Having got together our apparatus, which, as we have seen, need be neither costly nor complicated, the next step will be to acquire some knowledge of the group from which the examples here treated of will be taken—the Ar´thropods, or animals with hollow-jointed limbs. These are the ‘Insects’ of the Linnaean classification, and, for the matter of that, of popular phraseology; for though few people would now venture to call a Lobster an ‘insect,’ we still style some of its near relatives Water ‘Fleas,’ as Swammerdam did two hundred years ago.

The Arthropods form a phylum, or main division of the Animal Kingdom. Above this phylum comes that of the Molluscs, or soft-bodied animals, such as the Oyster, the Snail, and the Cuttlefish. Still higher are the Lancelet, the Sea-squirts, and some few others, that bridge the chasm between the phyla without, and that phylum with, a backbone. And to this last Man himself belongs.

Two reasons contributed to the selection of the Arthropods as a subject for work with the pocket lens: (1) the great interest which surrounds many of the group; and (2) the ease with which specimens may be procured and kept under observation.

Every one has pretty clear notions as to the general ‘make’ of a Vertebrate or backboned animal. An Invertebrate animal has, of course, no backbone or the semblance of one; the nervecord, where present, lies on the under surface, and forms a ring round the gullet, and the heart lies on the upper surface or back. We may verify this by pulling to pieces a dead insect.

But a phylum, or main division, is much too large to be considered as a whole. It must, therefore, be broken up into smaller groups, which are called Classes, generally reckoned as five in number. These, again, may be grouped into two divisions, according as their members breathe by means of air-tubes (tracheae) or by gills. Our scheme then will stand thus:—

This scheme looks well on paper; and on the whole is workable. But among our examples chosen from the Class of Insects, we shall find some that breathe by gills in their larval stage, and by air-tubes when adult. And among the Crabs are some, the gills of which have ceased to perform their normal function, so that these animals cannot live in water for a single day. And then there are the Sand-hoppers and Woodlice.

The body of an Arthropod may be represented by a series of similar rings, thus:

This similarity is clearly apparent in the Centipede, but is concealed in the Beetle, the Shrimp, and the Spider. It seems, at first sight, to be altogether lost in the Crab, and does really vanish in the adult stage of some parasitic Crustaceans.

It may be plausibly objected that our ideal Arthropod resembles nothing so much as a worm. In many respects this is true. A primitive Arthropod was worm-like, as is a Centipede. And Arthropods and Worms were formerly classed together in one group, as Annulo´sa or ringed animals. The chief external difference lies in the nature of the appendages borne by the various rings or segments.

We may represent those of the Worms thus , for they are bristles, or groups, or modifications of bristles. Those of the Arthropods may be represented thus , for the appendages are really jointed, though, of course, in a fashion different from those of a backboned animal.

The jointed appendages of Arthropods may be modified to fulfil very different functions. They may serve as legs for walking, hands for climbing or seizing prey, jaws for masticating food, feelers or organs of touch and sense, and, strange as it may seem, in one group, as eyes.

It is well to get some notion of how these joints are formed. To take the body first: the skin connecting the segments is much thinner than that of the segments themselves, which is thickened by the deposition of chitine, and, in some cases, also of carbonate and phosphate of lime. A portion of the body, then, may be represented thus, where the heavy lines denote the segments, and the thin ones the spaces between the segments. It will be seen that this arrangement allows of considerable play, and also of a telescopic movement by which the segments can be brought close together.

It is easy to construct a kind of model that shall exemplify these movements. Make a tube of calico, some six inches long, and having stuffed it with cotton-wool, paste on it strips of brown paper one inch in width, leaving an interval between each, as in the last diagram. Then we shall be able to understand how Arthropods can bend the body or move it from side to side. And the limb joints are made on a similar plan.

The most archaic Arthropod—Perip´atus—must be mentioned. It is not found in Britain, nor even in Europe; so that, unless we travel, we shall only know it from books, or from museum specimens. But it is an extremely interesting creature, for it is of worm-like aspect, and breathes by air-tubes, opening all over the body, which has no external segments. The limbs are imperfectly jointed, and each of them bears two claws. Most naturalists make this genus a Class by itself, while some put it with the Centipedes. There are about a dozen species, four of which are African, two Australian, and the rest are found in South America and the West Indies. Besides these there are some doubtful species.

