Through a pocket lens

In this chapter we shall deal with a few aquatic insect larvae. Of these, some are aquatic also in the perfect condition, while others emerge from the pupa stage as aërial insects. It requires no great amount of care to keep these creatures, and some hints on this subject and on collecting are given in the first chapter.

The larva of Dytiscus is abundant during the greater part of the year, and is almost sure to be met with by the collector, who will find it an extremely interesting object for examination and study. No other creatures should be put in the same bottle with these larvae; and if there are several of them in one bottle, it is a good plan to put in plenty of pondweed, which will often keep them from attacking each other.

When full grown, the Dytiscus larva may attain a length of two inches or rather more. Its colour is dingy brown, and its aspect forbidding enough to justify the uncomplimentary names that have been bestowed upon it—Water-devil and Water-tiger. It certainly rivals the tiger in fierceness, and its method of stealing up to its prey and attacking it from behind led Swammerdam to call it the Sicarius or Assassin Worm.

One must not imagine that Swammerdam was ignorant of its nature; ‘worm’ with him was a general term for any larval form. Indeed, he says, ‘It is extremely probable that some peculiar species of the Water Beetle proceeds from this worm, when, having remained in the water a sufficient time, it betakes itself to the land to undergo its mutation; but this is mere conjecture.’ What was conjecture for him is fact for us.

Now let us put our larva into a small tube, and examine it more closely. The head is large and joined to the first segment of the thorax by a distinct neck. There are twelve small eyes, six on each side, a pair of antennae, two pairs of palps, and a large pair of sickle-shaped mandibles, which Swammerdam calls ‘teeth,’ and says that ‘it is perhaps to contain the muscles such teeth require that Nature has made the head so large.’ Behind the head come eleven segments, of which the first and last are the longest. They gradually increase in width till the sixth, the rest again decreasing, till the eleventh ends in a blunt point, from which diverge two appendages, thus , thickly fringed on both sides with hair, as are the tenth and eleventh segments.

There are six legs, one pair to each of the first three segments. These also carry fringes of hair, thus increasing their power as swimming organs; and, in addition, they bear numerous spines, and end in strong double claws, which must be of service in climbing over aquatic vegetation, and may assist in holding a struggling victim or in striking it down, so as to bring it within reach of the mandibles.

Spiracles will be found—seven on each side. These do not, however, in the larval condition, serve as breathing organs, though they fulfil their proper office in the pupa. The air-tubes of the larva open at the extremity of the last segment. When the larva wants to breathe it comes to the surface without an effort, for it is lighter than the water it displaces. The tail rises above the surface, and a fresh supply of air is taken in. When the larva wishes to descend, a stroke of the tail sends it downwards, and as it reaches the bottom of the tube it will cling with its claws to any weed we may have put in with it, or hold on with them to the glass itself.

The chief interest of this animal lies in its mandibles, and the method in which they are employed. It was formerly believed that the mouth of this larva was closed, so that it could not take solid food, and that it lived entirely on the juices of its prey, which it sucked up through its hollow mandibles.

Swammerdam says of this larva, ‘When about to eat he seizes with the two teeth (mandibles) the little creatures that come in his way, and pierces their body with the sharp crooked points. The teeth being perforated from the points to the roots, he in a surprising manner sucks through them into his mouth the blood of the unfortunate captive. This may be easily seen, especially when the blood of his prey is of a red colour, as the teeth are transparent.’

He then describes how he watched one of these larvae feed, and saw the blood, mixed with air-bubbles, travel up the mandibles. After this he tells us how, if we have a Dytiscus larva, we may ‘procure ourselves a very entertaining and surprising sight, by throwing to it a small earthworm; for let this last move, twine, and otherwise bestir itself ever so much, the other keeps its hold, and very calmly sucks the blood of its prisoner.’

We may, however, go to work in another fashion. We may dissect out the mandibles from a dead larva and pass a fine hair into the slit near the point, and it will come out at the orifice near the base. It is through this orifice that the nutritive juices of the prey are drawn into the true mouth. This practical proof that the mandibles are pierced is by no means so difficult as one might suppose.

