First, as regards the number of claspers, it will be seen from what has just been said that this is not always the same. Some caterpillars possess five pairs, thus making up the total number of walking appendages to sixteen. In fact, we must regard this as the _usual_ number. But there are at least a few hundred exceptions to the rule.
Many of the _Bombyces_ (page 217), for example, have only four pairs of claspers; and in others of the same group the fifth pair is present, but only partially developed, and quite useless for walking.
Look at the peculiar caterpillar of the Lobster Moth (fig. 23)--a creature that differs from most other caterpillars not only in its claspers, but in many other respects too. Observe its long and slender legs, its humped middle segments, and its upturned hindermost segment, of enormous size and mounted with a pair of clubbed "horns." This last segment you will observe, has no claspers.
[Ill.u.s.tration: FIG. 20.--WALKING LEG OF A CATERPILLAR.]
[Ill.u.s.tration: FIG. 21--LARVA OF THE YELLOW UNDERWING MOTH (_p.r.o.nuba_).]
[Ill.u.s.tration: FIG. 22.--LARVA OF THE CRIMSON SPECKLED MOTH (_Pulch.e.l.la_).]
[Ill.u.s.tration: Fig. 23.--LARVA OF THE LOBSTER MOTH (_f.a.gi_).]
Another allied caterpillar is that of the Iron Prominent Moth (fig. 24).
This one also has humped segments, and the claspers of the thirteenth segment are imperfectly developed.
A large number of other exceptions to the general rule are to be found in the caterpillars of the Geometer Moths (page 268), one of which is here represented. These have generally only two pairs of claspers, one pair on each of the tenth and last segments, so that there is a distance equal to the combined length of six segments between the hindermost true leg and the first pair of claspers. But even among the Geometers there are variations to be observed in the number of claspers, and some of these will be pointed out in our brief descriptions of the commoner species.
[Ill.u.s.tration: FIG. 24.--CATERPILLAR OF THE IRON PROMINENT MOTH (_Dromedarius_).]
[Ill.u.s.tration: FIG. 25.--LARVA OF THE BRIMSTONE MOTH (_Luteolata_).]
These limbs which we have been calling claspers are known by several other names. Thus they are termed "pro-legs," "temporary legs," "false legs," and "abdominal legs;" but if you watch a caterpillar as it walks up a stalk or along the edge of a leaf, you will certainly agree that the term "clasper" is everything that could be desired. But why not call them legs, seeing that they are used in walking? The reason is that they differ in many respects from the three foremost pairs of limbs as regards structure, persistency, and function. The true legs, as we have called them, continue to exist, though concealed, in the chrysalis state, and again appear, far more perfectly developed, in the b.u.t.terfly or moth, but the claspers are no more to be seen after the caterpillar has pa.s.sed into the quiescent stage. We have noticed, too, that the true legs are pointed and clawed, also that they are protected by a hard and h.o.r.n.y covering; but examine a large caterpillar, holding it between the fingers and thumb with its under side uppermost, and you will soon see that the claspers are not at all hard, but soft and fleshy; not pointed, but often terminating in a broad flat circular surface. You will also observe, as the creature struggles to escape from your grasp, and tries to get a hold on something with its claspers, that these limbs, if we may so call them, are retractile, and are sometimes completely drawn into the body. Finally, examine the broad end of a clasper with a magnifier, and you will see it surrounded by a circle of little hooks, turning in all directions. You will no longer wonder how it is that a caterpillar can hold so tenaciously to a piece of twig that it is often almost impossible to remove it without injury.
Now put your caterpillar down, so that you may observe its gait. If it happens to be one with the full complement of sixteen limbs, you see that at each stride it makes but little progress. The segments contract and relax alternately and in succession, thus sending a series of wave-like motions along the body, and urging onward the front segments while the claspers keep the hinder portion firmly fixed.
[Ill.u.s.tration: FIG. 26.--THE CLASPERS OF A CATERPILLAR.]