In habit they resemble the Centipedes, and they ensnare the insects on which they feed by ejecting sticky slime from the small processes near the mouth. The left process is shown in the illustration, just below the antenna of that side.

Professor Sedgwick, who described these animals in the Quarterly Journal of Microscopical Science (1888), and, more popularly, in the Cambridge Natural History, says, that ‘the exquisite sensitiveness and changing form of the antennae, the well-rounded plump body, the eyes set like small diamonds on the side of the head, the delicate feet, and, above all, the rich colouring and velvety texture of the skin, all combine to give these animals an aspect of quite exceptional beauty.’

Unfortunately, an illustration in black-and-white can only render form. We must take the beauty of the colouring for granted. One thing, however, cannot escape the most cursory examination of the picture—the resemblance of the creature, in some respects, to a worm, and, in others, to a caterpillar, which, as everybody knows, is the larval stage of a butterfly. If this resemblance sets us thinking how it came about, and what it means, Peripatus will, for the present, have done its work for us.

With these general notions of Arthropods, we may pass on to put our pocket lens to some practical use. Our first subject shall be the Margined Water Beetle (Dytis´cus margina´lis), which can be taken in almost any open pond in the country. Water covered with duckweed should be avoided in hunting for these beetles, which prefer ponds with a clear surface, so that they may easily come to the top to breathe.

Every one has a good general notion of the principal Insect-groups, technically called Orders—Beetles, Cockroaches and Grasshoppers, Butterflies, Bees and Wasps, and Flies. Insects may be defined as animals with hollow-jointed limbs, and divided into three regions—head, thorax, and abdomen. The head bears a pair of antennae; the thorax carries three pairs of legs, and (generally) two pairs of wings; the abdomen is without appendages. Insects when adult breathe by tubes that open to admit air. In Chapter VI we shall see that many larvae obtain an air supply in different ways.

Beetles may be taken as very good types of true Insects. They constitute the Order Coleop´tera, or Insects with sheathed wings, only the hinder pair being used for flight (Fig. 18), and at other times they are folded under the wing-cases, or el´ytra, as in Fig. 15.

We may advantageously compare our Beetle with Peripatus, and note the points of agreement and of difference.

Now, if our captive Beetles are to yield us the greatest possible amount of profit, we shall keep them under observation for some time, so as to watch their habits.

In keeping these Beetles we shall not require a large aquarium. A small gathering of aquatic weed will be necessary to keep the water in good condition and the aquarium ready for its tenants.

My interest in these Beetles was quickened by a letter in the Field (Oct. 28, 1893), in which a correspondent at Weybridge asked ‘for information as to what animal or bird bisects so neatly the shells of the Water Snail (Planorbis).’ I thought then, and know now, that the shells were ‘bisected,’ if that is the proper word, by Water Beetles. From that time I have had, and still have, several living in small aquaria, but for a long time was unable to get direct evidence on the subject.

Many experiments were tried, and at last these proved successful. Several specimens of Dytiscus[7] were obtained, and put into a small aquarium in which was no other food for them than some snails and other molluscs. The Beetles were carefully watched, and were several times seen trying the snails. In crawling along the inner surface of the glass, Planorbis and Limnaea both protrude the foot to a considerable extent, and pieces were ripped out by the strong mandibles of the Beetles before the shells were actually broken up.

All the shells represented in Fig. 16 were taken from this aquarium, so that there is good evidence as to what creatures broke them up and devoured their inmates. In these, as in the specimen kindly sent me by Mr. Tegetmeier, the Natural History Editor of the Field, the bisection is not complete, though in all cases it is carried far enough to allow of the extraction of the mollusc. The large Limnaea shell in the centre has been attacked, but it seems to have been left when the beetles discovered it was empty. (The empty shell was noted before the Beetles were put into the tank.) Another Limnaea shell is figured, from which the snail has been picked out, and that of a fresh-water mollusc.