De Geer[57] seems to have been the first to suggest that there must be some kind of true mouth, and in support of his suggestion tells us that he saw this larva eating up the solid parts of a Water Woodlouse (cloporte), after having sucked up its juices. More than this, he places the mouth in what has proved to be the true position, though he did not solve the mystery as to why it is kept so firmly closed. This was done by Mr. Burgess, an American naturalist, from whose paper[58] the following particulars are condensed:—

‘Authors have described this creature as mouthless; and if we examine the slit where we should expect the mouth to be, we find that this slit ends in a perfectly closed seam. The methods of microscopical research were brought into play, and a longitudinal section of the head cut and mounted. This showed that the upper and lower lips were locked together by a peculiar joint—the upper coming over and locking into the under lip (Fig. 75).’

We may get some idea of this mouth-lock by placing the fingers of the right hand over those of the left, and then bending them.

Mr. Burgess concludes his paper thus: ‘We find that the Water-tiger, far from being mouthless, as ordinarily assumed, has in fact a very wide mouth, though its lips are closely locked together by a dove-tailed grooved joint developed for this purpose. Whether this joint can be unlocked by the animal itself is another question, which I cannot answer, though De Geer’s observation above quoted makes this probable. It is, at all events, easy to open the mouth by manipulation with a pair of forceps.’

Professor Miall[59] has verified Burgess’s observations, and carried them a step further. He found by actual experiment that ‘the mouth-lock acted automatically, opening when the mandibles opened, and closing when they closed.’

Both these authorities stand very high. Yet, with all respect to them, it will be well to test these statements before accepting them.

Something of this mouth-lock may be seen in any well-prepared slide of a Dytiscus larva. If we hold the slide up to the light and examine with a power of 10, we shall see a dark line—in some cases two nearly parallel thinner lines—running from the base of one mandible to the base of the other. The dark line is the closed mouth-lock. The two nearly parallel thinner lines are the edges of the lips drawn asunder in preparing the specimen or by the pressure of the cover-glass. If we get to see this much, we shall have advanced one step.

Next we may verify Professor Miall’s experiment, though not quite in his way, for such section-cutting as he speaks of is beyond our powers. Larvae as large as possible should be chosen, and killed by dropping them into boiling water. The mandibles of one should be allowed to remain closed, and those of the other plugged open with pith or a small piece of wood—a bit of a match will serve. By putting each in turn into an excavated slip with water, carefully covering this with a plain slip, and holding these slips together with an elastic band, we may examine the larvae as we did the slide.

The result will be practically the same. Where the mandibles are closed, we shall see the dark line; and where they are open, we shall see the two thinner ones nearly parallel.

One caution may be necessary. The mandibles bear at the base a rounded process, which fits into a chitinous cup. It is not difficult to mistake this for the mouth-lock, with which, however, it is not connected.

There is still one other method which we may adopt to see the mouth. The head of one of these larvae may be cut off, dropped into a strong solution of caustic potash, where it should remain for a day or two. After washing it well in water, cut it in half lengthways, and turning either half upon its side, so as to expose the part cut, examine with the pocket lens.

These experiments are by no means difficult. But if carried out step by step, it will be easy to understand how the larva can suck out the juices of its prey, and how it can open its mouth to swallow some of the solid parts.

The larva does not readily change into a pupa in confinement. If, however, we wish to rear a beetle, the larva must be plentifully supplied with food, and removed from a bottle to a flat dish, where earth can be placed against the side so as to rise above the water-level. Fig. 77, where a pupa is represented in a bank by the side of a pond, will give us a hint how to go to work. The time occupied in pupation will vary according to the temperature of the room—but is never less than a fortnight. In the open it is probable that the winter is passed in the pupal condition, the perfect insect emerging in the spring. Like Land Beetles, it does not assume its dark hue for some days, but its pale skin darkens by degrees.

