NSECTS are among the most interesting and available of
all living creatures for nature-study. The lives of
many of them afford more interesting stories than are
found in fairy lore; many of them show exquisite colors
and, more than all, they are small and are, therefore,
easily confined for observation.
While the young pupils should not be drilled in
insect anatomy, as if they were embryo zoologists, yet it
is necessary for the teacher, who would teach intelligently,
to know something of the life stories, habits and
structure of the common insects. Generally speaking,
all insects develop from eggs. To most of us the
word egg brings before us the picture of the egg of the hen or of some
other bird. But insect eggs are often far more beautiful than those of any
bird; they are of widely differing forms, and are often exquisitely colored
and the shells may be ornately ribbed and pitted, sometimes adorned
with spines, and are as beautiful to look at through a microscope as the
most artistic piece of mosaic.
The egg of the cotton moth, greatly enlarged.
From Manual for the Study of Insects.
The forest tent-caterpillar shedding its skin.
Photo by M. V. Slingerland.
From the eggs, larvæ (sing. larva) issue. These larvæ may be
caterpillars, or the creatures commonly called worms, or may be maggots
or grubs. The larval stage is always devoted to feeding and to growth.
It is the chief business of the larva to eat diligently and to attain maturity
as soon as possible; for often the length of the larval period depends
more upon food than upon lapse of time. All insects have their skeletons
on the outside of the body; that is, the outer covering of the body is
chitinous, and the soft and inner parts are attached to it and supported
by it. This skin is so
firm that it cannot stretch
to accommodate the increasing size of the growing
insect, thus from time
to time it is shed. But
before this is done, a new
skin is formed beneath
the old one. After the
old skin bursts open and
the insect crawls forth,
the new skin is sufficiently
soft and elastic to allow
for the increase in the
size of the insect. Soon,
the new skin becomes
hardened like the old one,
and after a time, is shed.
This shedding of the skin
is called molting. Some insects shed their skins only four or five times
during the period of attaining their growth, while other species may molt
twenty times or more.
Full-grown caterpillar of the luna moth.
Photo by M. V. Slingerland.
After the larva has attained its full growth, it changes its skin and its
form, and becomes a pupa. The pupa stage is ordinarily one of inaction,
except that very wonderful changes take place within the body itself.
Usually the pupa has no power of moving around, but in many cases it
can squirm somewhat, if disturbed. The pupa of the mosquito is active
and is an exception to the rule. The pupa is usually an oblong object
and seems to be without head, feet or wings; but if it is examined closely,
especially in the case of
butterflies and moths, the
antennæ, wings and legs
may be seen, folded down
beneath the pupa skin.
A luna cocoon cut open, showing the pupa.
Photo by M. V. Slingerland.
Many larvæ, especially
those of moths, weave
about themselves a covering
of silk which serves to
protect them from their
enemies and the weather,
during the helpless pupa
period. This silken covering
is called a cocoon.
The larvæ of butterflies
do not make a silken
cocoon, but the pupa is
suspended to some object
by a silken knob, and in
some cases by a halter of
silk, and remains entirely naked. The pupa of a butterfly is called a
chrysalis. Care should be taken to have the children use the words—pupa,
chrysalis and cocoon—understandingly.
A butterfly chrysalis.
After a period varying from days to months, depending upon the
species of insect and the climate, the pupa skin bursts open and from it
emerges the adult insect, often equipped with large and beautiful
wings and always provided with
six legs and a far more complex
structure of body than characterized
it as a larva. The insect never
grows after it reaches this adult
stage and, therefore, never molts.
Some people seem to believe that a
small fly will grow into a large fly,
and a small beetle into a large beetle; but after an insect attains its
perfect wings, it does not grow larger. Many adult insects take very
little food, although some continue to eat in order to support life. The
adult stage is ordinarily shorter than the larval stage; it seems a part of
nature's economic plan that the grown-up insects should live only long
enough to lay eggs, and thus secure the continuation of the species.
Insects having the four distinct stages in their growth, egg, larva, pupa
and adult, are said to undergo complete metamorphosis.
On the left: A young grasshopper, enlarged. The line shows its actual length.
On the right: The adult of the same grasshopper,
But not all insects pass through an inactive pupa stage. With some
insects, like the grasshoppers, the young, as soon as they are hatched,
resemble the adult forms in appearance. These insects, like the larvæ,
shed their skins to accommodate their growth, but they continue to feed
and move about actively until the final molt when the perfect insect
appears. Such insects are said to have incomplete metamorphosis,
which simply means that the form of the body of the adult insect is
not greatly different from that of the young; the dragon-flies, crickets,
grasshoppers and bugs are of this type. The young of insects with an
incomplete metamorphosis are called nymphs instead of larvæ.
Insect brownies; tree-hoppers as seen through a lens.