After these observations had been recorded in the Field[8], I found that I had been anticipated by about forty years. I picked up, at a bookstall, a copy of G. B. Sowerby’s Popular History of the Aquarium, and there I found that the author had distinctly seen Dytiscus at this kind of work. He says[9]: ‘I have only once witnessed him in the act of seizing an unfortunate Planorbis or Flat-coiled Water Snail. At first, the Dytiscus seemed to be roaming about in quest of something, first under, then over, the leaves of a water-lily. At last, in a rather dark corner, he seemed to perceive suddenly a Planorbis which was browsing upon the stem of a plant just under the shade of a broad leaf. He darted at this, seized it, and then, putting his tail out of water, for the purpose of taking in a fresh supply of air, moved slowly down, bearing the snail with him. He held it by his fore-feet, turning round the coil until the aperture of the shell was opposite his mandibles, then he began nibbling away at the animal. In vain did the poor mollusc try to withdraw within its shelly fortress, for the beetle picked off the edges of the shell bit by bit, so as to expose the body as fast as it was withdrawn. All the way down to the bottom of the tank was this process continued, air-bubbles rising to the top, and bits of broken shell falling, till the beetle with his burden reached a stone near the bottom, where I left him still busy at his work.’

This puts the matter beyond doubt, if any before existed. I at once wrote to Mr. Tegetmeier to let him know that my experiments had, unknown to me, been anticipated, long ago, by Mr. Sowerby. Had he rescued his Planorbis shell, it would have compared very well with those forwarded to the Field office in 1893. They had been exhibited at the Malacological Society, and no one was able to solve the mystery of their mutilation. This shows, to quote the Field[10] on the subject, ‘how easily statements that have been recorded may subsequently be overlooked and entirely forgotten.’

To return to our Beetle. The male is a handsome creature, from an inch to an inch and a quarter long, clad in olive-green, bordered with yellow, and exceedingly active. His mate is smaller, more soberly clad in brown, without the yellow markings, and the wing-cases are more or less furrowed.

The first thing to notice is the shape of the body, oval and smooth, offering no resistance to the water. The hind pair of legs are flattened and fringed with hairs, so as to make capital paddles. In swimming the right and left legs are moved together.

Now, though this Beetle lives in the water, it is made, so far as concerns its breathing apparatus, after the fashion of a Land Beetle, and consequently is compelled to come to the surface pretty frequently for a supply of air, which it obtains in this wise. Directly it ceases paddling it floats to the top of the water; and as the head is heavier than the tail the latter projects a little above the surface. Then the wing-cases are raised, and air flows in under them to the breathing holes on each side. The operation is not a long one, and as soon as it is over the Beetle is ready for another ramble round his dwelling-house.

But if we do not supply our captive with food that he may take for himself, it is only right that we should feed him, which may be done at intervals—say, every other day. ‘Little, and often,’ is an excellent motto to guide us in our feeding; and though its adoption may entail some trouble, it will be more than compensated by the success that will attend our endeavours to keep the inmates of our aquarium in good condition. And the operation of feeding our Beetle will show us that he has some capital sense-organs, which are of as much, if not of more, use to him than his eyes.

He is a flesh-eater. Let us take a small piece of meat or fish in a pair of forceps, or stuck on a pointed stick, and hold it at a little distance from his great eyes. The chances are that he will not see it. Even if we put it in front of him, he is quite likely to disregard it, for he has nothing corresponding to a nose, with which he may smell. From his head there spring a pair of long feelers—the antennae—and by means of these we will let him know that his dinner is ready. That is effected by drawing the food along the side of one of the antennae. The creature undergoes a sudden change. Till the antenna was touched with the food he was resting on his swimming legs. But in a moment down goes his tail and up goes his head, he stretches out his raptorial legs, and clutches wildly at the forceps or stick, as the case may be, holding so tight that he may be dragged round and round the glass vessel. Let go he will not, of his own accord; and it would be a difficult matter to shake him off. Similar experiments may be tried with other Beetles, and the result will be to impress on the mind the fact that the feelers are capital sense-organs.

If we are to turn our Beetle to the best account, we shall need to handle him. It may be inconvenient to wait till he dies, so we will kill him quickly and painlessly by plunging him into boiling water, and he may be preserved by putting him into a tube containing about equal parts of water and spirit, or a five per cent. solution of formalin.

