ENEMIES AND PROTECTION
Butterflies have many enemies. Even the eggs of butterflies are often
discovered by tiny, four-winged parasites, which pierce the egg-shell
and deposit their own microscopic eggs inside. These eggs produce
little grubs, which devour the contents of the butterfly’s egg, so that
the latter develops into a caterpillar.
The chief enemies of the butterfly tribe, however, are the
insectivorous birds, whose summer food often consists largely of
caterpillars. Caterpillars are also attacked by wasp-like parasites
which deposit their eggs in the victim’s skin; when the eggs hatch
the parasitic grubs feed upon the flesh of their unwilling host, who
usually dies about the time the guests are ready to pupate. One often
sees dead or dying caterpillars covered with the white cocoons of these
hymenopterous parasites. Certain larger wasps, too, use caterpillars as
food for their young. These insects paralyze their prey by stinging it,
lay their eggs on the helpless body, and seal it up in a hollow reed,
or in a nest of mud. When the wasp larva hatches it finds plenty of
living food at hand.
Chrysalids also are eaten by birds and by various carnivorous insects,
and are frequently killed by parasitic wasps and flies. Many an amateur
butterfly-hunter has been puzzled to see that some of his chrysalids
produce, not beautiful butterflies, but a lot of insignificant little
wasps.
Adult butterflies are not much troubled by parasitic insects, but they
are eaten by many birds, particularly those of the fly-catcher type, by
lizards, and by the larger dragonflies. Spiders kill a few, the great
gray robber-flies carry off a butterfly now and then, and frogs and
toads take them whenever possible. They are not much attacked by the
mammals, but I have seen a chipmunk devour a large _Papilio turnus_
with every symptom of satisfaction.
Long ages of struggle with these enemies have developed certain
protective devices--not through any supernatural intervention or any
conscious activity on the part of the butterflies, but simply by
the mechanical process called natural selection. There is a certain
degree of variation among all animals, and some of these variations
are transmitted to succeeding generations. Now, if certain butterflies
happen to vary in such a way that they are protected against their
enemies, they survive at the expense of their less fortunate fellows,
and such of their offspring as inherit the protective variation also
survive, until at last, by a gradual process of elimination, the entire
species is protected.
PROTECTIVE COLORATION
One of the commonest of the protective devices is called _protective
coloration_. It may be noted that many caterpillars are green, a color
which blends well with that of the leaves upon which they feed, and
so protects them in a measure from the prying eyes of their enemies.
Such chrysalids as are attached to green leaves or twigs are often
green also, but most caterpillars leave the foliage to pupate, and the
chrysalids are neutral gray or brown so as to be inconspicuous against
a background of bark or dead wood. Many butterflies pass the winter in
the chrysalis state, and these cold weather chrysalids are never green,
but usually some dull color which harmonizes with the winter landscape.
Another thing to be noted, particularly in caterpillars, is the
operation of the so-called _law of counter-shading_, which means simply
that the part of the body which gets the most light is usually darkest
in color. Some caterpillars habitually feed with their feet downward,
and their backs are darker than their bellies; others are accustomed to
feed in the opposite position, and the shading tones are reversed. One
has only to look at a few living specimens in their native haunts to
see the value of this arrangement.
In adult butterflies it may be observed that it is the upper side of
the wings which shows the bright colors, while the lower side is much
less conspicuous. The flying butterfly is not in much danger anyway,
and a display of color can do no great damage, but it might be fatal
to the same insect at rest. At rest, however, the wings are usually
brought together vertically, so that the highly colored upper side is
quite concealed, and only the dull under surface exposed to view. In
some butterflies--_Vanessa antiopa_, for example--the lower surface so
nearly resembles the bark upon which the insect is accustomed to rest
that it can hardly be distinguished, even when one knows exactly where
to look for it.
Some of these protected butterflies illustrate a minor protective
phenomenon known as _dazzling_ or _eclipsing coloration_. Many
observers believe that the sudden change from the bright colors of the
flying butterfly to the neutral tints of the same specimen at rest is
more confusing to a pursuer than the total absence of brilliant colors.
This dazzling and eclipsing effect is particularly noticeable in
various species of _Grapta_ and _Vanessa_.
