various colors and markings, and what are the lessons which they teach
us. For this purpose, I think I cannot do better than select the larvae
of the Hawk-moths, which have just been the subject of a masterly work
by Dr. Weissmann, from which most of the following facts are taken.
The caterpillars of this group are very different in color--green,
white, yellow, brown, sometimes even gaudy, varied with spots,
patches, streaks, and lines. Now, are these differences merely casual
and accidental, or have they a meaning and a purpose? In many, perhaps
in most cases, the markings serve for the purpose of concealment.
When, indeed, we see caterpillars represented on a white sheet of
paper, or if we put them on a plain table, and focus the eye on them,
the colors and markings would seem, if possible, to render them even
more conspicuous; but amongst the intricate lines and varied colors of
foliage and flowers, and if the insect be a little out of focus, the
effect is very different.
[Illustration: FIG. 3.--THE CATERPILLAR OF THE ELEPHANT HAWK-MOTH
(_Chaerocampa elpenor_). Full grown. Natural size.]
[Illustration: FIG. 4.--THE CATERPILLAR OF THE ELEPHANT HAWK-MOTH
(_Chaerocampa elpenor_). First stage.]
Let us begin with the Elephant Hawk-moth. The caterpillars (Fig. 3),
as represented in most entomological works, are of two varieties, most
of them brown, but some green. Both have a white line on the three
first segments; two remarkable eye-like spots on the fourth and fifth,
and a very faint median line; and are rather more than four inches
long. I will direct your attention specially, for the moment, to three
points:--What do the eye-spots and the faint lateral line mean? and
why are some green and some brown, offering thus such a marked
contrast to the leaves of the small epilobe on which they feed? Other
questions will suggest themselves later. I must now call your
attention to the fact, that when the caterpillars first quit the egg,
and come into the world (Fig. 4), they are quite different in
appearance, being, like so many other small caterpillars, bright
green, and almost exactly the color of the leaves on which they feed.
That this color is not the necessary or direct consequence of the
food, we see from the case of quadrupeds, which, as I need scarcely
say, are never green. It is, however, so obviously a protection to
small caterpillars, that this explanation of their green color
suggests itself to every one.
[Illustration: FIG. 5.--THE CATERPILLAR OF THE ELEPHANT HAWK-MOTH
(_Chaerocampa elpenor_). Second Stage.]
[Illustration: FIG. 6.--THE CATERPILLAR OF THE ELEPHANT HAWK-MOTH
(_Chaerocampa elpenor_). Just before the second moult.]
[Illustration: FIG. 7.--THE CATERPILLAR OF THE ELEPHANT HAWK-MOTH
(_Chaerocampa elpenor_). Third Stage.]
After five or six days, and when they are about a quarter of an inch
in length, they go through their first moult. In their second stage
(Fig. 5), they have two white lines, stretching along the body from
the horn to the head; and after a few days (Fig. 6), but not at first,
traces of the eye-spots appear on the fourth and fifth segments, shown
by a slight wave in the upper line. After another five or six days,
and when about half an inch in length, our caterpillars moult again.
In their third stage (Fig. 7), the commencement of the eye-spots is
more marked, while, on the contrary, the lower longitudinal line has
disappeared. After another moult (Fig. 8), the eye-spots are still
more distinct, the white gradually becomes surrounded by a black line,
while in the next stage (Fig. 9) the centre becomes somewhat violet.
The white lines have almost or entirely disappeared, and in some
specimens faint diagonal lines make their appearance. Some few assume
a brownish tint, but not many. A fourth moult takes place in seven or
eight days, and when the caterpillars are about an inch and a half in
length. Now, the difference shows itself still more between the two
varieties, some remaining green, while the majority become brown. The
eye-spots are more marked, and the pupil more distinct, the diagonal
lines plainer, while the white line is only indicated on the first
three, and on the eleventh segment. The last stage (Fig. 9) has been
already described.
[Illustration: FIG. 8.--THE CATERPILLAR OF THE ELEPHANT HAWK-MOTH
(_Chaerocampa elpenor_). Fourth Stage.]