The larva of Hydrophilus does not seem to be often taken in this country. It would make a capital subject for investigation, and the food-supply could be arranged easily enough. The repetition of the experiences of Lyonnet, who reared these larvae from the egg, would be of great interest. He says that he took about thirty larvae from the brood, and fed them with very small water-snails. These they devoured in the same way as the larger larvae do. Having seized the snail with their mandibles, they bent backwards, and supporting it on their back, which served them for a table, eat it there, without making any use of their legs to hold their prey. When the supply of small snails ran short, they did very well with large ones cut up into pieces, and with tadpoles. If, however, food was not supplied to them, they fed on each other. But, except when pressed by hunger, they lived together peaceably enough, and seemed to take pleasure in each other’s society[60].

The larva of Limnobia replicata, a Crane-fly allied to, but smaller than, the well-known Daddy-longlegs, is another excellent subject for investigation. It is not very often taken; perhaps because it is not often looked for. But it is common enough, for all that.

In general appearance these larvae resemble small caterpillars covered with spines. Some of these are simple and others forked, not much unlike the letter , with a short stem, and the arms slightly curved. There are no feet, and the last segment carries two pairs of hooks, one large and the other small. From their position they are called anal hooks.

The dykes of the Sussex marshes are an excellent hunting-ground. Probably the channels of water-meadows, or any other shallow standing water in which aquatic moss grows, would serve the collector’s purpose quite as well. And such pieces of water abound all over the country.

For taking these larvae the ordinary net and bottle are of little use, though a few may be captured by sweeping backwards and forwards among patches of aquatic moss.

There is, however, a far easier and surer method. A good handful of the moss should be gathered, and put into a shallow vessel half full of water—a developing-dish answers capitally—and then shaken to and fro or gently stirred with a small piece of stick. The larvae will curl into a ring and fall to the bottom, whence they may be picked up and dropped into a bottle or other receptacle to be taken home. A fair quantity of moss should be gathered, for this is their favourite food, and all larvae are greedy feeders.

Other water-plants, however, do not come amiss to them. Just before these lines were written I was examining a bottle in which some of these larvae were kept. It contained a few sprays of willow-moss and some ivy-leaved duckweed, which floated on the surface. One larva on a spray of moss was reaching upward, and it was distinctly seen to feed on the duckweed. This must have been from choice, for there was within reach plenty of what all observers consider to be its natural food. This, too, might have been more easily obtained; for, to reach the duckweed, the larva had to hold on to the moss by the anal hooks, and extend its body in a fashion analogous to that of the caterpillar of a geometer moth, which will hold on to a branch with its fore-legs and claspers and maintain the body at an angle of 45°, sometimes for hours together. I have also seen them feed on hornwort.

The larva of this small Crane-fly is not at all difficult to keep. It is exceedingly hardy, and will survive a great deal of rough treatment. In November, 1895, I sent three in a tube—securely packed, as I thought—to a friend in Yorkshire. A few days afterwards I heard, with regret, ‘that the bottle was broken in transit, and that the larvae arrived dead.’ Three days later I was gratified by receiving a letter, from which the following is quoted: ‘The Limnobia larvae have come to life again. I put them into water as soon as they arrived, and after lying motionless for many hours they have begun to creep about and feed.’

This is excellent testimony to their powers of endurance, but it is weak in comparison to that which De Geer supplies[61].

He was Marshal of the Court of Sweden, and one November, before leaving his country house for his official duties at Stockholm, he put four of these larvae into a vessel of water, and left them to take their chance. The water froze into a solid mass. When he returned in the following May he found about half the water thawed, and two of the larvae dead. The others, though they had been enclosed in ice all the winter, were lively and in good condition. He put them into another vessel with fresh water and some aquatic moss, and at once they began to move about and commenced to feed. Both pupated by the fifteenth of the month, and the perfect fly emerged after six days in the pupal stage.

The following description of this larva is principally condensed from De Geer, whence the figures are also taken. The body is long and cylindrical, and divided into eleven segments, of which the first and largest is sub-triangular. The second and third segments are shorter than the rest.