Another thing to be remarked is the fact that many butterflies
protectively colored, such as those mentioned above, usually alight
on some object similar in color to the lower side of their own wings.
These butterflies are accustomed to rest upon the trunks of trees, and
almost invariably select one with dark-colored bark, avoiding green or
light-colored trees such as birches and sycamores. It is not claimed
the individual butterfly, after examining the colors of its wings,
casts about for a perch to match, but it is quite conceivable that
those which were attracted to light-colored bark have been gradually
weeded out of the species.
OFFENSIVE ODORS AND TASTES
Besides the method of protective coloration, some caterpillars are
protected against birds by sharp spines or hairs; others by peculiar
markings and attitudes said to approximate the appearance of serpents
or other dangerous objects. Many butterflies, it has been observed,
are protected by still other methods. The common Monarch (_Anosia
plexippus_), the Zebra butterfly (_Heliconius charitonius_), and the
Red Silverwing (_Dione vanillae_) are very conspicuous butterflies, yet
they flutter leisurely about unmolested by birds and other enemies.
This protection is due to what Alfred Russell Wallace called “a strong,
pungent, semi-aromatic odor, which seems to pervade all the juices of
their system.”
It has long been known that certain butterflies produced disagreeable
odors, but Fritz Müller, working in Brazil as early as 1876, was the
first to give the matter any very serious attention. Since Müller’s
time extensive studies have been made by Colonel Longstaff and Dr.
Dixey, two English entomologists, but very little has been done
in North America. It is certainly true, however, that the three
butterflies mentioned above (and doubtless many others) are possessed
of some taste or odor offensive to their enemies.
_Dione vanillae_ is more abundant in South America than in the United
States, and was one of the species investigated by Müller, who writes:
“The males ... when seized, open wide the anal valvulae, from the inner
side of which there appear two glands yielding a strong and nauseous
smell. The females, on the contrary, emit a similar smell from a
yellow gland extruded on the dorsum between the last and penultimate
segments.” Longstaff, who studied this butterfly in Jamaica, describes
it as “a beautiful but ill-smelling Fritillary” and says it smells like
cow-dung.
WARNING COLORATION
It has been noticed that many dangerous and distasteful insects are
rendered conspicuous by their brilliant colors, and examples of this
so-called _warning coloration_ are not lacking among the butterflies.
The Swallowtail caterpillars, which produce a very disagreeable odor,
are usually marked by two great staring eye-spots on the back of the
thorax. Some of the protected butterflies, such as the ill-smelling
Zebra and Red Silverwing, are extraordinarily conspicuous by reason
of striking color-combinations. Many entomologists believe that these
peculiar color-schemes have been developed by protected butterflies
as an advertisement of their inedible character. This view is not
as popular as it used to be, but there may be something in it; it
certainly seems to explain the structure and habits of some of the
higher animals--the rattlesnake for example--better than any other
hypothesis yet advanced.
PROTECTIVE MIMICRY
There seems to be a tendency among certain insects which are edible
and unprotected, to bear a superficial resemblance to inedible or
distasteful species. Thus certain harmless flies have developed a
remarkable likeness to wasps and bumblebees, although in structure and
habits they are really very different. Several diurnal moths, too,
have transparent wings, and yellow bands about the body which give
them the appearance of gigantic and singularly ferocious hornets. It
is certainly a great advantage for a harmless insect to resemble some
dangerous and inedible species, and the whole phenomenon of resemblance
has been called _protective mimicry_.
The best example of mimicry in American butterflies is the case of the
Monarch (_Anosia plexippus_) and the Viceroy (_Basilarchia disippus_).
The Monarch belongs to the subfamily _Euploeinae_, all the members of
which are provided with secretions which render them distasteful to
birds and predaceous insects. The Monarch advertises its inedibility by
its bright brown and black wings, and its leisurely manner of flight.
The Viceroy belongs to an altogether different group, the members of
which are readily eaten by birds, and which do not resemble the Monarch
either in form, coloration, or manner of flight. The Viceroy, however,
has gradually developed so remarkable a resemblance to the Monarch that
it is difficult to distinguish them at a little distance, although the
Viceroy is much the smaller of the two, and has a traverse black band
on the hind wing that is lacking in the Monarch. There is no doubt that
the Viceroy benefits by this resemblance, as birds (doubtless mistaking
it for the inedible Monarch) appear to give it a wide berth.
