The two silk-worm moths which we figure this month both possess a point
of excellence far in advance of any other of our native silk-worm
moths; Luna on account of its graceful form and delicate colors, and
Polyphemus for the silk of its cocoons.
It seems that most persons who speak of the Luna moth (Tropaea luna)
feel called upon to give a more or less poetic description of it. This,
I hope, has been rendered unnecessary by the colored plate, so that it
will suffice simply to mention that the beautiful shade of green is of
very rare occurrence among our larger moths, and that no other has the
long, graceful "tails" on the hind wings, a characteristic which adds
greatly to the beauty of this insect.
This moth does not seem to be very abundant anywhere, but when once
seen will long be remembered on account of its great beauty. The green
and yellow colors are evidently very closely related, because either
one may, to a greater or less degree, replace the other, so that some
of the moths have quite a strong, yellowish tinge. One of our common
swallow-tail butterflies (Iphiclides ajax) possesses a very similar
green color in its wings, but does not seem to show this tendency to
replace the green by yellow. On the wings are four eyespots which
are also found in Polyphemus. These are remarkable in that they are
transparent in the center. This clear area in Luna is quite small,
while in Polyphemus it is about as large as the entire eye spot of
Luna. The legs are brown and colored like the front edge of the fore
wings. The hairs on the body and at the base of the wing are very
long and are white or yellow. The wing expanse ranges from three and
three-fourths to five and one-half inches.
During April or May the mother moth lays her dark-brown or
chocolate-colored eggs upon hickory, walnut, beech, oak, and a few
others of our forest trees. The limited number of food plants is
doubtless one reason for the rarity of the moths, as compared with such
a common and almost omnivorous larva as Cecropia. A single moth may lay
about one hundred eggs, which are smaller than those of Polyphemus.
These hatch in about ten or fifteen days, the larva making its escape
by eating a circular hole in the shell. Occasionally a young larva may
be seen crawling about for a short time, carrying upon its head or tail
the empty shell.
The adult larva is about three inches long, of a delicate pale green, a
color very difficult to preserve in the dead larva. Those on the plate
have lost this delicate green and have become yellow, but show the form
perfectly. This larva is very much like that of Polyphemus, but may be
distinguished from it by possessing a longitudinal pale yellow lateral
line, which is not found in Polyphemus. Since the cocoon is quite thin
and contains but little silk, it is considered of but little value.
This cocoon is spun among two or three weaves, and is about two inches
long. Some authors claim that the cocoon falls to the ground with
the autumnal falling of the leaves; others that it transforms on the
ground among the fallen leaves. The cocoon is quite similar to that of
Polyphemus, but not so firmly attached when fixed to a stem. The moths
emerge in April and May, there being only a single brood in the
north, while there are two in the south.
[Illustration: LUNA MOTH.
(Tropaea luna.)
Adult Male. Pupa. Adult Female.
Larva. Cocoon.
POLYPHEMUS MOTH.
(Telea polyphemus.)
Pupa.
Adult Male. Eggs on Maple Leaf. Adult Female.
About 1/2 Life-size.
Larva. Cocoon.
COPYRIGHT 1900, BY
A. W. MUMFORD, CHICAGO.]
The color of the cocoon seems to be influenced in some way by the kind
of food eaten by the larva. Cocoons made by larva which have been fed
on hickory leaves have a darker color. In the true silk worm moth this
same influence has been noticed; larvae fed upon the vine producing
red cocoons, on lettuce emerald green cocoons, while those fed upon
white nettle produce yellow, green or violet cocoons. It is necessary
in order to procure these results, that the larvae be fed upon the
mulberry till about twenty days before the formation of the cocoon.
Polyphemus. The life history of this native silk worm (Telea
polyphemus) is by far the best known, because many years ago it was
very carefully studied with the hope that it would prove an important
silk insect. This hope unfortunately has not been realized.
The moths, as shown by the plate, are really beautiful; the large eye
spots on the hind wings contributing much towards this effect. The
transparent, window-like centers in the eye spot are also of quite rare
occurrence among our moths. These transparent areas do not possess the
very minute scales found on the other parts of the wing. Almost all
of the wonderful variety of colors found in the wings of butterflies
and moths are due either to coloring matter in these scales, or to
the breaking up of the white light by minute lines on these scales,
such as are seen in the play of colors on a soap-bubble. These
fine lines on the scales are only on the upper side, and are about
one-sixteen-thousandth of an inch apart.
The eggs of Polyphemus are very much flattened, about the size of those
of Cecropia, and are deposited on leaves and twigs singly or in small
groups. These hatch in about ten days and usually in the morning. The
young larva often devours the shell which a few moments before afforded
it shelter. This larva feeds upon oak, hickory, apple, maple, elm and a
variety of other trees, and thus has a larger range of food plants than
the Luna larva. The rate of growth is prodigious, as has been shown by
Mr. Trovelot. When the larva hatches it weighs about one-twentieth of
a grain; in ten days it weighs one-half of a grain, or ten times its
original weight; in twenty days it weighs three grains, or sixty times
its original weight; when a month old it weighs thirty-one grains, or
six hundred and twenty times its original weight, and has consumed
about ninety grains of food; after fifty days it weighs two hundred
and seven grains, or over four thousand times the original weight.
