Kitabı oku: «Butterflies and Moths (British)», sayfa 8

It is certainly advisable to take a friend, whether an entomologist or not, on such expeditions; and if you intend working on private grounds, always make previous arrangements with the property owner, that you may fear no foes and dread no surprises; for a sugarer is far more sure of success in his work if he keeps a cool head and has nothing to think about for the time being but his moths and his boxes.

A few hours at this interesting employment pass away very rapidly, and when midnight arrives there is often no great desire to leave off, especially when it is known that some species of moths are not very busy till very late at night. Still it is not advisable to surfeit oneself with even the sweets of life. Perhaps it is better as a rule to work the early species only on one night, and reserve another for the later ones. The searchings are then always carried on with vigour throughout, and the labours that are thus never made laborious ever retain their attractiveness in the future.

It has often been observed that, when sugaring has been carried on for a few successive nights in the same locality, the success is greater each night than on the one preceding it. Hence it is a common practice to work a chosen 'run' for two, three, or more nights in succession; and some collectors even go so far as to lay on the bait for a night or two previous to starting work. For the same reason it is often advisable to continue the use of a fairly productive beat rather than to wander in search of a new one.

In the neighbourhood of large towns one may often meet with patches of sugared bark that mark the course and extent of a brother entomologist's beat, and such are valuable to an inexperienced amateur in that they give him some idea of the nature of the localities that are chosen by more expert collectors. But it must be remembered that each entomologist has a moral right to a run he has baited, and that it is considered ungentlemanly, if not unjust, to take insects from sugar laid by another. I have sometimes seen cards, bearing the names of the collectors and the date of working, tacked on to baited trees and fences, thus establishing their temporary exclusive rights to the use of their runs. Such precautions are not necessary in large tracts of forest land, where the choice of runs is practically unlimited.

There are two other modes of capture available to the moth collector – the use of decoy females, and the employment of 'sugar traps' – and both these may be used on the sugaring run, or at other times either in the woods or in your own garden.

Fig. 55. – Cage for Decoy Females.

Fig. 56. – A Sugar Trap.

The wonderful acuteness of the sense by which the males of certain species are enabled to seek out the females has already been alluded to, and the possession of a suitable decoy will often bring you a number of beautiful admirers without the least trouble, except that taken in securing the decoy and preparing her temporary abode. It is absolutely necessary that the female moth be one that has recently emerged, and consequently you had better secure her in one of her earlier stages, either by previous rearing or by collecting the pupæ.

A little cage composed of a framework of wire covered with gauze must now be made. Perhaps the simplest pattern is that illustrated. Here the gauze is attached to two wire rings, only a few inches in diameter, and suspended by a string. Such a cage answers every purpose in the field, and has the advantage of folding into an exceedingly small space when not in use. It may be suspended in your garden or taken into the field whenever you have a suitable decoy at your disposal.

The sugar trap may be of much the same pattern as that in which a light is used, but if intended for field work it should be of a convenient size for portability. A lighter and far more convenient form may be constructed as follows:

Procure a large cylindrical tin box, and cut a circular piece of perforated zinc just small enough to drop into it. Then make two wire rings, one a little larger than the top of the tin, and the other only about an inch in diameter. Next make a conical net of leno, open at both ends, and of such a size that the two rings may form the frames of its two extremities. When the trap is required for use, cut a circular piece of flannel or other absorbent, steep it in sugar that has just been flavoured with rum, and place it in the bottom of the tin. Then place a few pebbles of equal size around the sides to support the zinc partition, drop in the partition, and then allow the net to hang on the rim as shown in the sketch.

This arrangement will explain itself. The moths, attracted by the sweet perfume, flutter about in the net till at last they find their way through the small ring. Once in, they make further attempts to reach the sugar; and, at last, finding all efforts fruitless, and, like Paddy at the fair, not being able to discover the 'entrance out,' they finally settle down in a disappointed mood awaiting your pleasure.

