_PRESERVING OVA, LARVAE AND PUPAE_
Many young entomologists give their attention almost solely to the
perfect forms of insects, often collecting and studying a very large
number of species without regard to their earlier stages and
metamorphoses. This is decidedly a very great mistake. Although the
lifeless form pinned in a cabinet may be a most beautiful object in
itself, yet a study of this alone is uninteresting compared with that of
the wonderful changes it has undergone since the time it was a very
young larva.
The different stages of the insects should be known as far as possible,
and these, as well as the perfect forms, should be included in the
collection for future study and reference. A good cabinet, according to
my own opinion, is one that possesses, among other good features, a
number of complete sets illustrative of the life history of at least the
more typical forms; and as it is not a difficult matter to preserve the
earlier stages, there is really no excuse for their omission from the
collection.
The empty shells of ova are in themselves sometimes interesting objects,
especially when they illustrate some peculiar instinct on the part of
the parent. Sterile eggs, also, often fall into the hands of breeders
and rearers, and these, though in other respects unprofitable, are
useful in the cabinet.
If fertile eggs are to be prepared for a collection, they must be
killed. This is easily done by thrusting into each one the point of a
very fine needle, or by immersing them for a moment in boiling water, or
by shutting them up in a bottle with camphor. In drying they often
contract more or less, and frequently change their colour; still these
are useful, providing notes have been taken of the characters thus lost.
The larger eggs are capable of special treatment where the owner has the
necessary time and patience, and where the highest results are desired.
By means of a surgeon's injector of small size the contents of the eggs
can be removed; and then, by the same instrument, a warm solution of
gelatine, coloured in such a way as to restore the natural tint, may be
forced into the empty shell. As the gelatine cools and hardens, it
prevents any shrinking of the shell, and thus both form and colour are
well preserved.
For the preservation of larvae you will require one or two simple
appliances.
The first of these is a suitable glass blowpipe, one form of which is
here illustrated. It consists of a glass tube, one end of which has been
drawn out very fine; a piece of watch spring tied to it in such a manner
that it will hold the skin of a larva at the small end, and a piece of
india-rubber tubing at the other end, pressed by means of a brass spring
clip.
[Illustration: FIG. 64.--A BLOWPIPE FOR LARVAE.]
A little drying oven is also very useful, but not absolutely necessary.
If you decide to have one, any square box of sheet iron (not soldered
tinned iron) may be readily converted into one. It must be provided with
a hinged door in the front with a ventilator at the bottom, a hole for
the escape of hot air at the top, and a tripod wire stand inside on
which to rest the specimens while drying. The whole should be supported
on a wire stand, so that heat may be applied below.
Each larva to be preserved should be dealt with in this way. First kill
it by means of any one of the killing bottles or boxes already
described, or by immersion in spirit of wine. When quite dead, enlarge
the anal orifice by thrusting a needle into it, and then lay it on a
piece of blotting paper with its head toward you. Now take a round
ruler, previously covered with blotting paper, and roll the larva gently
from head to tail till all the contents of the skin have been expelled.
Next fix the skin on the fine end of the blowpipe, by thrusting the
point of the latter into the opening, and allowing the spring to press
gently on its edge. Gently blow into the skin till it is inflated just
to a _little below_ natural size, then either hold it near a fire or
rest it in the drying oven till it is quite dry and rigid.
If you have done your work neatly, the skin and blowpipe will both be
quite air-tight when the clip is closed; and the air, finding no outlet,
will still further inflate the skin when it expands on exposure to heat.
This is the reason why you are directed to blow it out to something
short of the natural dimensions. If you find that this expansion causes
the skin to stretch beyond its normal size, a little of the air must be
allowed to escape while it is yet soft and flexible.
The front of the larva is generally the last portion to become dry, and
when this is quite rigid the skin may be removed from the blowpipe. This
is a matter that requires the greatest care; for the skin is so very
thin and brittle that a little rough handling will break it to pieces.
As a rule it may be easily pushed off the pipe by a slight pressure
behind, or a gentle twisting motion will loosen its hold; but this
latter method can hardly be applied to hairy larvae without breaking off
the hairs, now rendered very brittle by the heat.
If you find the slightest difficulty in detaching the skin of a valuable
specimen, it is far better to damage the blowpipe than to risk spoiling
the skin. Supposing your blowpipe is a glass one, you can easily break
off the end of it after making a cut with a very small triangular file,
and the portion thus removed may be left attached to the skin. Then,
after softening the glass blowpipe in a gas flame or the flame of a
spirit lamp, it can be drawn out thin again for future work. Those who
can manipulate glass tubing in this way will find it far better to lay
in a stock of suitable material, drawing it out when required, than to
purchase blowpipes ready made at the naturalist's shop.
Very fine hollow stems, such as those of the bamboo cane, may be used
instead of glass; and these possess the advantage of being easily cut
with a sharp knife when there is any difficulty in removing the skin.
Again, whether glass or fine stems are used, a little grease of any kind
placed previously on the end will allow the dried skin to be slid off
with less difficulty.
Preserved larvae should preferably be mounted on small twigs or
artificial imitations of the leaves of the proper food plants. A little
coaguline applied to the claspers will fix them very firmly on these
twigs or leaves, which are then secured in the cabinet by means of one
or two small pins.
It is much to be regretted that the natural colours of many caterpillars
cannot be preserved in the blown skins. Some are rendered much lighter
in colour on account of the withdrawal of the contents, while others
turn dark during the drying. In the smooth-skinned species the natural
tints may be restored by painting or by staining with suitable aniline
dyes, but these artificial imitations of the natural colours are always
far less beautiful than the hues of the living larvae.
Very few words need be said on the preservation of pupae. Many of them do
not alter much in form and colour, and therefore they require no special
preparation.
If a pupa has to be killed for the purpose of adding to the value of the
collection, simply plunge it into boiling water, and it is ready to be
fixed in the cabinet as soon as it is quite dry.
The empty pupa cases, too, from which the perfect insects have emerged,
are often worth preserving, especially if the damage done by the imago
on forcing its way out is repaired with the aid of a little coaguline.
Let all larvae and pupae be preserved in their characteristic attitudes
and positions as far as possible, so that each one tells some
interesting feature of the life history of the living being it
represents. Further, enrich your collection by numerous specimens of the
various kinds of cocoons constructed by the larvae, pinning each one
beside its proper species; and never refuse a place to any object that
relates something of the life history of the creatures you are
studying.