Dissections should properly be made under water. The Beetle should be fastened, back upwards, to a piece of cork weighted with lead, and placed in a deep saucer, or dissecting dish, and covered with water. But a good deal of rough dissection, as is ours, may be done in air, and the Beetle may be fastened to any convenient piece of board, or even held in the palm of the left hand. Very little practice is needed to run over the external parts of a large Beetle in this manner.

First, let us look over our Beetle, and get some general notions of its make. As it lies, back upwards, it is clear that it consists of three parts or regions — —— ———, the first of which is the head, the second the thorax, and the third the abdomen. Not only in our Beetle, but in Insects generally, these parts correspond to the words that denote them, in that the thorax is longer than the head, and the abdomen longer than the thorax, as shown by the three dashes, a few lines above.

These divisions are well shown in Fig. 17, where other parts are also marked. It will pay to go over our own specimen with this figure before us, and so make acquaintance with the several parts, to some of which we shall return in greater detail.

At this point, if we have not done so before, it will be convenient to fasten our Beetle, in the position figured, by a stout pin driven between the thorax and the abdomen, just above the suture (h). We want to raise one of the wing-cases.

If a needle be taken in each hand, between the thumb and first two fingers, and that in the left hand be used to steady the creature, the wing-case on the right may be raised with the needle in the right hand, and then cut off. The small filmy membrane, of somewhat triangular shape, which comes off with the wing-case, is the winglet. There is one on each side; and their vibration causes the humming noise made by these insects in flight. When the water dries up in one pond, or food becomes scarce, they will leave and fly off to another.

The wing lies folded upon the abdomen. A good deal of very interesting matter has been written on the way in which Insects fold their wings, but we can see for ourselves how this Beetle folds them. All we have to do is to take the wing, and draw it gently away from us, and so unfold it. We may use finger and thumb, or a small pair of forceps. When let go, it will spring back to its old position. Reference to the expanded wing in Fig. 18, and to the diagrams Figs. 19 and 20, will show how the wing is folded.

The cross-mark in the diagram represents a joint in the chitinous rod that forms the wings. This lies just above the cell (which is left white in Fig. 18). The shorter part of the rod is bent down, forming an acute angle (Fig. 20); of course, carrying with it the membranous part of the wing.

This may seem a little difficult. But if it be tried on a specimen, no real difficulty will be experienced. When the wing has been unfolded, it will, if let go, spring back to its old position, the shorter part lying underneath, and the chitinous rod fitting into a groove formed by the projecting sides of the segments of the abdomen.

To this point the sum of our knowledge about Dytiscus amounts to this: It is aquatic in habits; its body is divided into three regions; and it has a pair of membranous wings, covered by chitinous wing-cases, or sheaths, technically called el´ytra (each being an el´ytron). Wing-cases of this kind are the distinguishing mark of the Beetles, or Coleop´tera, though they are not always so well developed as in the specimen with which we are dealing. This we can discover for ourselves by examining all the Land Beetles met with in a country ramble or in a stroll round the garden.

Now let us unpin our Beetle, turn it on its back, and examine it from the under side. Head, thorax, and abdomen may be made out more clearly than before, and we can see that the last two regions are divided into segments.

Let us deal with the head first. This may be easily separated from the thorax with a dissecting needle, or with a pocket-knife—an exceedingly handy tool. The huge goggle-eyes cannot escape observation; and, even without a magnifier, they may be seen to be compound—that is, made up of a number of facets, which show like a fine network.

Just in front of the eyes are the antennae, which serve as organs of touch and perhaps also of other senses.

Kirby has recorded facts which seem to show that the antennae (in some cases) are also organs of hearing. Other authorities, after many observations, have come to the same conclusion. The matter, however, is beset with difficulty. It is certain that some Insects have their ears in their legs; and for the present, at any rate, we may be satisfied to know that the antennae are sense-organs, certainly of touch, probably of smell, and, in some cases, of hearing. An excellent authority on the subject is Sir John Lubbock’s book, The Senses of Animals[11], which contains references to very many original papers.