Perhaps another word of explanation is necessary here. Why not allow the poor creatures to reach the sugar that attracted them to the spot? The reason is this. They sometimes gorge themselves to such an extent that their bodies, dilated to the fullest capacity with syrup, are a bit troublesome when the insects are placed in the cabinet. It is therefore advisable to see that the zinc is so far above the sugar that the moths are unable to reach the latter by thrusting their extended proboscides through the perforations. A few dead leaves scattered on the zinc is also a useful addition, since it affords shelter to such of the insects as prefer it.

This is a very useful trap to keep in one's garden throughout the season. It may not attract large numbers, but it has the advantage that it requires no watching. It is simply necessary to set it at dusk, and remove the captives in the morning or at your leisure.

CHAPTER VII
COLLECTING OVA, LARVÆ, AND PUPÆ

We have already observed that insects should, as a rule, be set as soon as possible after their capture; and it would therefore seem that this is the proper place for instructions in this part of the work. But it so happens that butterflies and moths are to be obtained by means other than those already described, and we shall therefore consider these previous to the study of the various processes connected with the setting and preserving of our specimens.

Were we to confine our attention to the capture of the perfect forms only, our knowledge of the Lepidoptera would be scanty indeed, for we should then be ignorant of the earlier stages of the creatures' lives, and have no opportunity of witnessing the wonderful transformations through which they have to pass.

Such an imperfect acquaintance with butterflies and moths will, I hope, not satisfy the readers of these pages; so it is intended, in the next two chapters, to give a little assistance to those who would like to know how to set to work at the collection of their eggs and larvæ, how to search for the pupæ, and how to rear the insects from the stage at which they are acquired till they finally emerge in the perfect form.

These portions of an entomologist's work certainly take up a great deal of his time, and also require much patience and perseverance; but the advantages derived cannot be over-estimated, for in addition to the knowledge gained of the early stages of insect life, this kind of work will enable him to place in his cabinet a number of gems he would otherwise have not and probably know not. Occasionally a prize may be obtained in the form of a cluster of eggs (ova) of a rare species, in many instances the larvæ are to be obtained with comparative ease, while the perfect insects of the same species are not often seen or not easily captured, and many a rare pupa has been dug out of its hiding place during a season when the entomologist had but little other work to occupy his time.

These and other similar subjects we shall now consider in turn.

Collecting Ova

The collection of ova may be carried on more or less throughout the year. A number of moths are out in February, and even in January if the weather is mild. These soon lay their eggs, which are hatched about the time that the buds of the food plant are breaking. From this time till late in the summer the ova of various species are being deposited, the average period from laying to hatching being from two to three weeks. Then, during the autumn, when the leaves of food plants are turning brown and crisp, ova are still being laid, but these remain unchanged till the new buds of the following spring are bursting.

Of course if you intend searching for the ova of particular species you must previously ascertain the favourite haunts of those species, become acquainted with their food plants, and also with the season or seasons during which the eggs are laid. But the few following hints will suffice as general instructions for the search.

In nearly all cases we must expect to find ova on the food plants of the respective species, but at times, especially with certain moths, we may come across them in the most unlikely spots. Thus, it sometimes happens that a moth settles on a street lamp, and lays her eggs on the framework round the glass, or even on the glass itself. The same thing may take place on the sash or glass of a brightly lighted window.

Such occurrences, however, we must regard as accidental and comparatively rare, and therefore we confine our searchings for ova to the food plants of the species we require.

As a rule the under sides of the leaves will yield the most, but we have already noticed (page 18) that some moths leave their eggs exposed on the upper surface. Again, some larvæ feed on flowers and seeds and fruit, and the eggs of such are deposited on these parts. Those insects which feed on the leaves of shrubs and trees often lay their eggs on trunks, branches and twigs. Sometimes these are laid singly, sometimes in dense clusters; and it is not unusual to find them arranged in rings or spirals with great regularity. When examining the trunks of trees for ova it is necessary to look well into the crevices of the bark, for some insects take particular care to lay them in deep sheltered chinks; but others take no such precautions, and deposit them on exposed ridges or plain surfaces where they are easily discovered.

