_REARING LEPIDOPTERA_
_Management of Ova_
In the management of insects in all stages the strictest attention must
always be paid to one general rule on which the success of the work
almost entirely depends; and that is--keep every specimen as far as
possible under the same conditions as those in which you find it in
nature.
Applying this principle to ova, we store them in airy and light places,
protected from the direct rays of the sun, and avoid handling and rough
treatment of any kind. It is also advisable in most cases to maintain a
slight amount of dampness corresponding with that of the open air at the
particular season of the year.
They do not require much space, and it is certainly desirable not to
give much, otherwise the newly hatched larvae, when their time arrives,
will actively wander all round their premises in search of food, and
give you no end of trouble in gathering them up.
Chip boxes are, as a rule, very good and very convenient receptacles for
ova. After placing the eggs in these, cover them over with very fine
muslin, held in place by elastic bands; and label each as far as you can
with the name of the species contained, and other particulars worth
remembering. The boxes may then be put in front of a window facing
north, or in any situation within or out of doors where rain and sun
cannot reach them. A greenhouse is an admirable place in which to keep
them, the natural dampness of the atmosphere being apparently a
considerable assistance to the tiny larvae just as they are striving to
escape from their shell.
Whatever place is selected, it is absolutely necessary that the ova be
carefully watched, so that each brood may be supplied with the required
food plant within a few hours of quitting the shells.
When ova are kept in a warm room, very great inconvenience and even loss
is sometimes caused by the appearance of larvae before the necessary food
plant shows its buds. Yet, on the other hand, it is sometimes a great
gain to the entomologist to get certain broods off early in the season,
providing the food is at hand; for in this way he can not only get some
of his work over during a slack season, but also, if he desires it,
secure an additional brood; that is, one brood more than the usual
number. Thus, supposing a certain species he is rearing is naturally
double-brooded, he can, by judicious management, secure three successive
broods before the food plant casts its last leaves.
This hastening of the natural events of insect life is known as
_forcing_, and merely consists in subjecting the species concerned to a
reasonable amount of artificial heat, such as that of a room in which a
fire is always kept, or of a hothouse.
It is interesting at all times to note the dates on which eggs are laid
or collected, and the times at which the young larvae appear. In addition
to this all changes that take place in the colours or forms of eggs
should be carefully observed; for such changes will assist you in
distinguishing between fertile and sterile ova, and also enable you to
judge approximately as to the date of the appearance of future broods.
_Rearing Larvae_
The main point in connection with the rearing of larvae is certainly the
selection and construction of the cages or their substitutes. For newly
hatched and all very small caterpillars a small bottle with a wide mouth
makes a very fair abode. Put a layer of sand or sifted soil in the
bottom, fix in this a small twig of the food plant or lay a few leaves
on the top, and then, after the larvae have been introduced, cover the
top with a piece of muslin, held in place by an elastic band.
The great drawback with this arrangement is the lack of any provision
for keeping the food moist and fresh, thus rendering a change necessary
at very frequent intervals; but this may be obviated by using damp sand
as a foundation for the little twig of food plant. With this
improvement, if you cover the top of the bottle with apiece of glass, a
saucer, or any impermeable substance, you may keep the twig fresh for
several days, generally until the disappearance of the last leaf calls
for a fresh supply; but it is very doubtful whether the damp atmosphere
resulting from this inclosure is not injurious to the larvae. It
certainly does not seem to have much influence on some, but the
unhealthy conditions that result must be detrimental to the inmates. It
must also be remembered that many species require a _dry_ soil in which
to burrow when about to change.
When the time comes for the change of food, great care must be taken not
to injure young and small larvae. In many cases they need never be
touched, for if a fresh twig be placed beside the stale one, they will
readily find their way to it; and to facilitate this, and also to afford
a convenient foothold to those larvae that accidentally fall from the
twig, the layer of sand at the bottom of the bottle should be covered
with moss or cocoa-nut fibre.
[Illustration: FIG. 57.--A LARVA GLASS.]
[Illustration: FIG. 58.--A LARVA GLASS.]
If you find it necessary to move the larvae yourself from the stale food,
never touch them with your fingers, but lift them gently by means of a
small camel-hair brush. Larger larvae need never be moved at all. They
will always search out fresh food for themselves, and the stale may be
removed after they have quitted it.