Now we may pass to the mouth parts. It will be good practice to dissect these out, either in air or in water. We may hold a Beetle between the finger and thumb of the left hand, and separate all the parts with a needle held in the right. It is a good plan to gum these parts on a card, for comparison with the figures in our favourite book—whatever that may be—on Natural History, and also with the mouth parts of insects of other Orders. For however much these may differ in form, and in the uses to which they are put, they are really modifications of the same parts.

In Fig. 21 we have the upper side and in Fig. 21A the under side of the head represented, so that we may easily get acquainted with the different parts, and the names given to them. The cut should be gone over several times, and the parts in the picture compared with those in the specimen under consideration. It is good practice to endeavour to draw what is seen from the specimen itself, and then to compare the result with the work of the trained artist. And the mouth parts of Dytiscus may be compared with the mouth parts of the Cockroach (Fig. 33).

Returning to practical work, the first thing is to separate the labrum, or upper lip, from the head. Then the large mandibles should be dissected out, and cleaned (by soaking in caustic potash) from the muscles which will come away with them. Behind these are a smaller pair of jaws, the maxillae, furnished with a pair of palps, called maxillary palps from their position. These are to be dissected out; and then the lower lip, or labium, may be separated by passing a sharpened needle along the line where it joins the chin. The palps on the lower lip are called labial palps.

When these parts are cleaned and dried, they should be gummed on card, as shown in Fig. 22, where the long lines represent the upper and lower lips respectively, and the shorter ones the mandible and maxilla of each side.

So much for the head. Now we discover that what appeared to be the thorax, when we were looking at the upper surface of the Beetle, and what is called the thorax in descriptions of Beetles, is really but a portion of that region, which is seen to be divided into segments. The covering on the upper surface protects only the first segment, the middle and hinder ones being covered by the wing-cases and the scutellum (a triangular piece jutting backward from the second segment, and meeting the suture). This is not represented in Fig. 17; but we may put in with our pen a tiny triangle, with its base towards the head, and its apex towards the tail—this will meet the case.

The first segment bears no appendage above, but to the under side is attached the first pair of legs. The middle segment also carries a pair of legs, and on its upper surface are the wing-cases, to the under side of which, and to the body, the winglets are joined. The last segment bears the wings above, and the last pair of legs below, these being placed very far back, so as to give them greater power in propelling the animal through the water.

It will be convenient to examine the legs next. First, however, it will be well to look at a normal leg of an Insect (the Cockroach), and learn the names of the different parts. First comes the coxa (a) or haunch, next the trochanter (b), then the femur (c) or thigh, the tibia (d) or shank, and the tarsus (e) or foot, ending in a pair of claws. There are three pairs of legs in perfect Insects, and usually the same number in larval forms, though in some of these legs are entirely wanting.

In the males of the Margined Water Beetle and many of its near relations the first pair of legs deserve special attention. The first three joints of the tarsus have coalesced to form a disk or cup, which in our specimen bears two smaller ones on its inner surface. A power of 20 will show the disk nearly as well as it appears in Fig. 24. The purpose of this disk, or clasper, which is absent in females, is obvious. It was formerly supposed to act as a sucker, but Professor Lowne and Professor Miall[12] have shown that it does not act by atmospheric pressure, but by a viscid secretion discharged from the cup-like hairs with which the inner surface is set.

The middle pair of legs in the male also bear cup-like hairs on the corresponding joints of the tarsus, and in very much greater number. Professor Miall quotes Simmermacher to the effect that while the large disk on the fore-leg has 170 sucking-hairs, the enlarged joints of the tarsus of the middle leg bear no less than 1590. These hairs are plainly discernible with the half-inch Steinheil, and I have made them out with the inch, and think that I could show them to anybody else with that power. I have not looked for these sucking-hairs on the middle leg of other Beetles of the same family which have disks on their fore-legs, but they do exist in some other genera.