One difficulty of the ova collector lies in the fact that many insects lay on the upper branches of large trees. Of course a search for these is out of the question; but in places where the trees have been cut down a few years previously, and where a consequent undergrowth has developed, there are considerable chances of success with these species. Young saplings of trees often yield well, especially in places where tall trees of the same species are absent. It may be mentioned, too, that some moths (page 294) actually lay their eggs beneath the surface of water, depositing them on the under surfaces of floating pond weeds; and others (page 298) even enter the nests of wasps and bees for the same purpose. It is clear, then, from these few remarks, that the work of an ambitious collector of insects' eggs is by no means a monotonous task; for his employment takes him into the meadows and woods, leads him to the banks of ponds, and even compels him to tear down banks and hedges for the nests of Hymenoptera at the risk of a sting or two.

One of the most productive sources of eggs is undoubtedly the possession of captured females. When you are out netting butterflies you often see a female that is evidently engaged in her matronly duties. Instead of seeking food from the various flowers in her path, she pays attention only to the foliage, looking out a suitable leaf on which to deposit her eggs. Should you meet with an insect thus engaged which you would like to rear at home, or of which you would like to know the egg, secure it in a perforated pill box with a leaf of the proper plant; and it will often supply you with abundance of eggs for your purpose, in many cases depositing them in the box before you arrive home. The eggs of numerous species of moths are also to be easily obtained from captured females.

Some insects do not seem inclined to deposit their eggs in captivity as freely as when at large, and in order to induce them to do so we must, as far as possible, put them in their natural conditions. Let them have plenty of room, and supply them with fresh twigs of their food plants, kept green by standing them in vessels of water. It is also advisable, supposing you are not well acquainted with the dispositions of the species you have, to keep a portion of the box well shaded from direct sunlight, and allow another part to be as bright as possible; for some species will not lay in a bright light, while others will not do so without it.

Again, while some deposit their eggs within a few hours of quitting the pupa case, others do not lay for several days. With regard to the latter, it is frequently necessary to feed them while in captivity, by placing in the box a piece of rag or sponge that has been dipped in honey or syrup.

Each batch of ova should be carefully examined with a view to knowing them by sight on a future occasion. A sketch should be made in your note book, showing every detail that you can make out with the aid of a good lens. Then observations concerning the season, colour of the eggs, the situation in which deposited, arrangement, and any other useful particulars, should be entered.

In the next chapter some hints will be given concerning the management of ova and the rearing of the larvæ from the time of hatching.

Collecting Larvæ

This occupation is generally far more productive to the entomologist than searching for ova. The latter are very small, usually well concealed, and to be detected only by a careful scrutinising use of the eyes; but the superior size of the larvæ, the frequent bright colouring, and the fact that they are easily beaten from their hold, render the searchings of their hunters comparatively easy and fruitful.

Before setting out on a larva-hunting expedition, there are a few requisites to prepare. These include not only the implements for your work in the field, but also the cages in which you intend to rear your little captives. The latter are described a little later on under the head 'Rearing Lepidoptera,' and the former we will now briefly summarise.

The outfit must consist of a quantity of suitable boxes, a stout hooked stick, a strong net, and a white material to place under the herbage while you are 'beating.'

'Larva boxes' are usually made of zinc, and have little sliding doors in the lids, so that the lids need not be removed while out of doors after the fragment of the required food plant has been inserted. Such boxes are not by any means essential. Small tin boxes will answer all purposes nearly as well, providing a number of small holes be made in them for the admission of air. Chip boxes are also fairly satisfactory, but these also should be perforated. The best way to do this is to push a red-hot iron wire through the chip, making about half a dozen small holes in each box. This method will give you clean holes of a uniform size without otherwise injuring the boxes.