For rearing larger species ordinary bottles are hardly satisfactory, and
we must either use large jars or construct cages of some kind.
An ordinary bell jar such as is used for covering ferns or for aquaria
makes a very useful 'larva glass.' Place a small bottle of water at the
bottom, and then introduce sufficient dry clean sand or sifted soil to
reach up to its neck. On the top of this place a layer of moss or
cocoa-nut fibre. Next introduce the food plant, fixing it firmly in the
bottle of water, and plugging up the space between the stem and the rim
with cotton wool. This precaution is to prevent the larvae from falling
into the water as they attempt to pass up or down the stem, and the wool
also helps to keep the twig in a vertical position. The glass is now
ready for the caterpillars, but it is advisable to keep a covering of
muslin or gauze over the top in all cases even though the larvae
contained are unable to creep up the surface of glass, for the great
enemies of caterpillars--the ichneumon flies--are always on the alert,
and will often take advantage of an open window to 'sting' the larva
rearer's pets.
Another form of larva glass can easily be made out of a large glass jar
if you know how to cut off the bottom, or of a chemist's bell jar which
is open both at top and bottom. In this case the bottle of water and the
soil are arranged as before in a pan of unglazed earthenware, and then
covered over with the glass. This is shown in fig. 58, and is an
exceedingly convenient larva house, since the lifting of the glass
enables you to get at the insects without any trouble.
[Illustration: FIG. 59.--A LARVA CAGE.]
Wood larva cages are very commonly used for the larger species after
they have attained a fair size and require more food than can be stocked
in bottles and glasses. These cages have glass fronts, either sliding or
in the form of a hinged door, and sides of perforated zinc. They are
kept in stock by all dealers in entomologists' requisites, but equally
useful ones are easily constructed. If you select a box of suitable size
at the grocer's, cut out large pieces from the lid and sides with a fret
saw, and fix in the glass and zinc, you will have a cage that will
answer all purposes.
The internal arrangements consist of a shallow tray filled with soil, in
which stands the bottle of water for the food, and a layer of moss
sufficiently high to cover the bottle completely.
A series of such boxes standing on end on a shelf, or hanging on a wall,
will form a very satisfactory nursery for your pets, and will occupy but
little space.
We have already observed that some larvae burrow into soil when about to
change, while others creep to a sheltered corner, or suspend themselves
from the food plant itself. It will be seen that the larva cage just
described supplies all these demands, and care must be taken not to
disturb the occupants while they are undergoing their metamorphoses.
Those that suspend themselves on the food plant should be allowed to
remain where they have fixed themselves, and when it is necessary to
remove the stale food in order to give a fresh supply to the later
larvae, let it be fixed in an airy place where it can be watched till the
perfect insects emerge. Those which suspend themselves on the sides or
top of the cage, or spin cocoons in the corners, should never be
disturbed unless you are greatly in want of the same cage for the
accommodation of another brood; and even then it is possible that their
presence will not in any way interfere with the new species. But if
their removal becomes a necessity, let it be carried out as carefully as
possible, and not until the change to the pupal stage is known to be
complete.
The species that burrow into the soil or bury themselves in the moss
need never be disturbed till the rearing season is quite over, and then
they may be transferred to a box specially kept for the accommodation of
pupae.
There is yet another method of rearing larvae to which we must refer--a
method known as 'sleeving'--particularly useful when you happen to have
the required food plants in your own garden. The ova or larvae are placed
on the plant, the whole or part of which is then covered with a bag or
'sleeve' of gauze. The larvae thus imprisoned have the full benefit of
fresh air and light, and are also free from the attacks of ichneumon
flies. They have a fair amount of liberty, and yet cannot get beyond
your reach; also abundance of fresh food without further trouble on the
part of the rearer.
But even this arrangement is not perfection. It will not suit the night
feeders that like to hide beneath the soil during the day, and it
interferes somewhat with the burrowing tendencies of those which pupate
underground. These little difficulties, however, can be overcome by
placing the food plants in large pots or tubs of soil, and tying the
mouth of the 'sleeve' round the outside of this utensil. If this cannot
be done, those insects that pupate underground must be removed from the
plant when their restless disposition shows that the changing time has
arrived, and then be transferred to a box of soil where they can find
the seclusion they seek.