The head (t) is very small and completely retractile within the first segment, the anterior margin of which completely closes the orifice, so that, in this condition, the creature appears to be headless. The body is covered with spines, some simple and others branched. On the first three segments there are only simple spines; but from the fourth to the tenth segment inclusive there are also on each segment two forked spines—that is, fourteen in all. On the last segment the spines are simple, and here are also four chitinous hooks, one pair much larger than the other. These simple spines carry a white vessel, which extends throughout their whole length; but in the forked spines there are two such vessels placed side by side in the stem, and diverging one to each branch.

He comes to the conclusion that these spines are probably the breathing apparatus of the larva, for they are similar to those which he observed in an aquatic caterpillar. Walker[62] calls these spines ‘long filamentous processes, which appear to be internally supplied with air-tubes,’ but he does not seem to have taken the trouble to break up a specimen, or he would have been in no doubt as to their real nature. This is shown by the fact that the larva never comes to the surface to take in a supply of air, but contents itself with the oxygen dissolved in the water.

The pupa is quite as remarkable as the larva, though the breathing apparatus does not assume the form of spines, but consists of two ‘trumpets,’ one on each side of the head, as is the case with the pupae of gnats, using that term in a wide sense. The colour is a greenish-brown, dotted with black. The abdomen is capable of a good deal of motion from side to side; and by this means the pupa can raise itself to the surface of the water to take in a supply of air.

De Geer remarked that when the pupa rose for this purpose it lay with its body horizontal, having the lips of the trumpets a little above the surface. It appeared not to like being placed on its back, because in that position the trumpets cannot be raised above the surface. When he tried the experiment of putting the pupa back downwards, it wriggled over by bending the abdomen.

On the abdomen there are several chitinous hooks, which serve in some sort as substitutes for limbs. By their means the pupa can moor itself to the stems of aquatic plants; and this is necessary, since its specific gravity being less than that of the water it would be always at the surface, if it had no such means of anchorage. And of course, at the top of the water, it would be exposed to the danger of being snapped up by birds.

De Geer’s specimen passed six days in the pupal state, and then emerged as a perfect Crane-fly. My specimens did not emerge till after ten and eleven days of pupahood, which seems strange, as they were plentifully supplied with food in their larval stage.

The fly is a little more than half an inch long, and may be distinguished from the common Daddy-longlegs by the character of the wings, which are folded somewhat like those of a wasp, but with this difference: that the wings of this fly are folded outward, while those of the wasp are folded inwards. When the insect wishes to fly, it opens the fold so that the whole wing presents a plane surface, but the fold reappears directly the Crane-fly ceases its flight (Frontispiece).

De Geer’s allusion to the wing of the wasp might have been extended to most of the Hymenoptera, as we may see by examining the fore and hind wings of a bee or a sawfly. Dr. Sharp[63] says, ‘The wings [of the Hymenoptera] are remarkable for the beautiful manner in which the hinder one is united to the anterior one, so that the two act in flight as a single organ. The hind wing is furnished with a series of hooks, and the hind margin of the front wing is curled over so that the hooks catch on to it. In some of the parasitic forms the wings ... have no hooks. The powers of flight, in these cases, are probably but small.’

If we were taking our subjects in consecutive order, the larva of Ptychoptera would properly come here, as being that of a Crane-fly. But since it will be convenient to examine another larva which resemble Limnobia in its breathing apparatus, we will take next the larva of Parap´onyx stratiota´ta, one of the China Marks, for it is extremely interesting and by no means hard to come by.

The China Marks are small moths, with white wings bearing dark markings, which have been supposed to resemble Chinese characters. Their larvae are aquatic in varying degree—that is, some breathe air in the usual way, by means of spiracles; while others, by means of gills, breathe the oxygen dissolved in the water.

Many collectors have, no doubt, taken these larvae, and cast them away in the belief that they were caddis-worms. Such was the experience of the Rev. Gregory Bateman, the author of Fresh-water Aquaria, who says (p. 259): ‘While hunting for fresh-water animals, one not seldom comes across an insect wrapped up in two or more green leaves, or pieces of leaves, of some aquatic plant. The leaves and the animal have somewhat the appearance of a caddis-worm in its case; in fact, for a time, before I knew what it was, I mistook it (and I daresay others have done so also) for a caddis-worm.’

