Part 5

Réaumur saw large districts of grassy swamps in Poictou, which, in certain years, furnished very little grass for the cattle, on account of the ravages caused by these larvæ. They had also much injured the harvest in the same districts during those years.

These larvæ appear to require no other food than vegetable mould. Their excrements are, in fact, according to Réaumur, nothing else than dried earth, from which the stomach and intestines of the insect have withdrawn all nourishing matter.

Old trees have often hollow cavities occasioned by the decay of the trunk. When these cavities are old, their lower parts are full of a sort of mould, which is in fact half-decayed wood. It is there that the _Tipulæ_ often lay their eggs. Réaumur frequently found the larvæ in the trunks of elms or willows, and also in the fleshy parts of certain kinds of mushrooms. He carefully observed the habits of one, which lived under the covering of a mushroom, the Oak agaric (_Agaricus quercinus_). This larva is round, grey, and resembles an earth-worm. It does not walk, but crawls; and the places where it stops, or which it passes over, are covered with a sort of brilliant slime, like that left by the snail or slug.

M. Guérin-Méneville has published some very interesting remarks on the migrations of the larvæ of a particular kind of _Tipula_, known by the name of _Sciara_. We will borrow from that entomologist the following curious details, which will initiate us into one of the most wonderful phenomena in the whole history of insects. These small larvæ are without feet, hardly five lines in length, and about the third of a line in diameter. They are composed of thirteen segments, and have small black heads.

In some years, during the month of July, may be found on the borders of forests in Norway and Hanover, immense trains of these larvæ, formed by the union of an innumerable quantity fixed to each other by a sticky substance. These collections of larvæ resemble some sort of strange animal of serpent-like form, several feet long, one or two inches in thickness, and formed by the union of an immense number, which cling to each other by thousands, and move on together. The whole society advances thus with one accord, leaving a track after it on the ground, as a material indication of its presence.

These strange collections of living creatures form societies, sometimes only a few yards long; but at other times it happens that they form bands from ten to twelve yards in length, of the breadth of a hand and the thickness of a thumb. M. Guérin-Méneville observed columns as many as thirty yards in length. These troops advance as slowly as a snail, and in a certain direction. If they encounter an obstacle--as a stone, for instance--they cross over it, turn round it, or else divide into two sections, which reunite after the obstacle is passed. If a portion of the column be removed so as to divide it into two parts, it is quickly reunited, as the hindmost portion soon joins that which precedes it. Lastly, if the posterior part of this living ribbon be brought into contact with the anterior, a circle is formed, which turns round and round on the same ground for a long time, sometimes even for a whole day, before breaking, and continuing to advance. They are never met with in bad weather, but only when the sun is warm.

The curious and astonishing phenomenon of an assembly of larvæ without feet, advancing with an equal movement resulting from the individual motion of thousands of little worms, was remarked for the first time, in 1603, by Gaspard Schwenefelt. This naturalist says that the inhabitants of Siberia consider this phenomenon as an indication of a bad harvest if they go towards the mountains; whereas, if they descend towards the plains, it is the sign of a good one. In 1715 Jonas Ramus mentioned the same phenomenon, recalling a superstition attached to it by the peasants of Norway. This writer informs us that the peasants of that country, on meeting one of these moving columns, throw down their belts or waistcoats on the ground before it. If the _orme-drag_ (that is the name given to the moving column) crosses over this obstacle, it is a good sign; but on the other hand, if the column turns round the obstacle, instead of crossing it, some mischief may be expected.

The same animals were observed in 1845 at Birkenmore, near Hefeld, by M. Rande, Royal Inspector of the Forests of Hanover.

M. Guérin-Méneville is of opinion that these larvæ, which exist in great numbers in certain districts, sometimes devour all the nutritive substances contained in the ground. After having done so, they are obliged to come out of it, in order to seek at a distance a place where they will find food, or perhaps only a suitable place to undergo their metamorphosis. It is then that this singular journey commences. As regards the uniting of these myriads of individuals into columns, M. Guérin-Méneville thinks that it can be explained by the necessity these insects feel for mutual protection against the drying effect of the atmosphere when they are forced to leave the ground. United into masses, and moistened by the glutinous matter which connects them, they can leave their former place of abode without danger; if each were by itself, they would soon perish. Here, as in other cases, union is strength; and the strength of these larvæ lies in this protecting moisture. However it may be explained, the migration of these troops of insects are among the most astonishing phenomena of Nature.

BRACHYCERA.