The larvae that hybernate throughout the winter are rather more
troublesome, especially those which are inclined to take a ramble on
certain mild days in search of food when none is at hand. Still there is
no reason why even a beginner should not attempt the rearing of these.
They will require food in the autumn until the cold weather sets in, and
again early in spring as soon as the new leaves appear; but this is not
of much consequence to those who reside in districts where the required
food plants abound.
Wood feeders also require some special treatment and precautions, and
the successful rearing of some is a matter of no little difficulty. A
wooden cage is, of course, quite out of the question with these, unless
you wish to test the power of their jaws. They must be kept in large
pots or jars, covered over with wire gauze or perforated zinc, and
supplied with fresh stems or logs of wood, or with moist sawdust fresh
from their favourite tree. A few of them--the 'Goat' (page 224), for
example--will eat dead and rotting wood, and may be fed on old palings
and other waste providing the right kind is selected.
The troubles and disappointments of larva rearers are numerous and
varied, and commence with the earliest moments of the young insects.
Even the hatching period sometimes proves a trial, for it occasionally
happens that the young larva has not sufficient strength to bite its way
through the shell that surrounds it, and dies with nothing but the
surface of its head exposed to view. This may be the result of keeping
the ova in too dry a spot, the shell having become too hard and horny
for the little creature's jaws.
Then the moulting seasons are always periods of trial to the larvae, and
often of loss to the rearer. Some of the hardier species may pass
through all their moults without appearing to suffer anything more than
a slight inconvenience at each, but in other cases the greater part of a
brood may fall victims to these ailments of the growing stage.
Apart from these sources of loss, however, larvae are subject to numerous
diseases, infectious and otherwise, about which we know but little. A
fever may rage in one of our cages; a fungoid growth may establish
itself on the bodies of our pets, or we may see them cut down, one by
one, through a fatal attack of diarrh[oe]a.
In many such cases we are at a loss as to what to do. Blue pills and
black draughts are not to be prescribed, and the modern practices of
surgery and inoculation have not yet been applied to insect patients
with very great success; but we must do our best to adopt hygienic
principles, paying the greatest attention to proper means of ventilation
and to a regular and wholesome dieting. In the case of diarrh[oe]a--a
very common insect malady--the best we can do is to avoid the young and
juicy leaves of the food plant, and substitute the older, and drier
foliage.
Ichneumon flies have already been mentioned as great enemies of larvae.
These flies either deposit their eggs on the skins of caterpillars, or
thrust their sharp _ovipositors_ into the creature's flesh and lay their
eggs beneath the skin. When the young ichneumons are hatched, they
immediately begin to feed on the fatty matter that is usually stored in
comparative abundance under the skin of the caterpillar, and thus they
grow at the expense of their host, within whose body they lie completely
hidden from view.
The poor caterpillar, though being eaten alive, often shows no external
signs of the mischief wrought within, and, even though its substance is
really decreased by the hungry internal parasites, yet the rapid growth
of these robbers maintains the general plumpness of a healthy larva. But
the ichneumons, having at last devoured the store of fat, and avoided
the vital organs of the caterpillar, as if with a view to preserve their
living home to the latest moment, now commence to attack the latter,
speedily reducing the vitality of their host to the lowest ebb, and
finally causing its death.
This untimely end may come before the caterpillar is full grown, or the
insect may change to the pupa before the ichneumons have done their
worst, but it rarely occurs that the unfortunate creature has sufficient
strength to carry it on to the final stage.
A large number of the collected larvae will have been 'stung,' much to
the disgust and disappointment of the rearer; and hence the advantage of
rearing your specimens from ova wherever possible, providing you keep
them so well under cover that the ichneumons cannot visit your broods.
_The Management of Pupae_
The disappointments connected with the rearing of _Lepidoptera_ are by
no means at an end when all have passed successfully into the pupal
condition, and the number of perfect insects obtained will often fall
far short of the number of pupae in your boxes; but we must now see what
can be done to minimise the death rate of the captives.