If we watch a male Dytiscus in life, in a small aquarium, we shall soon be convinced that Lowne and Miall are correct in their statement that the cup-hairs discharge an adhesive substance. We shall see this all the more plainly if there is much floating vegetation. For, in swimming about, the Beetle will often come in contact with some of this, and it will adhere to the cup-hairs. His struggles to free himself from the encumbrance will show that the attachment is not altogether under his control. The offending weed is rubbed against the spines of one of the other legs till it is removed.

The spines with which the legs are set are worthy of a good deal of attention, and, like the adhesive cup-like hairs, though in different fashion, they doubtless assist the animal in holding its prey. The first and middle legs end in strong claws; those of the last pair are not so well developed.

The last pair of legs are the swimming organs. The tibia and tarsus are fringed with long stiff hair behind, so as to hold the water when the Beetle swims. A peculiar arrangement of the first joint of the tarsus allows the edge to be presented to the water when the limb is carried forward for the return stroke, thus offering the least possible resistance. This Dr. Sharp has compared to the action of a rower in feathering his oar. There is, however, this difference, which it is well to note. The oar is feathered after the stroke; the Beetle feathers its legs before the stroke. It is the first motion when it begins to swim, and the action is not peculiar to the male.

We now come to the third region, the abdomen. Like the thorax it is visibly divided into segments, though the division between them is not so great. Much difference of opinion exists as to the number of segments in the abdomen of a typical insect. Some authorities maintain there are eleven, while others put the number as low as five. This, however, is theoretical rather than practical. It is enough for us to know that the number apparently varies greatly, owing to the coalescence of two or more of the segments.

The head in Insects, we have seen, carries the eyes, antennae, and feeding organs. The thorax bears the legs and wings. The abdomen bears no appendages, except in some cases, on the last segment; these are called cerci. It may be, however, that the stings of bees and the ovipositors of saw-flies and other insects are modified appendages.

On examining the abdomen of Dytiscus we shall probably be struck with the difference in appearance between the upper and the under surfaces. The latter is hard, smooth, and shiny; the former, when the wings are removed, is seen to be covered with felt-like hair.

Our interest is with the upper surface. Along the abdomen on each side lie spiracles, stigmata, or openings to the breathing tubes. The first and last are larger than the rest, and their general form can be readily made out with an inch magnifier, and with the half-inch we may get some idea of the detail shown in Fig. 27.

Dytiscus breathes in this way. Floating up to the top of the water, the end of the abdomen projects above the surface. If one watches the Beetle the wing-cases will be seen to rise a little. The air retained by the felted hairs is given off, and a further supply taken in. Then the wing-cases are lowered again; the Beetle gives two or three strokes with its swimming legs, and descends below the surface to ramble round the tank in search of food.

This air-supply between the wing-cases and the abdomen is taken in at the spiracles and distributed through the tracheal tubes throughout the body. These tubes branch and subdivide till they end in small twig-like vessels comparable to the capillaries of the human body. They consist of two layers—the inner strengthened by what probably is a spiral fibre, though Packard believes that, in some cases at least, it consists of similar rings. But we must not pursue this subject. It would lead us beyond our appointed limits.

Another Beetle fairly common in stagnant waters round London and in the southern counties is that to which the name Great Water Beetle (Hydroph´ilus pic´eus) of right belongs. This name is sometimes wrongly applied to Dytiscus, with which its rightful owner has little in common, except its aquatic habitat. Its scientific name is Hydrophilus piceus; but we shall speak of it as Hydrophilus.

It is not a very easy matter to take this Beetle with a net, by sweeping in the ordinary way, for it likes to get into the middle of a mass of vegetation, where it is sure of a good food supply, and is probably safe from the attacks of Dytiscus, who not unfrequently makes a meal of his larger relation. A good plan is to pass the net under a mass of weed and then shake it to and fro in the water. By this means any Beetles in the weed will be dislodged from their hiding-places, and fall down into the bottle.

They have, in confinement, the same habit of making a snug place for themselves; and more than once I have fancied that a Beetle of this species had escaped from the aquarium, when all the time it was hidden in a thick patch of water-moss. They are practically vegetable feeders, though Dallas says that they are not such strict vegetarians as to deny themselves a meal of animal food when they meet with a dead mollusc or larva in the course of their wanderings. I have never known them to indulge in animal food, dead or living, but I have known them refuse it.