The Brachycera, from [Greek: brachus], "short;" and [Greek: keras], "a horn"--these Diptera having short antennæ--are divided into four groups. In this subdivision the sucker is composed of six bristles. Amongst other families it includes that of the _Tabanidæ_; the insects belonging to which family are of remarkable strength, and possessed of daring and courage in the highest degree. Their wings are provided with powerful muscles, their feet are very strong, and their trunk is provided with six flat, sharp lancets. Distributed over the entire world, their instinct is everywhere the same, it is the desire for blood, at least in the females; for the males are not so warlike, they do no harm, but live on the juices of flowers. They are chiefly found in woods and pastures, and during the hottest part of the day in summer may be seen flying about seeking for their prey.

M. de Saint-Fargeau has described the manner in which the males fly. They may be seen flying hither and thither in the glades of woods, remaining for some time suspended in the air, then darting quickly and suddenly away a yard or two, again taking up the same immovable position, and in each of these movements turning the head to the opposite way from that in which they are going. This naturalist is certain that on these occasions they are watching for the females, which they dart upon. When they have succeeded in doing so, they rise so high as to be out of sight.

To this group belongs the genus _Tabanus_.

The first species we shall mention, _Tabanus autumnalis_ (Fig. 33), a common species, is eight or nine lines in length, and of blackish colour. The palpi, the face, and the forehead are grey; the antennæ black; the thorax grey, striped with brown; the abdomen spotted with yellow; the legs of a yellowish white; and the outer edge of the wings brown.

Another species (_Tabanus bovinus_) is twelve lines in length, and of a blackish brown. The palpi, the face, and the forehead are yellow; the antennæ black, with a whitish base; the thorax, covered with yellow hair, is striped with black; the posterior edge of the segments of the abdomen pale yellow; the legs yellowish, with the extremities black, and the exterior edge of the wings yellow. This species is frequently met with in woods.

A third species, _Chrysops cæcutiens_ (Fig. 34), which belongs to the same family, and of which the generic name _Chrysops_ signifies golden-eyed, torments horses and cattle very much by biting them round the eyes. Its thorax is of a yellowish colour, striped or spotted with black; the abdomen yellow, and the eyes golden.

In the next group of the Brachycera the sucker is composed of four bristles, and the antennæ generally terminate in a point which appears to be rather a development than an appendage.

This group includes a number of genera, but the following only possess sufficient interest to claim our attention. From the _Tanystomæ_ we select the families of the _Asilidæ_, _Empidæ_, and _Bombylidæ_. As types of the _Brachystomæ_ we select the _Leptidæ_ and _Syrphidæ_.

The chief characteristic of the _Asilidæ_ is strength. All their organs combine to produce this quality, which they display only too much, being as formidable to cattle as the _Tabani_, but even surpassing those insects in natural cruelty.

The _Asilidæ_ unceasingly attack other insects, and even those of their own kind. Their trunk is strong; one of the fibres of the sucker is furnished with small points, turned back, which are intended to hold firmly to the body into which it has entered. They carry on their devastations in the glades of woods and on sunny roads.

We will mention in this group _Asilus crabroniformis_ (Fig. 35), an insect ten to twelve lines long, having a yellow head, black antennæ, and thorax of a brownish yellow. The three first segments of the abdomen are black, the second and third having a white spot on each side, the remaining segments are yellow. The wings are yellowish, spotted with black on the inner and hind margin. This species is common over the whole of Europe, and lives at the expense of caterpillars and other insects, of which it sucks the blood with the greatest voracity.

The _Empidæ_ live in the same way as the _Asilidæ_, but the males are chiefly nourished by the juices of flowers.

"They wage war on other insects," says M. Macquart, in his "Histoire Naturelle des Diptères," "either when flying or running, and they seize their victims with their feet, which are formed in various ways, and well adapted for their purpose, but it is in the air that their hunting, as well as their amours, chiefly take place. They unite together in numerous companies, which during fine summer evenings whirl like gnats about the water's edge. A singular observation, however, that I have made on the _Empis_, is, that among the thousands of pairs that I have seen resting on hedges and bushes, nearly all the females were occupied in sucking an insect; some had hold of small _Phryganeæ_,[13] others of _Ephemeræ_,[14] and the greater part of _Tipulæ_."

[13] The insects produced from the caddis or case-worm.--ED.

[14] May-fly family.--ED.

The _Empidæ_ have the trunk bent down, and resembling the beak of a bird; but the _Bombylidæ_, on the contrary, have the trunk extended straight in front.

The typical genus which has given its name to this latter group is easily to be recognised by the elegance of the fur which covers its body, the slenderness of its feet, and the length of its wings, which extend horizontally on each side of the body.

Much more common in hot climates than in the North, these insects, the larvæ of which are not yet known, take flight in the middle of the day, when the sun's rays are hottest. They fly very fast, making a dull buzzing sound, and hover over flowers, from which they draw the juices without settling.