But if your caterpillar is one of the Geometers, with only two, or perhaps three, pairs of claspers, the mode of procedure is very different. The creature stretches its body out at full length, often raising its head high in the air, and swinging its long body right and left with a most furious motion, as if to hastily scan the neighbourhood. Then, having satisfied itself as to the _direction_ of its proposed course (which, by the way, is often changed considerably at almost every stride), it holds on by the true legs and pulls its hinder quarters forward till the body forms almost a closed loop, with the fourth segment nearly touching the ninth. The claspers now become the holdfasts. The little hooks with which they are provided are firmly fixed to the surface on which it is walking; and the body being again straightened out to its utmost length, the same man[oe]uvre is repeated.
So, you see, the insect progresses by strides equal in length to about six segments of the body, and these the longest segments generally; and the rate at which the strides succeed each other, especially in some of the smaller species, is really astonishing.
We have seen the caterpillar in the act of taking its walk, and now we will give it a twig of its food plant so that we may see it feed. It walks up the twig without hesitation--for caterpillars (excepting those which feed on roots) always seem to move upward when in search of food--and soon finds itself on a leaf. Over this it walks till it reaches the edge; and, grasping the edge firmly between the claspers, so as to give perfectly free play to its legs and head, it stretches its body at full length, and takes a series of bites as it brings its head backward in a curve. When the head has thus been brought close to its fore legs, the body is again extended, and the same ground is gone over again.
If the caterpillar is a fairly large and hardy one, it will bite through the smaller veins, and perhaps even the larger ones; but the smaller species often change their position on reaching a moderately thick vein, and so devour little else than the soft cellular substance of the leaf.
In any case, it is astonishing to see how rapidly the leaf disappears under the influence of the powerful jaws and marvellous digestive apparatus of the hungry grub.
Those who take a delight in watching the _movements_ of caterpillars are sure to be interested in observing them when at rest; for at such times the various att.i.tudes a.s.sumed are as pleasing and instructive as are their active moments. And these att.i.tudes are all the more interesting on account of the mimicry by which the creatures often baffle their numerous enemies. We may profitably spend a little time in studying a few cases in point.
Many species, when at rest, fix themselves by means of their claspers to a small twig or leaf stalk, or on the midrib of the leaf itself. Here they remain perfectly still, with their bodies perfectly straight or with head slightly raised. I need hardly say that these generally fix themselves on the _under_ side of the leaves and stalks, thus securing themselves against the attacks of the feathered foes above. But some birds are equal to the caterpillars in this matter; and it is really amusing to see them hopping about beneath the leaves in our gardens, every now and again slyly turning one eye upward, and smartly plucking an unwary grub from its resting place.
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 antic.i.p.ated 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 ma.s.s of food.
[Ill.u.s.tration: FIG. 27.--THE HOMES OF LEAF MINERS AND LEAF ROLLERS.]
We must conclude this brief account of resting and hiding places and att.i.tudes of caterpillars by a few observations on the leaf miners and leaf rollers.
The former are very small caterpillars--the larvae 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 cl.u.s.ters 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 cl.u.s.ters, sometimes several scores in each, and all help in the spinning of a complicated ma.s.s 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 _tracheae_.
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 tracheae will show that its walls are supported by an elastic spiral of a firm substance.
This arrangement serves to keep the air pa.s.sages 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.
First segment--head Two short antennae, two jaws, and twelve eyes.
Second " Legs and spiracles.
Third " Legs only.
Fourth " Legs only.
Fifth " Spiracles only.
Sixth " Spiracles only.
Seventh " Spiracles, and sometimes claspers.
Eighth " Spiracles, and sometimes claspers.
Ninth " Spiracles, and sometimes claspers.
Tenth " Spiracles, and generally claspers.
Eleventh " Spiracles only.
Twelfth " Spiracles only.
Thirteenth " Claspers only, and these occasionally absent.
We must now watch the caterpillar through its later days, to see how it prepares for pa.s.sing 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.