Butterflies and Moths (British). Furneaux William Samuel
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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.
We have already observed the general arrangement of the caterpillar's limbs; but perhaps it may be interesting and even convenient to the reader to give here a little table that will show at a glance the disposition of both limbs and spiracles.
We must now watch the caterpillar through its later days, to see how it prepares for passing into the pupal stage, and to witness the various interesting changes that take place at this period.
When fully grown, it ceases to eat, and begins to wander about in search of a convenient spot for the coming event. Its colours fade, and the body becomes appreciably smaller, especially in length, as it ejects the whole contents of its digestive apparatus. According to some accounts, it even evacuates the lining of the intestines with their contents.
A great variety of situations are chosen by the different species at this time. Some will fix themselves on their own food plant, and there remain till they finally emerge in the perfect state, suspending themselves from a silken carpet, hiding themselves in a rolled leaf, or constructing a cocoon of some kind. A large number walk down the food plants, and undergo their changes in moss that happens to lie at the foot; or construct a cocoon on the surface of the ground, utilising for the purpose any decayed leaves, fragments of vegetable matter, or pieces of earth or small stones. Many seek a further protection than this, and burrow into the soil, where they either lie in a little oval cell that they prepare, or in a cocoon constructed by spinning together some particles of earth. Again, there are those caterpillars, chiefly of butterflies that frequent our gardens, which find their way to the nearest wall or fence, and there secure themselves in a sheltered nook. We will watch a few of these varied methods of procedure, taking as our first instance the caterpillar of the common Large White or Cabbage Butterfly.
When fully fed, this larva seeks out a sheltered spot, generally selecting the under surface of some object, or of the ledge of a wall or fence. Sometimes it will not even leave its food plant, though it generally walks some considerable distance before a suitable shelter is found. Having satisfied itself as to the site of the temporary abode, it sets to work at spinning a silken carpet. At first the threads spread over a rather wide area, and seem to be laid in a somewhat irregular and aimless manner; but after a little time its labours are concentrated on one small spot, where it spins several layers of silk fibres.
This done, it fixes the little hooks of the claspers firmly in its carpet bed, and then proceeds with a highly interesting movement. It is not satisfied with only the one mode of suspension. In fact, this alone would hardly be safe, for when it casts its skin, as it is shortly about to do, its claspers will all disappear; and although it afterwards secures itself by the 'tail,' it would be dangling in such a manner as to swing with every breeze – a very unsatisfactory state of affairs, especially with those that pupate late in the summer and remain in the pupal state throughout the winter storms.
Its next procedure, then, is to make a strong silk band round the middle of its body, so as to keep it close to the surface against which it rests. But how is this to be done? It bends its head round till the spinning organ can be applied to a point close beside the middle of its body. Here it fixes one end of a thread; and then, gradually twisting its body, brings its head round to the other side, still keeping it close to the same segment, and fastens the other end of the thread exactly opposite the point at which it started.
The head is now brought back to its