The silkworm eggs came in a very small box. They were sent by a friend
of Uncle George’s who lived in the south of England. The eggs were stuck
to pieces of paper in scattered groups. Some pieces of paper had only
one or two eggs on them, others had as many as forty. They were small,
round things, flattened on the top, and coloured light green. They
reminded the boys of small mustard seeds, but they were not so round.
In answer to Frank’s question as to why the eggs were laid in irregular
groups, Uncle George answered:
“When the silk-moth emerges from her pupa case, she lays her eggs
anywhere on the floor of her cage. As the eggs are coated with a gluey
material when newly laid, they stick firmly to the floor.
“Those people who rear silkworms always have a sheet of clean paper
covering the floor of the cage before egg-laying commences. After the
eggs are laid, they are found adhering to this sheet of paper. The paper
is taken out of the larva cage, and those parts containing eggs are cut
out with scissors.”
“How many eggs are laid by one moth,” Frank asked.
“From four to five hundred,” his uncle replied. “Let us get our larva
cage cleaned out, and we will put these eggs in the lower part of it and
await their hatching.”
“Why in the _lower_ part,” Frank asked, as he set about cleaning the
cage.
“Because its floor is a drawer which can be pulled out. If you put them
in the upper part, you must lay a sheet of paper on the floor first, for
your silkworms must not be handled on any account.”
“And how long do we have to wait for these eggs to hatch?” asked Tom.
[Illustration: Larva Case.]
“That depends altogether upon the amount of warmth they receive, Tom.
They will remain unhatched ever so long if kept in a cool place; but
whenever they are placed where they can get the necessary amount of
warmth, they begin to hatch. Let us put them in the front window, where
they can get most sunshine.”
About a week after the larva cage was placed in the window, Frank
reported to his uncle that some of the eggs had turned white.
“Take your lens and examine carefully those eggs which have lost their
colour,” said Uncle George.
Lifting out one of the small pieces of paper with eggs on it, Frank
examined it with the glass.
“Oh,” he said, “these colourless eggs are just empty shells, and there
are two small creatures crawling along the edge of the paper. How small
they are! They are no longer than the breadth of a pin-head. They are
almost black in colour, covered all over with hair, and have very large
heads.”
“Well, these are newly-hatched silkworms,” said Uncle George, “and they
are already looking for food. Run out to the garden and bring in some
fresh lettuce leaves.”
Uncle George carefully dried the lettuce leaves before putting them into
the cage. In two days all the eggs were hatched, and in their place were
as many black, hairy, crawling little creatures with clumsy heads.
Although they did not seem to eat much, they grew very rapidly. When
three days old, they were fully twice as big as when hatched.
In five days the silkworms seemed to have increased in number as well as
in size. In fact, there seemed to be twice as many grubs as there were
of eggs.
Then Uncle George explained that half of these were not grubs, but
simply empty skins.
“The silkworm, in order to grow, must cast its skin,” he said. “It does
this four times during its lifetime. Look out for another _moult_, or
skin-casting, in four days’ time.”
Four days later the grubs cast their skins again, just as Uncle George
had said. They were now about half an inch in length, and much lighter
in colour.
In six further days they moulted a third time. Instead of dingy, hairy
grubs, they were now hairless caterpillars, yellowish-white in colour,
and with round black heads. Their fourth and last moult took place in
another week. By this time they were over an inch in length. They now
began to eat the lettuce leaves ravenously, and grew very fast. For ten
days they ate much and grew rapidly. Then they seemed to get tired of
feeding, and began to crawl lazily up the sides of their cage.
[Illustration: Silk Moth.]
1. Eggs.
2. Larva.
3. Silk Cocoon.
4. Chrysalis.
“They are full grown now, boys,” said Uncle George, “and will soon begin
to spin their silken cocoons.
“They are now about three inches long, and very different from the tiny
hairy grubs which came out of the eggs over a month ago.”
“We have been comparing drawings of them with drawings we made of the
caterpillar of the white cabbage butterfly,” said Frank, “and we find
that they are very much alike.”
“That is good,” said Uncle George. “They are, indeed, very much alike.
The body is made up of thirteen segments, and they have the same number
of legs. They differ from the cabbage caterpillar, however, in size and
colour. Notice the little horn which sticks up out of the twelfth
segment. Your cabbage caterpillar did not have that. If you look closely
you will notice a long silk thread issuing from the head of one of these
silk-spinning caterpillars. This one has just begun to make his cocoon;
and, in order to prevent his threads becoming entangled with those of
the other caterpillars, I will place this small paper cone over him.”
Next day Uncle George covered several of the caterpillars with paper
cones. On the roof of the cage the boys saw one caterpillar surrounded
with a flimsy web of silk of a golden-yellow colour. As they watched the
creature turning its head round and round inside the silky veil, their
uncle told them that the creature was spinning silk at the rate of about
a foot per minute.