Metal boxes possess the advantage that they keep the food plants moist for a long time, while chip boxes allow them to dry rather rapidly. Yet there are some larvæ that do far better in the latter, since such a quantity of moisture exudes through their skins that they soon become uncomfortably wet if their apartment is not well ventilated. Under these circumstances perhaps it is better to take a supply of both, so that changes may be made as found necessary.

One grave objection to chip boxes, however, is the weakness of the material. They are easily crushed by pressure, and a bottom or a top disc of wood often falls out; but this is easily overcome by gluing narrow strips of calico round the top and bottom edges. Chip boxes should always be treated in this way, and they will then last five or six times as long.

Your supply of boxes should always include one large one of metal in which to bring home a supply of food for the larvæ. If you have a botanist's vasculum, by all means take it, for nothing can serve this purpose better. If not, any rather large square tin box will do, and this may be carried in your satchel, or a couple of hooks may be soldered to it so that a leather strap can be fixed for slinging it over your shoulder.

The net required is that commonly known as the 'sweep net.' It must be very strong, for it has to submit to rather rough usage. The frame must be made of thick wire; and the bag, which need not be more than a foot deep, should be of strong calico or holland.

Now with regard to the white material previously mentioned. This may be a square of calico, hemmed round the edges. Nothing is more convenient than this, as it occupies but little room in the pocket when not in use, if neatly folded. The material need not be thick, but the larger it is the better. Many prefer a white umbrella or an ordinary umbrella with a white lining, but as this is only a matter of taste and convenience you must decide for yourself as to which you will use.

If your field of operations is only a little way from your head quarters, and quantity of luggage therefore not a serious consideration, you may provide yourself with a heavy mallet, loaded if necessary with a pound or two of lead. This will prove very useful in shaking larvæ from trees and large branches. Lastly, take a pencil and a note book or writing paper for your observations in the field.

Now for the choice of the season. Larvæ are to be found all the year round. Early in the spring, as soon as the buds are bursting, some break out of the eggs recently laid by the moths that appear in February and March. Later on, during April and May, a host of both butterflies and moths are busy arranging for their broods. Then, throughout the whole of the summer, thousands of caterpillars of all sorts and sizes are to be met with everywhere. And finally, during the bleak winter months, you may amuse yourself by digging the hybernators out of their hiding places where they rest themselves till the spring sun again calls them out to refresh them with the young and tender leaves of a new year. Thus, unless you are merely intending to search out certain species you happen to require, there is not much difficulty in settling on the season.

The day selected should be dry, for your work lies among the herbage of banks, meadows, and woods, and nothing is more unpleasant than wading through a wet and dense vegetation, or beating down on yourself a shower of large drops from the branches of trees and shrubs.

Having reached the hunting ground, the first thing to do is to look out for signs of the presence of larvæ rather than for the larvæ themselves. Healthy vegetation with sound leaves must be passed by as untenanted; but the presence of partly eaten foliage immediately arouses suspicion.

A little experience will soon enable you to distinguish between the ravages of larvæ and of slugs, snails, wasps, &c. Some of the smaller larvæ certainly eat out clean holes like those cut by Hymenopterous insects, but as a rule they bite away at the edges, leaving the midrib and the larger veins standing out almost naked.

By looking well into the edges of the eaten leaves, it is easy to see whether the marauders have been recently at work. If they are dried up and discoloured, it is not of much use to search; but if still green and moist, you may feel almost sure that the hungry larvæ are not far off.

In this case you will carefully turn over the leaves to examine the under sides, and also the leaf stalks and branches or stems; but you must be prepared for all kinds of protective mimicry. Little green caterpillars will be seen lying on the midrib or veins, so straight and so still that they are scarcely perceptible. Others are snugly tucked in a depression of a leaf with the same result. Then we must also be prepared for the artful little tricks of the larvæ of Geometræ (p. 268), by which they imitate stalks and twigs so closely that a sharp eye is necessary to discriminate between the two.