This novel situation is supposed to have come about as follows: In
the remote past the Viceroy was a blue-and-black butterfly like its
relatives, but because of the abiding principle of variation the
individuals of the species were not exactly alike--some specimens were
lighter than others. For some reason or other these lighter-colored
butterflies had a slight advantage in the struggle for existence, and
so in time the entire species was of this type, the darker specimens
having been exterminated. Finally, some individuals chanced to bear
a slight resemblance to distasteful butterflies of the Monarch type,
so that birds avoided them, but continued to feed upon their less
fortunate relatives. Thus, through a long process of natural selection,
the Viceroy has come to resemble the Monarch.
HELIOTROPISM AND LIST
It was long ago observed that plants respond definitely and
mechanically to the direction of rays of light; the leaves and flowers
of many plants always turn toward the sun and even follow its daily
course, so that the flower which turned to the East in the morning
faces due West at sunset. Similar phenomena are now known to occur
in animals. Among butterflies a good example is that of the Mourning
Cloak (_Vanessa antiopa_). This butterfly, when it alights in the
sunlight, almost invariably turns about until its body lies parallel
with the rays of light and its head points directly away from the sun.
This phenomenon is known as _negative heliotropism_, and a number of
plausible explanations of it have been advanced. Probably it is merely
another method of blending with the background, akin to protective
coloration. When the Mourning Cloak alights upon the ground, or upon a
log, it closes its wings and becomes well-nigh indistinguishable from
its surroundings, because the under side of the wings is protectively
colored. Now, if the body lies at right angles to the sun’s rays, the
wings cast a large shadow, much more conspicuous than the butterfly
itself, and hence attracting unfriendly eyes to the butterfly. If
however the body is in line with the rays of light, and the wings
brought together vertically, the shadow is insignificant. One has
only to pin a few dead butterflies on a smooth neutral background in
full sunlight to see the force of this theory. It is very probable,
then, that _negative heliotropism_ is simply a method of reducing the
too-conspicuous shadow to its lowest terms.
Other butterflies reduce the shadow by what is known as _list_, leaning
far over to one side like a sail-boat in a storm--hence the name.
The best American example of _list_ is found in the behavior of the
Wood-nymph butterfly (_Satyrus alope_), which is often seen to topple
over to one side, presenting the entire wing surface to the source of
light, lowering the top-line and thus reducing the shadow.
FEIGNING DEATH
Many insects, particularly beetles, frequently escape their enemies
by feigning death. Whether this behavior is an instinctive ruse or
a genuine paralysis induced by something akin to fright we do not
know, but it is doubtless of considerable value to the insect. Some
butterflies have been known to play possum when in a tight place.
_Vanessa cardui_, known as the Painted Lady, sometimes closes its
wings, folds its legs close to the body, and falls motionless to the
ground. Usually it is lost in the weeds or grasses, but even if found
and picked up it often allows itself to be handled without any sign
of life. Similar behavior has been reported in _Satyrus alope_, the
Wood-nymph butterfly.
Transcriber’s Notes
In the Table of Contents, the page number for Chapter VI. has been
corrected from 34 to 38.
Page 11: Changed “Fritz Muller” to “Fritz Müller”.
Page 16: In the illustration caption for Fig. I, “9. rectum, 10” was
replaced with “9. rectum; 10”.
Page 17: Changed “slender Malphighian tubules” to “slender Malpighian
tubules”.
Page 24: Changed “a reddish-brown color.” to “a reddish brown color.”.
Page 25: Changed “far from the eggshell” to “far from the egg-shell”.
Page 28: Added a missing “ before “6:00 p. m.”.
Page 34 & 38: Changed “Hesperidae” to “Hesperiidae”.
Page 48: Changed “ajax). a typical representative” to “ajax), a typical
representative”. Changed “A egg” to “A, egg”.
Page 58: Changed “green or light colored trees” to “green or
light-colored trees”.
Page 64: Changed “Wood-nymph butterfly,” to “Wood-nymph butterfly.”