The cases are usually, but not invariably, made from the food-plant of the insect. Mr. Bateman has noted that these larvae ‘do not always confine themselves to the same weed, either for food or for building material.’ This has also been my experience. A larva of the Brown China Mark, recently taken on the Norfolk Broads—an excellent collecting ground for aquatic larvae—was put into a tube. The case had been damaged, and the only vegetation in the tube was a spray of bladderwort. On examining the tube, after some days, I failed to find the larva. The reason was evident on removing the cork, a small part of which had been gnawed away to procure material for the repair of the larval case, which was affixed to the under side. The larva was dead and too much decayed to be put into pickle, a circumstance I much regret, as I should like to have preserved the larva in such a strange dwelling. As it is, I have only been able to keep the house without its tenant.

Pondweed is the usual home of the larva of the Brown China Mark, and from the leaves of this plant the first larval case is generally fashioned. This was the species upon which Réaumur made his interesting observations, most of which have been confirmed by succeeding observers. In well-grown larvae the contrivance by which the animal is protected from contact with the water in which it lives should be noted, as it may be easily, with the hand lens. The skin is thickly set with tiny protuberances between which the water cannot penetrate, the surface film stretching from tip to tip of these prominences, just as it does over the hairs that cover the body of a water-spider.

De Geer[64] describes an aquatic larva of one of the China Marks (Paraponyx stratiotata), which has its breathing apparatus fashioned on a similar plan to that of Limnobia, though there is some difference in the details. He found his specimens on the leaves of the Water Soldier, and his interesting account recalls the fact to memory that this remarkable plant was at one time called the Marsh Aloe—an exceedingly appropriate name.

He describes the filaments on the body of the larva, and concluded that they were probably breathing-organs, because of the dark-coloured vessels within them. These he traced, as we will presently do, to their union in the stem of the gill, and thence to the air-vessels running down each side of the body of the larva. He fed them on leaves of the Water Soldier, and kept them through the winter. In the following June he found them preparing to undergo their transformation into the pupal stage, and at the end of the month the moths came out. He was gratified by seeing the congress of these insects. The females deposited their eggs on the floating vegetation and on the sides of his aquarium, a little below the surface of the water, and in about eight days the young larvae were seen.

These larvae must be very abundant, though they do not seem to be often taken by collectors. In describing an allied (American) species, which is found on Vallisneria and pondweed, Mr. Hart says[65], ‘They feed at first exposed on the leaf, but later two or even three leaves are loosely webbed together, face to face, by each larva, between which it remains concealed while feeding. They are, therefore, hard to discover unless their hiding-places are broken up by seining, or the like, when the larvae may be seen swimming about.’ This is, no doubt, the reason why these larvae are not more often taken. Anything like a seine net is of course out of the question for us; but masses of vegetation may be readily broken up by vigorously working the bottle and net backwards and forwards amongst them. Specimens I have seen were taken among duckweed; and Mr. Hart mentions one instance of part of the larval case being constructed of ivy-leaved duckweed, ‘which was abundant there at that time.’

Now let us bring our lens to bear, so that we may make out the external structure, and recognize the similarity of the breathing-organs of this Moth-larva to those of the Crane-fly larva already treated of (p. 168).

In order to make out the scheme of the gills, which is somewhat complicated, one should first of all distinguish the spiracles, remembering that they are not functional. And it is best to begin with those on the middle segments of the body. They may be detected as little dark spots, sometimes enclosed in a ring. The head, the first segment of the thorax, and the last segment of the body, bear no gills; the second segment of the thorax has but two pairs on each side; and there is but a single gill on each side of the ninth segment of the body. On the other (nine) body-segments there are the full number of five gills on each side, arranged two above, and three below the spiracle. The upper pair are called supra-stigmals, or gills which lie above the spiracles; the lower pair are called infra-stigmals, or gills which lie below the spiracles; and the single one, the lowest, is known as the pedal or foot gill. These technicalities need not give us any trouble here, in our examination of the larva; nor do they present any real difficulty. But it is worth while to master the arrangement as soon as we get hold of one of these larvae, and then we shall be able to take up and understand technical descriptions of this aquatic caterpillar and its allies, in so far, at least, as they refer to the breathing apparatus.