Now, the principal points to which I wish to draw attention are (1)
the green color, (2) the longitudinal lines, (3) the diagonal lines,
(4) the brown color, and (5) the eye-spots.
[Illustration: FIG. 9.--THE CATERPILLAR OF THE ELEPHANT HAWK-MOTH
(_Chaerocampa elpenor_). Fifth Stage.]
As regards the first three, however, I think I need say no more. The
value of the green color to the young larva is obvious; nor is it much
less clear that when the insect is somewhat larger, the longitudinal
lines are a great advantage, while subsequently diagonal ones become
even more important.
The next point is the color of the mature caterpillars. We have seen
that some are green, and others brown. The green ones are obviously
merely those which have retained their original color. Now for the
brown color. This probably makes the caterpillar even more conspicuous
among the green leaves than would otherwise be the case. Let us see,
then, whether the habits of the insect will throw any light upon the
riddle. What would you do if you were a big caterpillar? Why, like
most other defenceless creatures, you would feed by night, and lie
concealed by day. So do these caterpillars. When the morning light
comes, they creep down the stem of the food plant, and lie concealed
among the thick herbage, and dry sticks and leaves, near the ground;
and it is obvious that under such circumstances the brown color really
becomes a protection. It might indeed be argued that the caterpillars,
having become brown, concealed themselves on the ground; and that we
were, in fact, reversing the state of things. But this is not so;
because, while we may say, as a general rule, that (with some
exceptions due to obvious causes) large caterpillars feed by night and
lie concealed by day, it is by no means always the case that they are
brown; some of them still retaining the green color. We may then
conclude that the habit of concealing themselves by day came first,
and that the brown color is a later adaptation. It is, moreover,
interesting to note that while the caterpillars which live on low
plants often go down to the ground and turn brown, those which feed on
large trees or plants remain on the under side of the leaves, and
retain their green color.
Thus, in the Eyed Hawk-moth, which feeds on the willow and sallow; the
Poplar Hawk-moth, which feeds on the poplar; and the Lime Hawk-moth,
which frequents the lime, the caterpillars all remain green; while in
those which frequent low plants, such as the Convolvulus Hawk-moth,
which frequents the convolvulus; the Oleander Hawk-moth, which feeds
in this country on the periwinkle; and other species, most of the
caterpillars turn brown. There are, indeed, some caterpillars which
are brown, and still do not go down to the ground--as, for instance,
those of the Geometridae generally. These caterpillars, however, as
already mentioned, place themselves in peculiar attitudes, which,
combined with their brown color, make them look almost exactly like
bits of stick or dead twigs.
The last of the five points to which I called your attention was the
eye-spots. In some cases, spots may serve for concealment, by
resembling the marks on dead leaves. In one species, which feeds on
the hippophae, or sea buckthorn, a gray-green plant, the caterpillar
also is a similar gray-green, and has, when full grown, a single red
spot on each side--which, as Weissmann suggests, at first sight much
resembles in color and size one of the berries of the hippophae. This
might, at first, be supposed to constitute a danger, and therefore to
be a disadvantage; but the seeds, though present, are not ripe, and
consequently are not touched by birds. Again, in another caterpillar,
there is an eye-spot on each segment, which mimics the flower of the
plant on which it feeds. White spots, in some cases, also resemble the
spots of light which penetrate foliage. In other instances, however,
and at any rate in our Elephant Hawk-moth, the eye-spots certainly
render the insect more conspicuous.
Now in some cases, this is an advantage, rather than a drawback.
Suppose that from the nature of its food, from its being covered with
hair, or from any other cause, a small green caterpillar were very
bitter, or disagreeable or dangerous as food, still, in the number of
small green caterpillars which birds love, it would be continually
swallowed by mistake. If, on the other hand, it had a conspicuous and
peculiar color, its evil taste would serve to protect it, because the
birds would soon recognize and avoid it, as has been proved
experimentally. I have already alluded to a case of this among the
Hawk-moths, in a species which, feeding on euphorbia, with its bitter
milky juice, is very distasteful to birds, and is thus actually
protected by its bold and striking colors. The spots on our Elephant
Hawk-moth caterpillar do not admit of this explanation, because the
insect is quite good to eat--I mean, for birds. We must, therefore, if
possible, account for these spots in some other way. There can, I
think, be little doubt that Weissmann is right when he suggests that
the eye-spots actually protect the caterpillar, by frightening its
foes.