While thus searching we may meet with the cast skin of a caterpillar. This gives us fresh hopes, and so we continue our careful examination. At last, on grasping a leaf in order to turn it over for inspection, we feel something hairy or something soft and smooth. But lo! it is gone. It is one of those numerous caterpillars that feign death and drop to the ground on the slightest sign of danger. We search below for it, but the density of the vegetation renders this hopeless, and we are just about to start off in search of a more productive locality when we espy a quantity of the excrement of larvæ lying on a little bare patch of ground close by. This gives us a new idea. Here is another indication of the presence of the creatures we require, one that we can put into practice; and by-and-by we learn that in many cases this is really the surest sign of their whereabouts.

We look at these little pellets of excrement, and gain at once some idea of the size of the larvæ that produce them. Then we observe whether they are fresh and moist, or dry and stale. If the latter, it is not of much use to examine the leaves above; but if otherwise, there is little doubt of our meeting with larvæ, as the present position they occupy is so truly marked. The leaves just over them are carefully examined, either by turning them over as before described, or, if the height of the foliage admits of it, by placing our heads below and looking upward.

If we find that the larvæ are some of those that endeavour to escape by feigning death and allowing themselves to drop at the slightest disturbance, the net is always kept beneath the leaves we are touching in order to intercept them in their downward journey.

Continuing the search, we meet with leaves that are rolled up and bound with silk threads, and others that are drawn together and similarly bound. These are carefully uncurled and pulled asunder with the result that active little larvæ are exposed to view, or, it may be, pupæ are discovered. In some cases flowers are drawn together in just the same way, and an examination reveals one or more of the species that prefer petals and other parts of flowers to the green leaves.

Silken threads always arouse our suspicions. These may be seen lying on the surfaces of leaves, and passing from one leaf to another, or they may be hanging perpendicularly from the branches of trees above. In the latter case a larva may be frequently seen on the lower extremity of the fibre, swinging gently in the breeze, and, should we require it, we have only to place the open box below for its reception.

Hawthorn and other trees are sometimes seen almost devoid of leaves, nearly every bit of green having been greedily devoured by a host of small larvæ. In such cases we often meet with dense clusters of silk fibres that may easily be mistaken for spiders' nests. But when we look more closely into the structure we observe that we have discovered instead nests of gregarious larvæ, such a large number being in each little community that the deplorable appearance of the tree is at once explained.

A little farther on we meet with a sickly-looking plant in the midst of a number of flourishing individuals of the same species, and stop to make inquiries into the cause of this strange occurrence. Is it due to a poorness of the soil? No, this cannot be the case; for intermingled with its roots are those of its flourishing companions. We pluck a stunted and half-shrivelled leaf and examine it. At first we do not notice the cause of its peculiar condition; but, holding it up to the light, and looking through it, we see a number of little galleries that have been eaten out of its internal soft substance, leaving the thin skin (epidermis) almost entirely intact. But nothing more is to be seen. Another leaf is examined in exactly the same way; and here we see the little destroyer, lying motionless in its burrow till a gentle pressure applied against it from outside causes it to wriggle along its narrow passage. This is the larva of one of the little leaf miners mentioned again on page 303.

Reaching a little marshy spot we see a number of water-loving reeds, most of them beautifully green and in a flourishing condition, but here and there in their midst is a poor stunted specimen – another result of the ravages of the larvæ of one or more moths. An examination of the blades reveals nothing; but on splitting open the stalk we discover some larvæ that have already devoured a quantity of the internal pith, and thus endangered the life of the plant. On inspecting other similar reeds we are at first puzzled as to how the larvæ could get inside the stems without damaging the outer portion; but at last we see in each one a little discoloured hole that was eaten out by the young caterpillar just after its escape from the egg. Once within the reed, it found a plentiful supply of food, and there grew at the expense of the plant without doing any further external damage save by causing a stunted growth.