The gills differ in their character: some few are simple, while most of them are more or less branched. In Limnobia the branching of the gills is into a simple fork; in Paraponyx this kind of division also occurs, and in others most of the gill branches are also given off from the main stem below one of the branches of the fork.

In Fig. 84 we have a representation of one of these branched gills. It will not be difficult for us to make out the details as there shown. But the vessels that run down into the filaments, constituting them breathing-organs, are smaller than those of Limnobia, and will consequently require a little more care and patience before we can distinguish them.

One would think that, with such an array of gills, this larva ought to be in good case for its air-supply. It may, perhaps, be doubted whether this is so. At any rate, the creature adopts the same plan as the larva of Chironomus, which has no gills at all, for driving away from its case water that has parted with its oxygen. Water charged with oxygen pours into the case, and so the air-supply is renewed. This plan is nothing more than keeping the fore-part of the body in undulating motion, the tail in both the larvae serving as a point of attachment. One or two that I have kept made their cases against the side of the bottle, and so afforded an excellent opportunity of seeing them in this motion. The Tanypus larva does the same thing. Against the side of one of my small aquaria a Tanypus larva and a Chironomus larva have both made tubes; and as I look up from writing these lines I can see them both hard at this work.

The larva of the Alder-fly (Si´alis luta´rius) is also furnished with tracheal gills, seven on each side. So little is known of the life-histories of common insects that it may be profitable to introduce the account of an observer who watched the deposition of the eggs and the emergence of the young larvae:

‘On April 25 I found, on the rushes round the margin of a small pond, a great many patches of eggs, and shortly after observed many of the Sialis lutarius depositing them.

‘They form large patches of from two to three inches in length, generally encircling the whole rush near the top, but sometimes deposited on one side only, and extended to about a line in breadth.

‘I counted 100 in a square line, so that each batch may be fairly considered to contain from 2,000 to 3,000 eggs; the greater portion of which must consequently perish either in the egg or larva state; as, common as the insect is, and widely distributed throughout the country, we should be perfectly overwhelmed with the swarms of the perfect insect if such were permitted, when it is considered that round this one small pond there could not have been less than 100 patches of them.

‘The eggs are of a very singular form, and placed in a slanting position.

‘The females, while depositing them, appeared perfectly motionless on the rush, and varied considerably in size, being from five lines to nearly double that in length. Some parts of the patches of eggs are of a much lighter colour than the rest.

‘On May 3 I found many of the eggs hatching, the little larvae tumbling about in great numbers, with their bodies erected like [the larvae of] the Staphylinidae.

‘On putting them into water they swam about with the greatest activity, wriggling and undulating their bodies about much like a serpent or the tadpoles, and working their legs at the same time[66].’

The author draws attention to the disproportionately large head of the larvae, which, however, he did not describe, as he had ‘brought some of them alive, and some eggs for exhibition.’

Sialis larvae occur in most ponds with muddy bottoms. They may be taken by scooping up some of the mud in a long-handled spoon—a most useful instrument for the collector—and washing it, or by throwing in the drag, and bringing to land a mass of water-weed, roots and all. A few may generally be detected near the roots. They may be picked up with a small pair of forceps, or with a brush, and dropped into a bottle; or, better still, into separate tubes; for they are fierce and voracious, and, failing other food, by no means indisposed to prey on each other.

Their general appearance, and especially their powerful mandibles, give them some resemblance to the larvae of a water beetle, for which a celebrated naturalist not unnaturally took them, when he began to study them. And this would seem to be the opinion of some mounters, for I have a slide of the mouth parts of this larvae, labelled ‘Mouth parts of the larvae of a water beetle.’ It was not till I had broken up a good many Sialis larvae that I found out what the slide really was.

These larvae may be kept alive in small bottles of water, if they are supplied with food. They will eat Chironomus larvae and those of Tanypus. Professor Miall has found that they will eat caddis-worms and May-fly larvae. Probably, no small aquatic creatures that they can overcome are safe from them.