Fig. 36 represents the _Bombylius major_, which is common enough throughout the whole of Europe. This insect is from four to six lines long, black, with yellow fur; the feet light yellow; and the wings have the edges bordered with a sinuous brown band.

The genus _Anthrax_, belonging to this family, has a different form to _Bombylius_. The body is much less hairy; the trunk is short and concealed in the mouth; the wings, which are very large, are clothed, at least in the principal genus, in a garb of mourning, sufficiently remarkable, in which the combinations of black and white are admirably diversified.

"Here," says M. Macquart, "the line which separates the two colours is straight; there it represents gradations, in other cases it is deeply sinuous. Sometimes the dark part shows transparent points, or the glassy part dark spots. This sombre garb, added to the velvet black of the body, gives the Anthrax a most elegant appearance; and while resting on the corolla of the honeysuckle and hawthorn to suck the juice, forms a most striking contrast, and sets forth its beauty no less than that of those lovely flowers."

_Anthrax sinuata_ is common in Europe.

The family of the _Syrphidæ_ includes three remarkable types, which we cannot pass over in silence. They are _Vermileo_, _Volucella_, and _Helophilus_.

_Vermileo de Geeri_ (Fig. 37), which inhabits the central and southern parts of France, is four or five lines in length. Its face is white; its forehead grey, bordered with black; the thorax of a yellowish grey, with four brown stripes in the male; the abdomen light yellow, spotted with black; and the wings glassy.

The larva of the _Vermileo_ has a thin cylindrical body, capable of bending itself in every direction; a conical head, armed with two horny points; and the last segment elongated, flat, elevated, and terminated by four hairy tentacles; at the sides of the fifth segment may be observed a little angle, from which projects a horny retractile point.

It is of very singular habits. It makes a small tunnel in the sand, having a conical mouth, where it waits, like the spider, immovable. As soon as an insect falls into the hole, it raises its head, and squeezing its prey in the folds of its body, devours it, and afterwards throws out the skin. It lives in this way for at least three years before attaining the perfect state.

The _Volucellæ_ (Fig. 38) have a strong resemblance to the humble-bee. Certain kinds make use and abuse of this resemblance to introduce themselves fraudulently into its nests, and to deposit their eggs therein. When these eggs have hatched, the larvæ, which have the mouth armed with two mandibles, devour the larvæ of their hosts, the bees. This is the return they make for the hospitality they have received!

The _Helophili_ (Fig. 39) deserve to be mentioned here on account of the singular form of many of their larvæ. The head is thick, fleshy, and varying a little in form. But the point by which they are easily to be distinguished from most other larvæ is, that they have always very long tails, sometimes, indeed, out of proportion to the length of the body. Réaumur called these larvæ "vers à queue de rat;" they are known in England as rat-tailed maggots, and their habits are aquatic. Having placed some of them in a bason of water, Réaumur saw that they kept in a perpendicular position at the bottom of the bason and parallel to one another, the extremities of their tails being on the surface of the water. He then increased the depth of the water by degrees; and, as it got deeper, observed that the tail of each worm became longer. These tails, which at first were only two inches long, at last attained to five.

It will be remarked that the body of each worm does not exceed five lines in length. The tail is a peculiar organ, by the aid of which the worm breathes, although its body may be covered by water to the depth of several inches. It is composed of two tubes, one of which shuts into the other, like a telescope. Réaumur calls it the breathing tube. It terminates in a little brown knob, in which, according to Réaumur, are two holes for the purpose of receiving the air, and which have five little tufts of hair, which float on the surface of the water. When the time comes for the metamorphosis of these worms, they come out of the water and bury themselves in the earth; the skin then hardens and becomes a sort of cocoon. In this cocoon the insect loses the form of a worm, and takes by degrees that of the pupa, which it keeps until circumstances cause it to throw off its last coverings, and to appear in the winged state.

What an eventful life! what a life full of changes and turns of fortune is that of these insects, which pass the first and longest period of their existence under water, another part of their life under the ground, and, finally, after having existed in these two elements, enjoy, high in the air, the pleasures of flight!

The third group of Brachycera is that of the _Dichæta_; that is, those flies having two-fibred suckers. Among these are classed the _OEstri_, the _Conopes_, and the flies properly so called.

The genus _OEstrus_, the Gad, Bot-fly, or Breeze, comprises those formidable insects which attack the horse, the sheep, and the ox.[15] The labours of Réaumur, in his admirable Memoirs, and those of M. Joly, Professor of Zoology to the Faculté des Sciences de Toulouse, who published some most valuable researches on this subject, in 1846, will guide us in the following brief explanation.