Every one must have observed that these large caterpillars--as, for
instance, that of the small Elephant Hawk-moth (Fig. 10)--have a sort
of uncanny poisonous appearance; that they suggest a small thick snake
or other evil beast, and the so-called "eyes" do much to increase the
deception. Moreover, the segment on which they are placed is swollen,
and the insect, when in danger, has the habit of retracting its head
and front segments, which gives it an additional resemblance to some
small reptile. That small birds are, as a matter of fact, afraid of
these caterpillars (which, however, I need not say, are in reality
altogether harmless), Weissmann has proved by actual experiment. He
put one of these caterpillars in a tray in which he was accustomed to
place seed for birds. Soon a little flock of sparrows and other small
birds assembled to feed as usual. One of them lit on the edge of this
tray, and was just going to hop in, when she spied the caterpillar.
Immediately she began bobbing her head up and down, but was afraid to
go nearer. Another joined her, and then another, until at last there
was a little company of ten or twelve birds, all looking on in
astonishment, but not one ventured into the tray; while one bird,
which lit in it unsuspectingly, beat a hasty retreat in evident alarm,
as soon as she perceived the caterpillar. After watching for some
time, Weissmann removed it, when the birds soon attacked the seeds.
Other caterpillars also are probably protected by their curious
resemblance to spotted snakes.
[Illustration: FIG. 10.--THE CATERPILLAR OF THE SMALL ELEPHANT
HAWK-MOTH (_Chaerocampa porcellus_).]
Moreover, we may learn another very interesting lesson from these
caterpillars. They leave the egg, as we have seen, a plain green, like
so many other caterpillars, and gradually acquire a succession of
markings, the utility of which I have just attempted to explain. The
young larva, in fact, represents an old form, and the species, in the
lapse of ages, has gone through the stage which each individual now
passes through in a few weeks. Thus, the caterpillar of _Chaerocampa
porcellus_, a species very nearly allied to the Elephant Hawk-moth,
passes through almost exactly the same stages as that species. But it
leaves the egg with a subdorsal line, which the caterpillar of the
Elephant Hawk-moth does not acquire until after its first moult. No
one can doubt, however, that there was a time when the new-born
caterpillars of the small Elephant Hawk-moth were plain green, like
those of the large one. Again, if we compare the mature caterpillars
of this group of Hawk-moths, we shall find there are some forms which
never develop eye-spots, but which, even when full grown, correspond
to the second stage of the Elephant Hawk-moth. Here, then, we seem to
have species still in the stage which the Elephant Hawk-moth must have
passed through long ago.
The genus _Deilephila_, of which we have three species--the Euphorbia
Hawk-moth, the Galium Hawk-moth, and the Rayed Hawk-moth--is also very
instructive. The caterpillar of the Euphorbia Hawk-moth begins life of
a clear green color, without a trace of the subsequent markings. After
the first moult, however, it has a number of black patches, a white
line, and a series of white dots, and has, therefore, at one bound,
acquired characters which in the Elephant Hawk-moth, as we have seen,
were only very gradually assumed. In the third stage, the line has
disappeared, leaving the white spots. In the fourth, the caterpillars
have become very variable, but are generally much darker than before,
and have a number of white dots under the spots. In the fifth stage,
there is a second row of white spots under the first. The caterpillars
not being good to eat, there is, as has been already pointed out, no
need for, or attempt at, concealment. Now if we compare the mature
caterpillars of other species of the genus, we shall find that they
represent phases in the development of the Euphorbia Hawk-moth. The
Sea Buckthorn Hawk-moth, for instance, even when full grown, is a
plain green, with only a trace of the line, and corresponds,
therefore, with a very early stage of the Euphorbia Hawk-moth; there
is another species found in South Russia, which has the line, and
represents the second stage of the Euphorbia Hawk-moth; another has
the line and the row of spots, and represents, therefore, the third
stage; lastly, there are some which have progressed further, and lost
the longitudinal line, but they never acquire the second row of spots
which characterizes the last stage of the Euphorbia Hawk-moth.