A larva that is full-fed, and ready to change to become a pupa, will measure about an inch in length or a little more (Fig. 85). The general colour is brownish, with dark markings. The legs are powerful, and our lens will show us that they end in two strong curved claws. From each of the first seven segments of the abdomen are given off a pair of jointed tracheal gills or breathing-organs, which are directed upwards and backwards when the larva is at rest—a rare occurrence—but which wave to and fro in the water when the creature is swimming.

From this fact has arisen the statement found in most books that the larva uses these gills not only for respiration, but for locomotion. Professor Miall has come to a contrary conclusion, and, as he has kindly informed me, is confirmed in his opinion by the weakness of the muscles.

It will be well to make repeated observations till we are satisfied on the subject. When these larvae are kept, the conditions necessary to provide them with food and to keep the water aërated by means of growing vegetation are unfavourable to close observation. It will, therefore, be necessary to remove one or more of these larvae to a bottle in which there is nothing but pure water.

The work is now rendered much easier. There is nothing to obstruct. As soon as the larvae reach the bottom they will walk round and round, giving us a good opportunity of watching them. In swimming—which may be backwards as well as forwards—the abdomen is waved from side to side. To see the backward motion one need only put a dipping-tube or a pencil, or the like, in front of the larva, so as to bar its progress. The creature will retreat a step or two, and then, with a flourish of the abdomen, dart back through the water. The larva will sometimes wave the abdomen up and down, just as one may see a Chironomus larva do when it has affixed its dwelling to the side of the glass, and this motion probably assists the process of respiration.

When the larvae have been watched under the conditions above described, I have never been able to detect independent motion of the gills. But it is better that every one should observe for himself, and draw his own conclusions from what he sees.

Now let us examine a specimen more closely with the lens, or under the dissecting microscope. The mouth parts may be broken up separately, or a little careful manipulation will enable us to see the chief features without injuring the specimen. The head is strong and massive, and the group of ocelli, or simple eyes, may be clearly made out. The antennae bear a small pencil of hairs, no doubt sensory in function, at the extremity, but careful management of the light will be required to distinguish them. The mandibles are extremely business-like instruments, and each bears two strong teeth on the inner side. Next come the maxillae, with their palps, each of which has an appendage, while each maxilla carries three strong spines. The labium, with its palps, and the mentum, with its saw-like notchings, may be plainly seen.

The three segments of the thorax offer no difficulty. The legs are worth more than a cursory examination from their apparatus of spines and double fringe of hairs. Nine body-segments succeed to the thorax, and behind these is a long tail-like organ, which some authors consider represents a tenth segment.

The gills are seven on each side, and are given off from the spaces in front of the first seven segments of the abdomen. Each gill is five-jointed—an unusual arrangement, for the gills of the larvae of Limnobia and Gyrinus are without joints. With the half-inch Leitz the branching tracheal tube in the gill may be seen, as well as the double fringe of hairs and the long hairs at the extremity. The tail-like organ, though without joints, bears a close resemblance to the gills. It has two tracheal tubes running through it, and it is fringed on both sides with hair. Indeed, Professor Miall, F.R.S., compares it to ‘two ordinary tracheal gills completely fused together.’ The first glance will convince the observer that the comparison is just.

The pupa (Fig. 87) need not detain us, for the larva undergoes its transformation in the ground, not in the water, where it could be watched. But it is interesting to notice that the legs and wings are enclosed in separate cases, and that the segments of the abdomen bear spines. These spines are extremely serviceable to the pupa when making its way out of its cell to emerge as a perfect insect, which is well known, especially to fishermen, as the Alder-fly. It may be found near streams, and rarely uses its wings.

Ptychoptera paludosa is a small Crane-fly, with an aquatic larva which will repay observation. It is one of the group generally called ‘rat-tailed maggots,’ from the peculiar character of the breathing apparatus, which consists of a retractile tube at the end of the abdomen. It is, I believe, better known to some dealers than the larva of the Drone-fly, the rat-tailed maggot of Réaumur. A few months ago I wanted some Drone-fly larvae, and asked a dealer to supply me. When the larvae arrived and were turned out for examination, they proved to be Ptychoptera larvae—which I did not want. I naturally wrote to point out the mistake; and was told, in reply, that the larvae sent were the only ‘rat-tails’ known to my correspondent.