At fifty-six days the larva has eaten eighty-six thousand times its
original weight in food! It is therefore not surprising that these
larvae can often be easily detected upon trees by the large number of
leaves which they have devoured.
To provide for this great change in size, the larva moults five times,
but the time between these moults is not always the same; there is
usually about ten days between the first four moults and about twenty
between the fourth and fifth. The larva stops eating a day before the
moult, spins a few threads upon the leaf to which it attaches its hind
legs, and waits for the transformation, which usually takes place in
the afternoon. The larva, when mature and ready to spin its cocoon, is
about three inches long. It is sometimes influenced in its color by the
food plant; the normal larva being of a golden green, although it has
been known to show more yellow coloring when found on red maple.
A short time before beginning its cocoon the larva ceases to eat and
selects a place for its cocoon. These cocoons are usually found upon
the ground among the leaves, but are frequently attached to twigs.
After about a half day's work the larva spreads over the inside of the
cocoon a gummy, resinous substance, which binds together the threads.
After four or five days more of almost continuous work, another coating
is smeared over the inside, which renders the cocoon practically
air-tight. The silk fibres become considerably finer as the cocoon
nears completion and the supply of silk begins to run low. For this
reason the inner layers of the cocoon are only about half as strong as
the outer ones. The larva, as the supply of silk diminishes in the
silk glands, becomes perceptibly reduced in size. It has been estimated
that the larva, in attaching the continuous thread of its cocoon, makes
two hundred and fifty-four thousand back and forward movements. The
cocoons are very strong and dense, of a dirty white color and generally
coated with a white powder, the female being the larger.
There is but a single brood in the north, while in the south there are
two.
In order to see if the pupa needed air, Mr. Trovelot sealed up some
cocoons over winter in shellac, but the moths emerged in due time
after being in an air-tight space for nine months. He also delayed the
emergence of the moth till twenty-one months after entering the cocoon
by placing it upon ice.
The silk in the spinning glands before it is spun is a clear,
transparent fluid. These glands seem to be of excessive size when
compared with that of the larva, since, when fully expanded, they reach
the great length of twenty-five inches, or about eight times the length
of the full-grown larva. These glands are paired, one being found on
each side of the body, are considerably folded and taper at each end.
The ducts leading from the anterior end of the glands unite to form a
single duct which opens below the mouth. The thread is double, being
really composed of two different fibres, one from each gland, as may
be shown by separating them. The silk in these glands is prepared and
sold as silk "gut" to anglers. On account of its transparency when in
water, it becomes invisible and thus aids in deluding the wary fish,
who does not see any connection between the line and the baited hook.
The "gut" is prepared as follows: Larvae which are ready to spin their
cocoons are cut open and placed in strong vinegar for eighteen hours;
the glands are then taken out, stretched and dried in the shade.
Six or eight days after beginning the cocoon, the larval skin is
moulted and the real chrysalic or pupal stage begins. This stage
normally lasts till the following spring or summer. A few days before
the time of emergence a pair of glands which open into the mouth become
very active and secrete an acidulated fluid which escapes and wets
the fore end of the cocoon, causing the resinous material binding
together the fibres to become soft. Even cocoons sealed up in shellac
and starch have been dissolved by this fluid, and thus the moths have
been able to escape. When the cocoon has become sufficiently soft, the
moth pushes its way between the fibres, but in doing so often breaks
some of the threads, thus making the silk of such cocoons useless for
commercial purposes. The moth at the time of emergence, with its folded
and crumpled wings, is quite a forlorn-looking object. These wilted
wings soon begin to fill up with fluids from the body, which is very
large at this time. In some cases, the fluid is driven into the wings
with so much force that they swell up, and if such a wing is punctured,
thus allowing some of the fluid to escape, the mature wing will be of
a smaller size than one from which no fluid has been lost. It must be
remembered that it is possible to inflate a butterfly or moth's wing,
because the wings of insects are not composed of a single layer, but
are sacs of two layers which are closely applied. It is thus possible
to split the wing into upper and lower halves, but this can only be
done at the time of emergence, when these two layers are not so firmly
cemented together as they are in a few hours after emergence.
The enemies of Polyphemus are numerous. Birds prey upon the larvae, in
addition to numerous parasitic insects which are very similar to those
which destroy Cecropia. The cocoon itself is not a complete protection
because rats and squirrels plunder them. We thus see that the life of
even an insect is full of dangers, and that it is really a wonder that
so many are able to become mature and reproduce.
The silk-worm moths are excellent illustrations of what is called
complete metamorphosis in insects. An insect like the grasshopper, when
it hatches from the egg, is very much like the adult insect in its
general form and appearance; the most evident difference being the lack
of wings. An insect which shows such slight changes in its growth to
maturity is said to have an incomplete metamorphosis. It is incomplete
in the sense that the change is not of a very radical nature. But
in the case of the silk worm moths, and moths and butterflies in
general, the larva which hatches from the eggs has not even the most
superficial resemblance to the adult insect, the fully-developed moth.
This necessitates a complete change or metamorphosis in the form and
structure of the insect before it can become mature. This great change
is accomplished during the quiet pupal stage in the cocoon. Because
the pupa is apparently passive when viewed from the exterior, one must
not conclude that it is so internally; far from it; the digestive
organs of the larva must be completely made over from those of a
chewing leaf eater to those of a moth which can only take liquid food.
Charles Christopher Adams.