Butterflies and Moths (British). Furneaux William Samuel
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Название: Butterflies and Moths (British)
Автор: Furneaux William Samuel
Издательство: Public Domain
Жанр: Природа и животные
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The precautions of the caterpillar, however, do not end merely with the selection of an under surface. You will find that the bright green species invariably settle on a leaf or a green stalk, while the darkly coloured insects often choose a twig covered with a brownish bark. Some even make for the trunk of the tree on which they feed, and here remain quite still in a vertical position, so that they look just like a ridge in the bark, the colour of which is faithfully imitated by their skin. Further, many of the caterpillars that resort to this stratagem have bodies that are notched or knotted and spotted in such a manner that the resemblance to their surroundings is so perfect as to defy any but the most experienced eye. And even this is not all, for a number of these mimics of the insect world never venture to feed by day, but take in their quantum of provisions during the dark hours, and practise their deceptions throughout the day.
Most of the Geometer caterpillars, of which we have already spoken, are well trained in the art of deception. You are out on a caterpillar hunt, and engaged in carefully turning over the twigs of the hazel or some other shrub, so that you may the more readily examine the under surfaces of the leaves. At last you lay hold of a small broken twig for this very purpose. To your astonishment it is very soft, and readily bends between your fingers. You look more closely at this peculiar piece of stick, and find, to your surprise, that you have grasped a looper caterpillar that was standing out at an angle just like a broken twig, supported by its two pairs of claspers, and coloured and knotted exactly like the little branch on which it rested.
At other times you meet with little green caterpillars of the same group, supporting themselves in exactly the same manner on a small twig, and looking just like a leaf stalk from which the blade had fallen or been devoured.
What a wonderful power is exhibited in the grasp of the claspers and the tension of the muscles, enabling the caterpillar to fix itself and retain its position for so long a time! Imagine an acrobat fixing himself by his hands on an upright pole, throwing out his body at an angle, and without any further support retaining his position motionless for several hours!
Other experiences of the larva hunter are equally interesting and, perhaps, even more tantalising. He is engaged in very cautiously turning over the leaves of a certain food plant from which he hopes to obtain the larva of a much-coveted species. Then, just as his eye catches a glimpse of the very object of his search, down falls the caterpillar, rolled up into a little ball, among the herbage below. This latter is diligently and patiently examined. But no, the anticipated prize is nowhere to be seen. It is probably a green one, and this adds to the difficulty of the patient entomologist. Then, as he carefully separates the low herbs, hoping to find the spot where the larva had fallen, the insect, rolled up into a compact little ball, only sinks deeper and deeper into the maze.
Many caterpillars avoid capture in this manner, while others seek to avoid detection by remaining perfectly motionless, even when roughly handled. They allow themselves to drop from their resting place on the slightest sign of danger, and, when the alarm is over and all is quiet again, they ascend the food plant and resume their position.
Some caterpillars not only rest, but even feed under cover, quite secure from most, if not all, of their enemies. Several of them feed on roots, and many a farmer can relate sad experiences of the havoc committed by these caterpillars on his turnips and other crops. Then there are those which feed on flowers and buds, completely burying themselves in the dense mass of food.
Fig. 27. – The Homes of Leaf Miners and Leaf Rollers.
We must conclude this brief account of resting and hiding places and attitudes of caterpillars by a few observations on the leaf miners and leaf rollers.
The former are very small caterpillars – the larvæ of certain small moths – that eat burrows into leaves without doing any considerable injury to the outer epidermis, and thus prepare a safe resting place within the substance of their food.
The latter, also mostly of small size, make themselves secure by curling a leaf or a portion of a leaf into a cylinder, and holding it in position by means of a number of silken threads.
If you examine a leaf thus curled you will soon be convinced that a considerable number of the extremely delicate threads must be necessary to hold it in position; but, if you would like to know how a very small and feeble caterpillar can manage to roll up a comparatively large and rigid leaf, you must watch the little creature at its work.
You need have but little difficulty in finding a willing worker, for such caterpillars are extremely numerous. Take a few out of their self-made homes, place them on a sprig of the food plant, and you will soon have the pleasure of seeing one start its extraordinary work.
At first it spins a number of threads stretching from the edge of a leaf to about the middle of the surface. These threads are not tight by any means, and the leaf is, as yet, unchanged in position. But now the little mechanic exhibits a tact that almost seems to prove a knowledge of the principles of its art. Each thread in turn is pulled at right angles at its middle, and then fastened by means of the creature's spinneret. Each time this is done the edge of the leaf is bent round a little; and when at last the cylinder is completed, a number of other threads are stretched across from the scroll to the flat part of the leaf to secure it firmly in its place.
Many caterpillars are solitary in their habits: that is, they are always found singly, whether walking, resting, or feeding. But a large number of species are gregarious, living in dense clusters either throughout their larval state or, perhaps, only while young. In many such cases it is difficult or even impossible to find any reason for this gregarious tendency – to discover any advantage that the insects may derive from the habit. Many species, however, are true co-operators in the defence of their communities. The caterpillars of such live in clusters, sometimes several scores in each, and all help in the spinning of a complicated mass of silk fibres, which, with the leaves and twigs they join together, form a safe home in which they can rest, feed, or change to the chrysalis state. In early summer hundreds of such caterpillar 'nests' are to be seen in many of our hawthorn and other hedgerows.
Before closing our general account of the caterpillar we must have a word to say about the breathing apparatus, more especially as in our future descriptions we shall frequently have to mention the colours and markings which surround the openings in its body through which the air supply is admitted.
If you examine the sides of the segments of a caterpillar, using a lens if the insect is a small one, you will observe some little round holes, often inclosed in a ring or a patch of some prominent colour. These are the spiracles or openings of a series of air tubes called tracheæ. These latter divide and subdivide within the body of the caterpillar, the branches of one often uniting with those of another, thus forming a really complicated arrangement of air pipes by which the supply of oxygen is distributed.
A microscopic examination of a portion of one of the tracheæ will show that its walls are supported by an elastic spiral of a firm substance. This arrangement serves to keep the air passages open, and secures for the caterpillar a free supply of air at times when a contraction of the segments would otherwise cause the tubes to collapse.
There are nine spiracles on each side of the caterpillar's body, and never more than one in the side of the same segment. The head, which we have been regarding as the first segment, has no spiracles. The second segment has a pair – one on each side. There are none in the third and fourth; but all the segments, from the fifth to the twelfth inclusive, have each a pair; the last (thirteenth) segment has none.
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