This larva is a mud-dweller, and is best captured by scooping up surface mud near the banks of pools and ditches, just where the water shallows on to the shore. This should be washed in a small dish or saucer, so as to carry away the mud and leave the larvae wriggling on the bottom. They may be picked up with a brush and dropped into a bottle for transport home.

There is not the slightest difficulty about keeping them for observation. A bottle of the capacity of six or eight fluid ounces will make a good aquarium for a dozen or even twenty. The bottom should be covered to the depth of about an inch with mud fairly rich in organic matter. My own plan has been to use the accumulation from the bottom of a large aquarium. In this the larvae will bury the body, and feed, the tail protruding and thrust up to the surface of the water, of which there should be about two inches above the mud.

This is a liberal allowance of space. A couple of these larvae lived with me for some months in a glass capsule two inches in diameter and three-quarters of an inch in height. The mud at the bottom and the water covering it together measured about half an inch. Both pupated, and in due time from the pupa cases a perfect insect came out.

But that larvae may pupate, they require to be well fed. How shall we know when the bulk of the nourishment has been extracted from the mud? From the castings of the larvae; and these, though of a different shape, are as easy to be distinguished as the castings of the earthworm in the garden or those of the lobworm on the seashore. All the mud that passes through the bodies of the larvae is discharged in the form of tiny hard, cylindrical pellets; and when the mass consists of these pellets it should be changed, or the larvae will go short of food. They will, however, support long fasts.

From Fig. 88 we may get a good idea of the appearance of the larvae when kept in confinement. The figures are rather less than natural size, and all the attitudes were sketched from life. One is seen extended on the bottom; two are partially buried in the mud, with the breathing-tube protruding. The larva on the mud, and bent into curves, is just about to rise to the surface; others are shown in the act of rising, while one has its breathing-tube raised above the surface, and another is attached by the breathing-tube to the side of the glass vessel. The larva with the star-like process at the end of the tail is that of Odontomyia, a large bee-like fly.

A larva of good size, like that of Ptychoptera, is especially easy to examine; and by reason of its transparency the tracheal tubes may be clearly traced. The under surface of the larva should be first looked at, and its adaptation for existence in the mud of a pond-bottom will be evident. The creature is legless, but possesses three pairs of false legs armed with dark-coloured hooks, and each body-segment bears a circle of stiff hairs, which enable the larva to travel through the mud, in the same way that the earthworm moves through the soil. Moreover, the segments between these circles are pretty thickly set with hairs.

The tracheal tubes run down on each side of the body, not in a direct line, for there is a most ingenious arrangement by which contraction and expansion of the larva, and the protrusion and retraction of the tail, are provided for. One can easily discern that in most of the segments the tubes are large, and that these large portions are connected by smaller tubes, whence others are given off into the body. These connecting-tubes are loop-like when the larva is of the normal length, but are straightened out, thus adding to their length, when the larva is extended.

The opening and closing of these loops may be observed at leisure if a larva be put in a long excavated slip, with some water, and then covered with a plain glass slip. The two slips, fastened together with small elastic bands, should then be laid on the stage of the dissecting microscope for examination; or they may be held in the hands, and the movements of the larva watched with the hand lens.

In the posterior segments of the body the tracheal tubes run side by side, while in the tail itself they are, so to speak, intertwined. When the tip of the tail pierces the surface-film a fresh supply of air is taken in.

At the base of the extensile portion are two processes which diverge, one on each side, at an angle of 45 degrees. These, according to a German observer, are tracheal gills, and they are absorbed just before the larva enters the pupal condition.

Réaumur found these larvae plentifully in the Bois de Boulogne, and gives a figure[67]. He was not, however, successful in rearing the fly. Lyonnet not only took the larvae and kept them in an aquarium, but watched their change into the pupal condition, and saw them emerge as perfect insects. An abstract of this description will probably be of interest.