Thus, then, the individual life of certain caterpillars gives us a
clue to the history of the species in past ages.
For such inquiries as this, the larvae of Lepidoptera are particularly
suitable, because they live an exposed life; because the different
species, even of the same genus, often feed on different plants, and
are therefore exposed to different conditions; and last, not least,
because we know more about the larvae of the butterflies and moths than
about those of any other insects. The larvae of ants all live in the
dark; they are fed by the perfect ants, and being therefore all
subject to very similar conditions, are all very much alike. It would
puzzle even a good naturalist to determine the species of an ant
larva, while, as we all know, the caterpillars of butterflies and
moths are as easy to distinguish as the perfect insects; they differ
from one another as much as, sometimes more than, the butterflies and
moths themselves.
There are five principal types of coloring among caterpillars. Those
which live inside wood, or leaves, or underground, are generally of a
uniform pale hue; the small leaf-eating caterpillars are green, like
the leaves on which they feed. The other three types may, to compare
small things with great, be likened to the three types of coloring
among cats. There are the ground cats, such as the lion or puma, which
are brownish or sand color, like the open places they frequent. So
also caterpillars which conceal themselves by day at the roots of
their food-plant, tend, as we have seen, even if originally green, to
assume the color of earth. Nor must I omit to mention the
_Geometridae_, to which I have already referred, and which, from their
brown color, their peculiar attitudes, and the frequent presence of
warts or protuberances, closely mimic bits of dry stick. That the
caterpillars of these species were originally green, we may infer from
the fact that some of them at least are still of that color when first
born.
Then there are the spotted or eyed cats, such as the leopard, which
live among trees; and their peculiar coloring renders them less
conspicuous by simulating spots of light which penetrate through
foliage. So also many caterpillars are marked with spots, eyes, or
patches of color. Lastly, there are the jungle cats, of which the
tiger is the typical species, and which have stripes, rendering them
very difficult to see among the brown grass which they frequent. It
may, perhaps, be said that this comparison fails, because the stripes
of tigers are perpendicular, while those of caterpillars are either
longitudinal or oblique. This, however, so far from constituting a
real difference, confirms the explanation; because in each case the
direction of the lines follows that of the foliage. The tiger, walking
horizontally on the ground, has transverse bars; the caterpillar,
clinging to the grass in a vertical position, has longitudinal lines;
while those which live on large-veined leaves have oblique lines, like
the oblique ribs of the leaves.
Red and blue are rare colors among caterpillars. Omitting minute dots,
we have six species more or less marked with red or orange. Of these,
two are spiny, two hairy, and one protected by scent-emitting
tentacles. The orange medio-dorsal line of the Bedford Butterfly is
not very conspicuous, and has been omitted in some descriptions. Blue
is even rarer than red; in fact, none of our butterfly larvae can be
said to exhibit this color.
Now let us turn to the moths. I have taken all the larger species,
amounting to rather more than one hundred and twenty; out of which
sixty-eight are hairy or downy; and of these forty-eight are marked
with black or gray, fifteen brown or brownish, two yellowish-green,
one bluish-gray, one striped with yellow and black, and one
reddish-gray. There are two yellowish-green hairy species, which
might be regarded as exceptions: one, that of the Five-spotted
Burnet-moth, is marked with black and yellow, and the other is
variable in color, some specimens of this caterpillar being orange.
This last species is also marked with black, so that neither of these
species can be considered of the green color which serves as a
protection. Thus, among the larger caterpillars, there is not a single
hairy species of the usual green color. On the other hand, there are
fifty species with black or blackish caterpillars, and of these
forty-eight are hairy or downy.
In ten of our larger moths the caterpillars are more or less marked
with red. Of these, three are hairy, one is an internal feeder, four
have reddish lines, which probably serve for protection by simulating
lines of shadow, and one, the Euphorbia Hawk-moth, is inedible. The
last, the striped Hawk-moth, is rare, and I have never seen the
caterpillar; but to judge from figures, the reddish line and spots
would render it, not more, but less conspicuous amongst the low
herbage which it frequents.