[15] Mr. Bates, in his interesting "Naturalist on the Amazons," mentions an _OEstrus_ as occurring in those regions, which deposits its eggs in the human flesh, the larva causing a swelling which resembles a boil.--ED.

The following is the description given by M. Joly of the Gad-fly (_OEstrus equi_) represented in Figs. 41, 42, which are taken from a drawing which accompanies that naturalist's Memoirs.

The head of this insect is large and obtuse; the face light yellow, with whitish silky fur; the eyes blackish; the antennæ ferruginous; the thorax grey; and the abdomen of a reddish yellow, with black spots. The wings are whitish, not diaphanous, with a golden tint, and divided by a winding band of blackish colour. The feet are palish yellow.

This species is found in France, in Italy, and also in the East, especially in Persia, and rarely in England. During the months of July and August the _OEstrus_ frequents pastures, and deposits its eggs chiefly on the shoulders and knees of horses (PLATE I.). In order to do this, the female suspends herself in the air for some seconds over the place she has chosen, falls upon it, and with her abdomen bent, sticks her eggs to the horse's hairs by means of a glutinous liquid with which they are provided, and which soon dries. This is repeated at very short intervals. It often happens that from four to five hundred eggs are thus deposited upon the same horse. Guided by a marvellous instinct, the female _OEstrus_ generally places her eggs on those parts of the horse's body which can be most easily touched with the tongue, that is, at the inner part of the knees, on the shoulders, and rarely on the outer part of the mane.

The eggs of the _OEstrus_, which are white and of conical form, adhere to the horse's hair, as shown in Fig. 43. They are furnished with a lid, which at the time of hatching opens, to allow the exit of the young larva, which takes place, according to M. Joly, about twenty days after they are deposited. In fact, it is not in the egg state, but really in that of the larva, that the horse, as we shall explain, takes into his stomach these parasitical guests, to which Nature has allotted so singular an abode. When licking itself, the horse carries them into its mouth, and afterwards swallows them with his food, by which means they enter the stomach. It is a remarkable fact that it is sometimes other insects, as the _Tabania_ for instance, that by their repeated stinging cause the horse to lick himself, and thus to receive his most cruel enemy. In the perilous journey they have to perform from the skin of the horse to his stomach, many of the larvæ of the _OEstrus_, as may be supposed, are destroyed, ground by the teeth of the animal, or crushed by the alimentary substances. There is hardly one _OEstrus_ in fifty that arrives safely in the stomach of the horse; and yet if one were to open a horse which had been attacked by the _OEstri_, the stomach would be nearly always found to have many of the larvæ sticking to its inside. Fig. 44, taken from a drawing which accompanies M. Joly's Memoirs, represents the state of a horse's stomach attacked by the Gad-fly larvæ.

The larvæ are of a reddish yellow, and each of their segments is armed at the posterior edge with a double row of triangular spines, large and small alternately, yellow at the base, and black at the point, which is always turned backwards. The head is furnished with two hooks, which serve to fasten the larva to the internal coats of the stomach. The spines with which the whole surface of the body is furnished contribute to fix it more perfectly, preventing the creatures, by the manner in which they are placed, from being carried away by the food which has gone through the first process of digestion.

It is probable that this larva, so singularly deposited, is nourished by the mucus secreted by the mucous membrane of the stomach, and that it breathes the air which the horse swallows with its food during the process of deglutition. It must be acknowledged, however, that it is in the midst of a gaseous atmosphere which is very unhealthy, for nearly all the gases generated in the stomach of the horse are fatal to man and to the generality of animals, as they consist of nitrogen, carbonic acid, sulphuretted hydrogen, and carburetted hydrogen. To explain how the insect can live under such circumstances, M. Joly has suggested the following ingenious hypothesis:--

"When the stomach which the larva inhabits," says this learned naturalist, "contains only oxygen, or air that is nearly pure, the insect opens the two lips of the cavity which contains the spiracles, and breathes at its ease. When the digestion of the alimentary substance generates gas which is unfit for respiration, or when the spiracles run the risk of being obstructed by the solid or liquid substances contained in the stomach, it shuts the lips, and continues to live on the air contained in its numerous tracheæ."

"Whatever may be the value of this explanation," adds M. Joly, "it is nevertheless very curious to see an insect pass the greater part of its life in an atmosphere which would be instantly fatal to most animals, and in an organ where, under the government of life, chemical processes bring about the most wonderful changes of the food into the substance of the animal itself. But how can the insect itself resist the action of these mysterious powers, and remain alone intact in the midst of all these matters which are unceasingly changing and decomposing? This is another question which it is difficult, or rather impossible, to explain in the present state of science; another enigma which humbles our pride, and of which He who has created both man and the worm alone knows the secret."