surface tends to move foremost, and the action of the limbs treading the water, helps to maintain the same direction of movement. As a rule the limbs are turned down, and the body con-