It may be that the stem eaters we have found are just about full grown. If so we examine a number of the stems with a hope that we may find one or two that are just about to change to the chrysalis state, or even a pupa already formed. By this means we may secure one of the perfect insects without the necessity of feeding larvæ at home. Such a consideration becomes a most important one when it happens that the required food plant is one that cannot be easily obtained.

Close by the reeds is another water-loving plant in the form of an old willow tree. This is always an attractive object to the entomologist, so it comes in for a share of our inspection. On its leaves we may find several species of the larvæ of Lepidoptera, including those of some of our largest insects. But a strange feature catches our eyes as we happen to glance at the bark of the tree. Here we see a few holes of different sizes, about which are a number of little fragments of wood that remind us of 'sawdust;' and, examining the ground below, we see quite a little heap of this dust, looking just as if a carpenter had been at work on the spot.

This is not the effect of a saw, however; it is a sure sign of the ravages of wood-eating larvæ (p. 224), whose powerful jaws gain them admittance into the very hearts of trees, and the application of the nose to one of the larger holes leaves no doubt of the presence of the large and beautiful caterpillar of the Goat Moth (p. 224).

If we require any of these wood-eaters, either for rearing or for preservation, we must be prepared for a little rather heavy work. A strong pocket knife is not sufficient, but with a good chisel the wood can be gradually cut away, and the galleries traced, till at last we come to the larvæ snugly resting in their burrows.

It often happens that the tree thus tenanted is half decayed, and consequently the work is rendered much easier. Also, while tearing away the wood, we often meet with a number of cocoons that have been constructed by the caterpillars for their winter quarters, or as a resting place while undergoing their transformations. These are composed of the wood dust bound together by strong silk fibres, and are often in such a good state of preservation that they form useful illustrations for the cabinet.

As further aids to larvæ searching we may mention that many species – chiefly of the Noctuæ– hide under the surface of the ground or among dense and low herbage during the day, and come out to feed only by night; that many others feed on roots, and are therefore seldom seen above the surface of the soil; also that a good number burrow into fruits, in the interior of which they spend the whole of their larval stage. The best way to secure the latter is to examine the 'windfalls' that lie scattered on orchard lands, for it is a well-known fact that the fruits that are infested with larvæ generally fall earlier than others – a result that must be attributed to the damaging work of the larvæ themselves.

All the larvæ collected should be carefully boxed at once, a separate compartment being used for each species, and a few fragments of the food plant being introduced in each case. It is also a good plan to have each box previously lined with moss as a further addition to the comfort of the captives. Without such a precaution some of the more delicate species are liable to injury during their transmission from field to home.

Hitherto we have obtained our larvæ by searching only, but there are times and occasions when our boxes may be far more rapidly filled by methods that are not such a tax on our time and patience. Suppose, for instance, that we reach a bush, the mutilated leaves of which seem to show that larvæ are present on its branches. We spread our white cloth or open out the white-lined umbrella just under a selected branch, and then tap that branch very smartly with our stick.

Down comes a host of living creatures! Spiders, larvæ, beetles, aphides, earwigs, and what not, struggling and running about on our white fabric in all directions, and all mingled with bits of stick, leaves, and fragments of all kinds. We leave the cloth or the umbrella, as the case may be, quite still for a few seconds to allow all the living creatures to get a good foothold, and then, raising it into a vertical position, allow all the rubbish to drop off.

We can now put the cloth down again, and select as many of the larvæ as we require, giving our first attention to the nimble runners and loopers that are already near the edge and just on the point of making their escape. This productive method of larva hunting is known as 'beating,' and is particularly applicable to tall herbs and the lower branches of trees and shrubs.