CHAPTER V
From biped to quadruped—Flies that borrow wings—Sit-o’-my-head—A novel cradle—Flies that kill bees—Nature’s sadness—Consolations of the future—The Tachina fly and the locust.
ALTHOUGH from the way in which the story is told, one might imagine that the fly here was merely enjoying a ride upon beetle-back, yet, from the efforts made by the latter to shake off its persecutors, and, still more, because these were of the female sex, the probability is that we have here to do with a case of parasitism. The fly, we may almost feel certain, was endeavouring to lay its eggs, and the reason why she took so long about it was that she required a certain spot upon the beetle in order to do so, and that the beetle’s efforts, though appearing futile, were more or less successful in guarding this spot. At any rate, if this was not the case here, it is so in many other instances, various flies being parasitic on various other insects. Not all of these are fatal to the object of their choice, which, if it affords them board as well as lodgings, may only do so to the extent of its blood. Such are the curious family ofHippoboscidæ, or Bird Ticks, who begin life with wings, but are so little appreciative of the powers which these confer that, having found the creature upon whom they elect to live, they bite them off, or otherwise wilfullyrid themselves of them, after the manner of ants and termites, thus offering yet another example in the insect world of
“one whose hand,Like the poor Indian, flung a pearl awayRicher than all his tribe.”
“one whose hand,Like the poor Indian, flung a pearl awayRicher than all his tribe.”
“one whose hand,Like the poor Indian, flung a pearl awayRicher than all his tribe.”
“one whose hand,
Like the poor Indian, flung a pearl away
Richer than all his tribe.”
For what can be imagined more glorious to possess, speaking of physical attributes, than the power of flight?
The course of life of these flies—if all be truth that is spoken of them—is, indeed, very extraordinary, for during the first or winged stage of their adult life they live on birds, but migrate from them to some quadruped—as, say, a deer—as soon as they find themselves within easy reach of it, and then, as having reached their final place of abode, do away with their wings. Thus, being too lazy or lethargic to fly themselves, they choose rather to stand indebted to another being for a power which they no doubt once possessed in perfection, and which they are still quite capable of exercising. What the larval stage of these flies is, whether they lay their eggs upon their first or last habitation—or on both, and if not, where or in what manner the larva passes its life, I do not know, and as my authority, who should be up to date, holds his peace upon the matter, I conclude that it is not yet made out. Possibly the grub is a vegetable feeder, or possibly, again, it is as fatal to some other insect as is that of a little fly with a big name—Apocephalus pergandeito wit—to ants. The victim here chosen—if there be not others also—is a black tree-climbing ant, common in Pennsylvania. As it runs over the ground or up and down the trunks of trees,the fly darts after it on tiny wings, intent on laying her egg upon its neck. The ant tries to elude her endeavours, butApocephalus—or Sit-o’-my-head—has a mission to fulfil, and will take no denial. The egg is laid, it cannot be detached, and, when hatched, the issuing grub bores, with enthusiasm, into the head of the ant. Coming to the brain he has nothing to do but to eat it, and he does so until the whole cavity of the skull has become an empty chamber, except for his own presence there. The movements of the ant during this process—of its feelings we have no record—have become more and more erratic, and it feels itself less and less capable of performing its duties as a member of an active and industrious community. At length it falls down, and not long afterwards its head falls off, giving to the maggot inside it its first opportunity of looking out into the world through the window of the neck-hole. Hitherto its life, however easy and pleasant, has been of a sedentary nature, but now it can enjoy the pleasures of a walk, and moves about something after the manner of a snail, dragging its cephalic shell behind it. But this active state is not of long duration. The time of change is at hand, and snug within the ant’s head and its own last larval skin, which, as is the way with fly caterpillars, serves it in lieu of a cocoon, the fortunate little creature turns into a chrysalis, and dreams away its time till, on some sunny day, it issues from its cradle a happy, active fly, feeling strangely attracted by ants.