Seven species only of the larger moths have any blue; of these, four
are hairy, the other three are Hawk-moths. In one, the Death's Head,
the violet color of the side stripes certainly renders the insect less
conspicuous among the flowers of the potato, on which it feeds. In the
Oleander Hawk-moth there are two blue patches, which, both in color
and form, curiously resemble the petals of the periwinkle, on which it
feeds. In the third species, the small Elephant Hawk-moth, the bluish
spots form the centres of the above-mentioned eye-like spots.
In one family, as already mentioned, the caterpillars are very often
brown, and closely resemble bits of stick, the similarity being much
increased by the peculiar attitudes they assume. On the other hand,
the large brown caterpillars of certain Hawk-moths are night feeders,
concealing themselves on the ground by day; and it is remarkable that
while those species, such as the Convolvulus Hawk-moth, which feed on
low plants, turn brown as they increase in age and size, others, which
frequent trees, and cannot therefore descend to the ground for
concealment, remain green throughout life. Omitting these, there are
among the larger species, seventeen which are brown, of which twelve
are hairy, and two have extensile caudal filaments. The others closely
resemble bits of stick, and place themselves in peculiar and stiff
attitudes.
And thus, summing up the caterpillars, both of butterflies and moths,
out of eighty-eight spiny and hairy species, only one is green, and
even this may not be protectively colored, since it has conspicuous
yellow warts. On the other hand, a very great majority of the black
and brown caterpillars, as well as those more or less marked with blue
and red, are either hairy or spiny, or have some special protection.
Here, then, I think we see reasons, for many at any rate, of the
variations of color and markings in caterpillars, which at first sight
seem so fantastic and inexplicable. I should, however, produce an
impression very different from that which I wish to convey, were I to
lead you to suppose that all these varieties have been explained, or
are understood. Far from it; they still offer a large field for study;
nevertheless, I venture to think the evidence now brought forward,
however imperfectly, is at least sufficient to justify the conclusion
that there is not a hair or a line, not a spot or a color, for which
there is not a reason--which has not a purpose or a meaning in the
economy of nature.
[Illustration]
PROTECTIVE RESEMBLANCES IN SPIDERS
BY ELIZABETH G. PECKHAM.[10]
[10] Abbreviated from the occasional Papers of the Natural History
Society of Wisconsin, Vol. I., 1889. By permission.
[Illustration]
There are, among spiders, two forms of protective modification: the
first, including all cases of protective resemblance to vegetable and
inorganic things--that is, all modifications of color or of color and
form that tend to make them inconspicuous in their natural
relations--I shall call direct protection. The second form, which I
shall call indirect protection, includes two classes, the spiders
which are specially protected themselves and those which mimic other
creatures which are specially protected.
Spiders are specially protected when they become inedible through the
acquisition of hard plates and sharp spines. The modification of form
is frequently accompanied by conspicuous colors, which warn their
enemies that they belong to an unpalatable class.
The second class of indirectly protected spiders--those that mimic
specially protected creatures--presents some difficulties, since it is
not always easy to determine whether the purpose of mimicry is
protection or the capture of prey. The resemblance may, as is
frequently the case in direct protection, serve both purposes.
In looking for instances of protective form and color among spiders we
encounter one difficulty at the outset. The meaning of a protective
peculiarity can be determined only when the animal is seen in its
natural home. The number of strangely modified forms depicted in
descriptive works on spiders is enormous. Bodies are twisted,
elongated, inflated, flattened, truncated, covered with tubercles or
spines, enclosed within chitinous plates, colored like bark, like
lichens, like flowers of every imaginable hue, like bird droppings,
like sand or stones, and in every one of these modifications there is
doubtless an adaptation of the spider to its surroundings which, when
it is studied out of its natural relations, we can only guess at.
It has been well said that in these protective resemblances those
features of the portrait are most attended to by nature which produce
the most effective deception when seen in nature; the faithfulness of
the resemblance being much less striking when seen in the cabinet....