They do not require much s.p.a.ce, 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 a.s.sistance to the tiny larvae just as they are striving to escape from their sh.e.l.l.
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 sh.e.l.ls.
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 a.s.sist 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 subst.i.tutes. 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 gla.s.s, 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.
[Ill.u.s.tration: FIG. 57.--A LARVA GLa.s.s.]
[Ill.u.s.tration: FIG. 58.--A LARVA GLa.s.s.]
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 gla.s.s." 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 s.p.a.ce 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 pa.s.s up or down the stem, and the wool also helps to keep the twig in a vertical position. The gla.s.s 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 gla.s.s, 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 gla.s.s can easily be made out of a large gla.s.s 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 gla.s.s. This is shown in fig. 58, and is an exceedingly convenient larva house, since the lifting of the gla.s.s enables you to get at the insects without any trouble.
[Ill.u.s.tration: 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 gla.s.ses. These cages have gla.s.s 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 gla.s.s 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 s.p.a.ce.
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 coc.o.o.ns 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 sh.e.l.l 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 sh.e.l.l having become too hard and h.o.r.n.y 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 pa.s.s 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 subst.i.tute 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 pa.s.sed 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 coc.o.o.ns. 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 coc.o.o.n, 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.