Another little fly belonging to this same family group—the hump-backed flies orPharidæ—has it fate linkedwith that of bees, in whose hive it is hatched and on whose eggs and larvæ it feeds; nor does the grown bee itself, though armed with its sting, escape from the more rapacious members of the order. These are known by the name of Robber Flies, though as the robbery involves the death of the victim, and consists of the juices of its body, murder would seem to be the better word. These flies, though of somewhat slender build, which the better fits them for their swift and darting flight, are excessively strong, as might be expected from their long muscular-looking legs and rough hairy bodies. All sorts of insects are their prey, for the despatch of which they are furnished with a hard tubular beak, enclosing, as in a sheath, a lancet-like instrument, which, being protruded at will, severely lacerates the body of the captive. The beak, or sheath, is also struck some way into the wound, and being tipped with bristles, these serve as so many barbs to keep it in position, whilst the blade continues to probe and hack the victim, on whose back the fly has descended, embracing it with its powerful legs. “These flies,” says Dr. Fitch—who seems strangely unalive to the moral beauty underlying the mere mechanical expression of it—“are inhuman murderers, they are savages of the insect world, putting their captives to death with merciless cruelty. Their large eyes, divided into such a multitude of facets, probably give them the most acute and accurate vision for espying and seizing their prey; and their long, stout legs, their bearded and bristly head, their whole aspect indicates them to be of a predatory and ferocious character. Like the hawk, they swoop upon their prey,and grasping it securely between their fore feet, they violently bear it away.”[17]Bees, beetles, butterflies, moths, even grasshoppers are thus treated, and sometimes, by a beautiful retributive arrangement—enough to throw one into ecstasies—they turn cannibals, and prey upon each other. Nay, there is even more than this to arouse our admiration, for so stern and unbending is the law of eternal justice, that even the softest feelings of nature are not allowed to interfere with it, and the female, wooed by the male, is frequently compelled to eat him. Thus the noble maxim offiat justitia ruat cælum, though, for a time, it may seem to be in abeyance, finds, at last, unconscious expression, if not in the breast, at least in the appetite of a cruel and murderous insect; and thus in the animal world, not less than in our human one, “the whirligig of time brings in his revenges.”
To bee-keepers—and to bees perhaps still more so—these terrible buccaneer flies are especially obnoxious. Poised in air, in the neighbourhood of some hive, they watch the issuing and returning stream, and, making swift choice of a victim, sweep, like the wingéd furies that they are, upon him. There is a sharp, shrilly sound, as the bee’s wings vibrate, for a moment, more rapidly, then the fatal legs wrap her round, and, pressed tightly to the oppressor’s body, she is borne to some shrub or flower, in the shade or pleasant fragrance of which the juices of her body are sucked out, through a hole specially made to allow of their passage. When nothing remains but the empty shell, the fly drops this, and returns to the scene of its labours. Through all the hot sunny hours these raids are continued, till hundreds of empty bee-shells strew the ground. As the sun declines the sport flags and gradually ceases, but it begins again the following morning as merrily as ever. America seems to be the homepar excellenceof these flies, but they are represented, under various forms, in many parts of the world. The United States has been accorded its fair share of them, and according to their numbers, each season, the labours of the bee-farmer are rewarded or otherwise. So much is this the case that the fact that “during certain seasons, in a bee-raising district of New York, not a single hive threw off a swarm”[17]has been attributed to this cause alone.
A BUCCANEER FLY, AND A LEAF-RESEMBLING INSECT.
A BUCCANEER FLY, AND A LEAF-RESEMBLING INSECT.
A BUCCANEER FLY, AND A LEAF-RESEMBLING INSECT.
Poised in the air, the buccaneer fly selects its victim from the bees issuing from a hive, pounces on it like a winged fury, and kills its hapless prey. The insect depicted beneath is protected from its enemies by its strange resemblance to a dead leaf.
It would appear from these facts either that no bee ever succeeds in stinging its assailant, or else that the latter is proof against the injection of poison. The former seems to me the most probable, since the system of the bee itself has no such immunity. It seems strange that so deadly a weapon should fail thus constantly, at a pinch, and it would be interesting to know if these redoubtable adversaries attack wasps as well as bees. As it is not stated that they do so—as wasps are pointedly omitted from the list made out of their victims—the contrary may, I think, be assumed, and also, as a corollary, that if wasps were attacked they would be able to use their sting, probably with fatal effect. This superior capability is, no doubt, owing to the superior flexibility of a wasp’s abdomen over that of a bee; and if we ask ourselves what is the cause of this—how and for what reason the superiority has been acquired—theanswer seems “as ready as a borrower’s cap,” viz. “as a means of self-defence through a process of natural selection.” Nothing could be better adapted to bring this process into play than the very ordeal through which the bee is passing; for if some could only succeed, through superior flexibility, in stinging the flies, they ought to increase at the expense of those unable to do so. As far as it goes, this seems to point to the wasp having gone through a longer course of development than the bee—to its ancestry dating farther back in time; but when we think of the latter’s more elaborate social organisation and the greater perfection of its cells, one feels inclined to reverse this opinion. As no bees possess such powers of twisting about and doubling round their abdomens as do wasps, though some can do so in a very fair degree, it seems probable that the common ancestor of all the species was more thickly built than that of the wasps, or at least that the potential capacity handed down by it of development in this direction was less. But precisely the same argument may be used in regard to the brain of the ancestral wasp, and thus we see that unless we have geological evidence on the subject it is very difficult to say which of two species has the more ancient descent.