One or more suitable boxes must be prepared for the reception of the
pupae, and the following suggestion will answer all purposes:
Get a wooden box, quite rough and unplaned inside, large enough to
accommodate your pupae with ease, and not less than eight inches deep.
Make several holes in the bottom, or else knock the bottom completely
out, and nail in its place a sheet of perforated zinc. Also make a lid
consisting of gauze attached to a light wood frame.
Place a layer of clean gravel, about an inch deep, in the bottom, and
over this a few inches of sifted soil or cocoa-nut fibre.
Now take all the pupae that are 'earthed' in your cages, and arrange them
on the prepared bed; also add to them the pupae you may have dug out
during your various excursions. Cover all with a layer of the material
selected for the bed, and then add a layer of moss.
Next come the pupae that are suspended by silky fibres, or are inclosed
in cocoons. These should be fixed with pins around the sides of the box,
running the pins either through the tuft of silk at the 'tail,' or the
outer layer of the cocoon, or through the portion of the dried food
plant to which they are attached.
Here your pupae will remain till they emerge, and the box may be kept in
any airy place where it is not likely to be forgotten, for it is
essential that the perfect insects should be removed as soon as possible
after quitting their cases. It does not matter much whether the pupae be
kept in or out of doors, providing they are sheltered from rain and very
severe frosts; but of course, if the former, the imagines will emerge a
little earlier, even if the room in which your specimens are stored has
no fire.
Even when protected in boxes such as that described the pupae are subject
to enemies and dangers. The soil and moss employed may contain slugs,
mites, or other creatures which prey on insects, and the amount of
moisture present in these materials and in the atmosphere may prove too
little for some species or too much for others.
The remedy for the former evil is a simple one. Bake the soil or fibre
well before fitting up the box, and boil and afterwards dry the moss.
You may then be sure that all life previously contained is quite
destroyed.
But the degree of humidity is a point not so easily settled, and so
variable are the experiences and opinions of different entomologists
that it is difficult to advise a beginner on the subject. The fact that
some strongly advise a perennial dampness, while others recommend no
attempt at the application of water, would seem to show that there are
probably important points to be urged on both sides.
Nothing can be better than a very careful observation of pupae in their
natural conditions. When engaged in pupa digging you will observe that
the larger number are to be found on the east and north sides of trees
where the soil is protected from the heaviest rains; on the other hand a
good many are certainly found in very moist and sometimes even in wet
situations.
Particular notice should be taken of such experiences, making every
allowance for the exceptions that prove the rule, and then let the
natural conditions be maintained in your nurseries at home. To carry
this out two pupa boxes should be kept, one for those species that seem
to require dry situations, and the other for the species that apparently
do best with moist surroundings.
But when it is desired to maintain the pupae in a moist condition, great
care must be taken not to allow any accumulation of stagnant water. The
box we have described, with its bottom of perforated zinc, is well
adapted for this purpose. Let it stand on a couple of strips of wood, so
that any excess of moisture may readily drain through. The perforated
bottom will also allow of a free circulation of air, thus securing the
ventilation that is desirable in all boxes, whether wet or dry.
If you have any insects that have pupated within moist stems, they
should be kept in a moist condition till they emerge. The simplest way
of doing this is to support the stems in a layer of wetted but
well-drained silver sand.
Forcing may be resorted to when it is required to obtain the imagines
for early setting in order to get them in the cabinet before the busy
season begins. The method is simple. Place the pupa box on a shelf in a
room where a fire is kept every day. By this means you may get all your
specimens out within a few weeks, even when you start the forcing at the
beginning or middle of the winter. If, however, you require the imagines
for breeding, you must be careful that the eggs are not laid long before
the buds of the necessary food plants are due.
When you are expecting the appearance of perfect insects, the pupa
boxes should be examined every day. A morning visit to your pupae (for
most insects emerge in the morning) may reward you with the sight of a
newly emerged imago, clinging to the rough surface of the box, thus
affording you an opportunity of observing the wonderful expansion of the
wings. But the greeting is not always of such a pleasant character, for
your disappointed eyes will sometimes be cast on a host of horrid
ichneumons that have just quitted a shell from which you were expecting
a prize of some specially valued species.