Hydrophilus is the largest British Water Beetle, and, with the sole exception of the Stag-Beetle, the largest British member of the Order. Its total length is very little less than two inches, and across the middle of the back it measures about half as much. It is more slenderly built than Dytiscus, and the contrast in the size and armature of the legs is very striking (Fig. 29). There is also a great difference in their method of progression through the water. Dytiscus moves both legs simultaneously, while Hydrophilus walks rather than swims, moving one leg after the other.

If we cannot collect this Beetle for ourselves—which we should endeavour to do, if possible—it may be bought of almost any dealer in what are called ‘aquarium requisites.’ But prices rule higher for Hydrophilus than for Dytiscus. Bateman says that this species is rarer than formerly, and that specimens cost from 1s. to 2s. 6d. a pair, ‘according to the dealer and the season.’ From this I gather that I must have gone to a shop where the prices were reasonable, for I have never paid more than 6d. for a Hydrophilus, and then have been allowed to pick out a male. At the same shop I have paid 2d. for Dytiscus.

In keeping this Beetle we shall need a larger vessel than was required for Dytiscus. (In both cases the aquarium should be covered, for if food be scarce, and sometimes for other reasons, both these Beetles may take to flight.) The aquarium should be well supplied with growing water-weed, but none that is choice or valuable should be put in, for in moving about over the weed the animal will damage almost if not quite as much as it eats. This difficulty can be easily got over by supplying it with anacharis, water-crowfoot, milfoil, or any other common plant that grows rapidly and is easily procurable.

The only specimen that I have taken myself was captured a few miles north of London. It exhibited a strange instance of depraved appetite. In the large tank into which it was put were growing vallisneria, frog-bit, and water-crowfoot in plenty. These it was never seen to touch. The tank, at one time, had been used for newts, and floating on the surface was a piece of virgin cork. It had served the former inmates as a kind of island continent, and had never been removed. To the under side of this the Beetle would moor himself, head downwards, and nibble away, as if cork were the natural diet of a British Water Beetle.

In a few days the Beetle died. It was put into spirit, and soon after became the subject of a post-mortem. But its strange diet was not the cause of its death, which was sufficiently accounted for by injuries inflicted before its capture, probably by a larval or an adult Dytiscus.

It would be mere waste of time to go over this Beetle and describe it point by point, as was done with Dytiscus. If what was there written was of any value, readers will be able to apply for themselves the method laid down. There are, however, some points of difference to which it will be well to invite attention.

It is a good plan to lay specimens of these Beetles side by side for comparison. Hydrophilus is the larger of the two; and differs in colour as well as in size. Its hue is black with an olive tinge; and in certain lights a blue-black metallic gloss may be seen on the outer margins of the wing-cases. These are marked with faint longitudinal lines, and each bears three rows of dots running in the same direction.

The greater length and more slender build of the legs of Hydrophilus are at once apparent. There is also a marked difference in the tarsal joints of the fore-legs of the male. The disks and cup-like hairs of Dytiscus are absent in Hydrophilus, but in their stead the last joint bears a sub-triangular plate, studded on the inner surface with spines, which probably serve a similar purpose. A great deal of valuable information about organs of this kind and their functions will be found in chapter X of Darwin’s Descent of Man. Simmermacher’s paper[13] should be consulted by all who have the opportunity. Our inch magnifier will show us these spines quite clearly; and also a curious little bunch of bristles, which Simmermacher says are probably organs of touch.

It is a good plan to take Hydrophilus out of the water, and lay it upon its back, so that the difference between it and Dytiscus may be clearly seen. The Beetle should be handled carefully, for on the thorax is a kind of keel, ending in a sharp spine, which extends over part of the abdomen. This spine is free, and may easily wound the hands of those who do not watch the motions of the creature pretty carefully. The fore part of the abdomen and the thorax are covered with short close hairs, and when the Beetle is in the water these parts entangle a layer of air, which gives it the appearance of being covered with quicksilver.

The two Beetles differ also in their method of exchanging impure for pure air. Dytiscus, as we have seen, takes in a fresh supply under its wing-covers behind; Hydrophilus takes in a fresh supply in front, employing for this purpose the antennae, which apparently do not function as feelers, as is generally the case.