The Robber Flies—whose scientific name I have forgotten—however disagreeable they may be, are at least not parasites. They attack their prey and kill it quickly, instead of handing it over to prolonged torture at the hands of the next generation. This last is what the Tachina flies—to say nothing of other kinds—do, who,as they principally attack caterpillars, may be considered beneficial to man. In the United States of America there is no greater destroyer of all sorts of trees than the so-called army-worm caterpillar, or rather grub—for it represents a fly merely—which gathers together in enormous numbers when about to enter the pupal state. “I have seen,” says Mr. Leland Howard, “vast armies of the army-worm, comprising, unquestionably, millions of individuals, and have been unable to find a single specimen which did not bear the characteristic eggs of a Tachina fly. These flies were present in such numbers that their buzzing as they flew over the army of caterpillars could be heard at some distance, and the farmers were unnecessarily alarmed, since they conceived the idea that the flies were the parents of the caterpillars, and were flying everywhere and laying their eggs in the grass and wheat. As a matter of fact, one great outbreak of the army-worm in northern Alabama in the early summer of 1881 was completely frustrated by the Tachina flies, aided by a few other parasites and predatory insects. They also attack grasshoppers, bugs, beetles, saw-flies and saw-fly larvæ, humble bees, and wasps. (How they avoid the sting of the latter we are not told; perhaps their insignificant size is a protection.) The eggs are stuck by some sort of gummy substance to the surface of the preyed-on insect; and the small white eggs are frequently seen sticking to the back of some unfortunate caterpillar. From the under side of each egg there hatches a little maggot, which bores its way through the skin of the host, and penetrates into its body, where it lives, nourishingitself upon the fatty matter and lymph until it reaches full growth, usually, if not always, destroying before it emerges some vital organ, so as to cause the death of the host insect. It almost invariably issues, when full grown, from the body of the insect attacked, and pupates, at or near the surface of the ground, within the last larval skin, which hardens into a brown oval puparium.”[17]There are some points of special interest about the parasitism of these Tachina flies, which seem to be directed by a less perfect instinct than that which guides other insects of similar habits; for instance, the Ichneumon flies, which, however, are such merely in name, being members of the orderHymenoptera, which includes the bees and ants.
These latter, by merely touching an insect with their antennæ, can tell if it is already occupied—in which case they withdraw—nor do they ever lay eggs in excess of the number of issuing larvæ that can be supported by the little world of provender into which they will be born. Neither do they choose a caterpillar to lay on, which is just about to cast its skin, by which manœuvre the host would escape, and the guests be left to perish. All these mistakes, however, are frequently made by the Tachina fly, the consequence being that many poor children die of starvation; whilst others, from wanting their necessary complement of food, have their growth checked and become poor pitiable objects, less than half the size that, with a more generous diet, they would certainly have attained to. It is painful to know that such privation exists and to have no means of relievingit; but nature is full of sadness, and it is best to look truth in the face. Some comfort may perhaps be derived by looking forward to a distant future, when the instinct which is now liable to these errors shall have been perfected. Such comfort, at any rate, lives in Mr. Leland Howard’s views that “the parasitic mode of life in the Tachina fly is one of comparatively recent acquirement, and that sufficient time has not elapsed since they began to take on this habit”[17]to allow of its having reached the final goal towards which it is always advancing. It is difficult, however, to console oneself for the imperfections of a work-a-day world in a far distant prospect of Elysium.
In the somewhat numerous list of insects distinguished by the attentions of the Tachina fly, grasshoppers have been mentioned. In Africa they, or, at any rate, one species of the family, attack the terrible plague locust, that has from time to time committed, and still apparently commits, such terrible devastations. The latter seems quite aware of the fate in store for it, and makes vigorous efforts to evade its destiny. Buzzing in the air, above the ravenous horde, the fly waits for one to hop or rise on the wing, and then darts swiftly upon it. To avoid her, the locust rises or sinks, tacks suddenly to right or left, scudding this way and that like a ship to meet a varying breeze. The Tachina, in the meanwhile, circles about her quarry, awaiting a favourable opportunity, which generally arises just as the locust alights, or is on the point of alighting, when, descending upon it before the lost impetus can be renewed, she clings lovingly,and deposits her eggs, either on the neck or under one of the wings,
“——and with a little pinBores through his castle wall, and farewell king.”
“——and with a little pinBores through his castle wall, and farewell king.”
“——and with a little pinBores through his castle wall, and farewell king.”
“——and with a little pin
Bores through his castle wall, and farewell king.”
It is not, however, as a rule, till after the grub or grubs have made their exit from the body that the locust dies, though it has drooped and become languid for some time. Of the vast swarms that darken the sky and descend upon the country, like a mantle, a very small proportion would seem to perish in this way, since everywhere the females may be seen drilling with their abdomens into the ground, preparatory to laying their eggs. The check upon their numbers, whatever it may be—and on the whole it must be very effective—supervenes, for the most part, at this early stage, before the egg is hatched, that is to say.