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 of _Hippoboscidæ_, 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 wilfully rid 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 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 pergandei_ to 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, but
_Apocephalus_—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 or _Pharidæ_—has it fate linked with 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 of _fiat 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 home _par excellence_ of 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.
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[Illustration: 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—the answer 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, nourishing itself 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 order _Hymenoptera_, 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 relieving it; 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 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.
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Footnote 17:
Leland O. Howard, The Insect Book.
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