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Ch. 8: Mimicry and Warning Colours in Grasshoppers

There is in La Plata a large handsome grasshopper (Zoniopoda tarsata),
the habits of which in its larva and imago stages are in strange
contrast, like those in certain lepidoptera, in which the caterpillars
form societies and act in concert. The adult has a greenish protective
colouring, brown and green banded thighs, bright red hind wings, seen
only during flight. It is solitary and excessively shy in its habits,
living always in concealment among the dense foliage near the surface of
the ground. The yonng are intensely black, like grasshoppers cut out of
jet or ebony, and gregarious in habit, living in bands of forty or fifty
to three or four hundred; and so little shy, that they may sometimes be
taken up by handfuls before they begin to scatter in alarm. Their
gregarious habits and blackness--of all hues in nature the most obvious
to the sight--would alone be enough to make them the most conspicuous of
insects; but they have still other habits which appear as if specially
designed to bring them more prominently into notice. Thus, they all keep
so close together at all times as to have their bodies actually
touching, and when travelling, move so slowly that the laziest snail
might easily overtake and pass one of their bands, and even disappear
beyond their limited horizon in a very short time.

They often select an exposed weed to feed on, clustering together on its
summit above the surrounding verdure, an exceedingly conspicuous object
to every eye in the neighbourhood. They also frequently change their
feeding-ground; at such times they deliberately cross wide roads and
other open spaces, barren of grass, where, moving so slowly that they
scarcely seem to move at all, they look at a distance like a piece of
black velvet lying on the ground. Thus in every imaginable way they
expose themselves and invite attack; yet, in spite of it all, I have
never detected birds preying on them, and I have sometimes kept one of
these black societies under observation near my house for several days,
watching them at intervals, in places where the trees overhead were the
resort of Icterine and tyrant birds, Guira cuckoos, and other species,
all great hunters after grasshoppers. A young grasshopper is, moreover,
a morsel that seldom comes amiss to any bird, whether insect or seed
eater; and, as a rule, it is extremely shy, nimble, and inconspicuous.
It seems clear that, although the young Zoniopoda does not mimic in its
form any black protected insect, it nevertheless owes its safety to its
blackness, together with the habit it possesses of exposing itself in so
open and bold a manner. Blackness is so common in large protected
insects, as, for instance, in the un-palatable leaf-cutting ants,
scorpions, mygale spiders, wasps, and other dangerous kinds, that it is
manifestly a "warning colour," the most universal and best known in
nature; and the grasshopper, I believe, furthermore mimics the fearless
demeanour of the protected or venomous species, which birds and other
insect-eaters know and respect. It might be supposed that the young
Zoniopoda is itself unpalatable; but this is scarcely probable, for when
the deceptive black mask is once dropped, the excessive shyness, love of
concealment, and protective colouring of the insect show that it is much
sought after by birds.

While setting this down as an undoubted case of "mimicry," although it
differs in some respects from all other cases I have seen reported, I
cannot help remarking that this most useful word appears to be in some
danger of losing the meaning originally attached to it in zoology. There
are now very few cases of an accidental resemblance found between two
species in nature which are not set down by someone to "mimicry," some
in which even the wildest imagination might well fail to see any
possible benefit to the supposed mimic. In cases where the outward
resemblance of some feeble animal to a widely different and
well-protected species, or to some object like a leaf or stick, and
where such resemblance is manifestly advantageous and has reacted on and
modified the life habits, it is conceivable that slight spontaneous
variations in the structure and colouring of the unprotected species
have been taken advantage of by the principle of natural selection, and
a case of "mimicry" set up, to become more and more perfect in time, as
successive casual variations in the same direction increased the
resemblance.

The stick-insect is perhaps the most perfect example where resemblance
to an inanimate object has been the result aimed at, so to speak, by
nature; the resemblance of the volucella fly to the humble-bee, on which
it is parasitical, is the most familiar example of one species growing
like another to its own advantage, since only by means of its deceptive
likeness to the humble-bee is it able to penetrate into the nest with
impunity. These two cases, with others of a similar character, were
first called cases of "mimicry" by Kirby and Spence, in their
ever-delightful _Introduction to Entomology--_an old book, but,
curiously enough in these days of popular treatises on all matters of
the kind, still the only general work on insects in the English language
which one who is not an entomologist can read with pleasure.

A second case of mimicry not yet noticed by any naturalist is seen in
another grasshopper, also common in La Plata (Rhomalea speciosa of
Thun-berg). This is an extremely elegant insect; the head and thorax
chocolate, with cream-coloured markings; the abdomen steel-blue or
purple, a colour I have not seen in any other insects of this family.
The fore wings have a protective colouring; the hind wings are bright
red. When at rest, with the red and purple tints concealed, it is only a
very pretty grasshopper, but the instant it takes wing it becomes the
fac-simile of a very common wasp of the genus Pepris. These wasps vary
greatly in size, some being as large as the hornet; they are solitary,
and feed on the honey of flowers and on fruit, and, besides being
furnished with stings like other wasps--though their sting is nok so
venomous as in other genera--they also, when angry, emit a most
abominable odour, and are thus doubly protected against their enemies.
Their excessive tameness, slow flight, and indolent motions serve to
show that they are not accustomed to be interfered with. All these
strong-smelling wasps have steel-blue or purple bodies, and bright red
wings. So exactly does the Rhomalea grasshopper mimic the Pepris when
flying, that I have been deceived scores of times. I have even seen it
on the leaves, and, after it has flown and settled once more, I have
gone to look at it again, to make sure that my eyes had not deceived me.
It is curious to see how this resemblance has reacted on and modified
the habits of the grasshopper. It is a great flyer, and far more aerial
in its habits than any other insect I am acquainted with in this family,
living always in trees, instead of on or near the surface of the ground.
It is abundant in orchards and plantations round Buenos Ayres, where its
long and peculiarly soft, breezy note may be heard all summer. If the
ancient Athenians possessed so charming an insect as this, their great
regard for the grasshopper was not strange: I only wish that the
"Athenians of South America," as my fellow-townsmen sometimes call
themselves in moments of exaltation, had a feeling of the samo kind--the
regard which does _not_ impale its object on a pin--for the pretty
light-hearted songster of their groves and gardens.

When taken in the hand, it has the habit, common to most grasshoppers,
of pouring out an inky fluid from its mouth; only the discharge is
unusually copious in this species. It has another habit in defending
itself which is very curious. When captured it instantly curls its body
round, as a wasp does to sting. The suddenness of this action has more
than once caused me to drop an insect I had taken, actually thinking for
the moment that I had taken hold of a wasp. Whether birds would be
deceived and made to drop it or not is a question it would not be easy
to settle; but the instinct certainly looks like 'one of a series of
small adaptations, all tending to make the resemblance to a wasp more
complete and effective.

W. H. Hudson