In three days each caterpillar had surrounded itself with a dense silken
cocoon as large as a walnut. Then Uncle George took one of the cocoons,
and, cutting it open, showed the boys the brown, shining chrysalis
inside. He took another cocoon, and, having dipped it into warm water,
began to wind the silken thread on to a piece of card.
Both Frank and Tom were amazed at the length of this silken thread. They
were more surprised when they were told that each caterpillar produces a
silk thread more than half a mile long.
“Tell us, please, Uncle George,” said Frank, “why the creature spins all
this silk, and how it makes it?”
“You are asking a difficult question,” said his uncle, “but I will tell
you something about it.
[Illustration]
S. Silk Fibres.
W. Wool.
C. Cotton.
(All highly magnified.)
“The creature spins this cocoon to protect its chrysalis, of course. The
silk is produced in two long, coiled tubes, which occupy nearly the
whole length of the creature’s body.
“Silk, before it issues from the caterpillar’s body, is a fluid.
Whenever it comes out into the air, it becomes horny. Two strands are
produced at once—one from each of the long coiled tubes. Before they
leave the caterpillar’s body they are pressed together to form one flat,
ribbon-like thread. Silk is one of the strongest fibres in the world. It
is nearly as strong as steel, and much stronger than many of our metals.
“In a fortnight your silken cocoons will produce white, fluffy moths.
‘Then you can supply your friends with hundreds of ‘silkworm’ eggs, and
still have enough over to stock your larva cage.”
[Illustration: Silk Moths—Male and Female.]
Exercises on Lesson XIV.
1. Are “silkworms” really worms? What creatures do they most resemble?
2. Many of our common caterpillars spin silk of a kind. Take in
“woolly bear” caterpillars. Feed them on the plants you find
them on. Compare the cocoons they make with those of the
“silkworm.”
3. Compare egg, larva, pupa, and perfect insect of the “silkworm” with
that of the white cabbage butterfly. Does the caterpillar of
the latter spin any silk?
4. Make a list of all the spinning insects you know, and try to
explain the use of the “silk” thread in each case.
APPENDIX.
HINTS TO TEACHERS.
STRUCTURE OF STEMS.—One- and two-year-old twigs of horse-chestnut, elm,
and willow should be cut into half-inch lengths and distributed among
the class. If cut straight across with a sharp pen-knife, the different
layers can be seen with the naked eye. Afterwards these pieces should be
slit longitudinally, and the various tissues _peeled off_ by the pupils.
BULBS AND CORMS.—Bulbs of snowdrop and tulip, and corms of crocus, can
be bought from seedsmen, from September to January, at from 6d. to 1s.
per 100. At any other season of the year the onion will make a good
substitute.
WILLOW TWIGS will root readily if placed in water and kept near the hot
pipes in school.
VEGETABLE FINGERS.—To illustrate this lesson, peas (of any kind) ought
to be grown in school. Branches of the many wild peas and vetches are to
be found in abundance in every hedgerow during spring, summer, and
autumn. Branches of Virginia creeper, clematis, tropæolum (Indian cress)
may be shown with advantage. The compound leaves of the rose, ash, or
mountain ash should be shown along with those of sweet-pea and edible
pea, and the amount of green (assimilating) area compared.
THE GREAT WATER BEETLE is found in nearly every pond. One or two should
be kept and fed upon small worms. These beetles can also be obtained
from any naturalist dealer. The larvæ of this beetle is found in ponds
in summer months.
“WATER BOATMAN” BEETLES (_notonecta_ and _corrixa_) are common to all
our ponds.
PLANT ARMOUR.—Branches of whin, bramble, hawthorn, and barberry, and
leaves of holly, thistle, and nettle will be found to illustrate this
lesson. Young whin plants are to be found plentifully in spring, growing
under the old bushes. One of these should be grown in a pot in school
and kept well watered. It will produce _no_ prickles.
TROUT EGGS are obtainable from any of the fisheries—_e.g._, Howietoun
Fisheries, Stirling, or Solway Fisheries, Dumfries, at about 1s. per
100, from October till April. They are easily hatched and reared
wherever running water can be obtained. If not hatched in a bottle, as
described in Lesson XIII., the ova ought to be placed in a shallow
trough of perforated zinc suspended in a small wooden box. The inside of
the box must be charred with a red-hot iron to prevent fungal growths.
When the yolk-sac is completely absorbed, the young trout should be fed
on yolk of hard-boiled egg, and, later on, on a mixture of this and
chopped raw beef. In each case the food should be squeezed through
perforated zinc.
“SILKWORMS.”—Eggs of the silk-bearing moth (_Bombex Mori_) can be had
from Messrs Watkins & Doncaster, 36 Strand, London, and other dealers,
at 4d. and 6d. per 100. The larva should be fed on mulberry or lettuce
leaves, and the leaves should never be given _wet_.
Caterpillars of Tiger Moth (_Arctia caja_), Emperor Moth (_Saturnia
carpini_), or Puss Moth (_Cerura vinula_) should be kept for comparison.