When Hydrophilus wants to take in a supply of pure air, it rises to the top of the water, slowly and deliberately. Unlike Dytiscus, it is never in a hurry. Then one of the antennae is pushed through the surface film, thus communicating with the air, which descends to the hair-covered thorax, whence it reaches the spiracles on the upper surface of the abdomen. To allow of this the wing-cases are slightly raised in front. The spiracles in Dytiscus are larger at the posterior end of the abdomen: in Hydrophilus the largest spiracles are in front. This is what might be expected, from the method adopted in each case for procuring a fresh supply of air.

These Beetles have frequently bred in confinement; but no better account than that of Lyonnet has ever been given of the operation of the female in making her cocoon and depositing her eggs. As his account is not generally available, a condensed translation of it is inserted with his illustration.

Lyonnet[14] wanted to find out how the female made the cocoons (Fig. 30), and this is how he set to work. He put some of these Beetles into a large aquarium, with a good quantity of water and some duckweed. On May 31 and the following day he noticed that one of the females was swimming about in every direction, as if in search of something. Thinking that this was because she had not the proper materials for her work, he then put into the aquarium some thread-like alga of a kind which he had seen attached to some cocoons, and on June 3 the Beetle began to make a cocoon, but soon gave up the task, apparently because she was troubled by other aquatic insects which had made a home in this weed. These intruders were removed, and the Beetle set to work once more. Lyonnet then noticed that, like a spider, she had her spinning apparatus at the posterior end of the body. She extended the last segments slightly, and opened the hindmost one, when he saw a nearly circular opening, in which was a whitish disk (Fig. 30A, a). On this disk were two little brown tubercles side by side, nearly at right angles to the longitudinal axis of the body. From each there projected a blackish-brown conical tube, about a line long, stiff towards the base, but flexible and elastic towards the tip. These tubes were the spinnerets, which acted together with a parallel movement, and from each proceeded a separate thread.

And this is how she made her cocoon. She lay near the surface of the water back downwards, the under part of the body and the second and third pair of legs buried in the thread-like weed. The front legs were free, and with these she shaped the weed over her abdomen. Then she spun a covering of white silk against the under side of the weed. While she was spinning, from time to time she used her front legs to press and flatten the work against her body (Fig. 30B), giving it the shape of a flattened arch, to which her body gave the requisite curve. This, forming the top of the cocoon, was finished in about half an hour. Then she turned (Fig. 30C), and spun the bottom of the cocoon, moulding this, like the top, on the curve of her abdomen, and uniting the top and bottom with silk which she spun. The work occupied about an hour and a quarter.

The Beetle then remained nearly in that position for some two hours. At first she was hidden in the cocoon quite up to the thorax. The body, however, was withdrawn almost imperceptibly. During this time she was busy laying her eggs in regular order, with the pointed ends upwards.

After this she came out of the cocoon, and closed the mouth (Fig. 30D), making the opening smaller by degrees. Then she made a little mast (Fig. 30D, b), of the use of which Lyonnet admits his ignorance, suggesting, however, that its construction may serve to use up the silky matter remaining after the work is finished, lest it should acquire harmful qualities in the body of the Beetle. The true explanation seems to be that it serves to convey air to the eggs inside the cocoon.

On July 17 Lyonnet was rewarded for his patient watching by seeing a larva come out of the cocoon, and the next day some fifty more appeared. What he saw and recorded it is in the power of others to see, if they will imitate his patient observation.

The Cocktail Beetle, or Devil’s Coach Horse (Ocypus olens), is an excellent specimen of a Land Beetle to examine, for it is of fairly large size and extremely common. Moreover it does well in captivity, so that there will be no difficulty in watching its habits in life, and pickling it for closer examination when dead.

During the day these animals usually lie concealed under stones or pieces of earth, coming forth at dusk and during the night in search of food. Occasionally, however, they may be met with in daylight, leisurely stalking a smaller beetle or a fly; then with a dash seizing the victim in their powerful mandibles, which are quite capable of making an impression on the human skin, as those who handle these Beetles unwarily will discover for themselves.