Homes Without Hands: Being a Description of the Habitations of Animals, Classed According to Their Principle of Construction - Page 34

Should an insect, an ant, for example, happen to pass near the pitfall, it will be sure to go and look into the cavity, partly out of the insatiable curiosity which distinguishes ants, cats, monkeys, and children, and partly out of a desire to obtain food. No sooner has the ant approached the margin of the pitfall, than the treacherous soil gives way, the poor insect goes tumbling and rolling down the yielding sides of the pit, and falls into the extended jaws that are waiting for it at the bottom. A smart bite kills the ant, the juices are extracted, and the empty carcass is jerked out of the pit, and the Ant-lion settles itself in readiness for another victim.

Sometimes, when a more powerful insect, such as a large wood-ant, or beetle, or perhaps a hunting spider, happens to fall into the pit, the Ant-lion does not obtain a meal on such easy terms. The victim has no idea of surrendering at discretion, but tries to scramble up the sides of the pit, and in its furious exertions, it brings down the sand in torrents, filling up the pit, making the slope of the sides shallower, and so rendering its escape easy. Then there is a battle between the Ant-lion and its intended prey, the one bringing the sand into the pit and the other flinging it out again so as to restore the steepness of the sides, and to deepen the pit.

Sometimes a quantity of the sand flung by the Ant-lion happens to fall on the escaping victim, knocks it over, and enables the devourer to grasp it in the terrible jaws, which never open but to reject the dead and withered carcasses; sometimes the insect is tired before the Ant-lion, and suffers itself to be captured; and sometimes, though very rarely, it succeeds in making its escape. In either case, the pitfall is quite out of shape, and instead of rearranging it, the Ant-lion deserts it and makes another. Some writers have said that the Ant-lion flings the sand at its escaping prey with deliberate aim and intention. It does nothing of the kind, but only tosses the sand out as fast as its head can work, without aiming in any direction, or having an idea except to prevent the pit from being filled up.

Its earth-burrowing life does not cease until it assumes the perfect state. When it has passed its full time in the larval condition, and is about to change into a pupa, it spins a silken cocoon of a globular form, and therein remains until it is about to assume its perfect condition. The pupa then bites a hole through the side of the cocoon, and projects its body half out of the aperture. The pupal skin then withers, bursts, and the perfect insect emerges. Scarcely has it taken the first few breaths of air, than its abdomen, which before was short, so as to be included within the cocoon, extends to nearly three times its original length, so as to resemble that of the dragon-fly; the curious antennæ unroll themselves, the wings shake out by degrees their beautiful folds, and in a short time the lovely insect is ready for flight. It is scarcely possible to imagine a more complete contrast than that which is exhibited by the larva and the perfect insect, and if the two were placed side by side, no one who was not aware of the circumstances would think that they are but two stages of the same insect.

If the reader will refer to the illustration on page 148, he will see a section of the pitfall, with the Ant-lion at the bottom, and a couple of ants falling into the trap. The Ant-lion belongs to the same order of insects as the dragon-fly, which it so much resembles.

CHAPTER IX. WOOD-BORING INSECTS.

Beetles.—The usual Form of the Wood-borers.—The Scolytus and its Ravages.—Mode of forming the Tunnels.—Curious Instinct.—Theories respecting the Scolytus.—Worm-eaten Furniture, its Cause, and the best Method of checking the Boring Insects.—Ginger and Cork Borers.—The "Petrified" Man.—The Mealworm and its Ravages.—Weevils.—The Palm Weevil of Jamaica.—Its Development, and Uses as an Edible.—Its Cocoons.—The Wasp-Beetle, its Shape, Colors, and tunneling Powers.—The Musk Beetle.—Its Beauty and Fragrance.—Difficulty of detecting the Musk Beetle.—Its Burrows and their Inmates.—The Rhagium and its Cocoon.—The Harlequin Beetle.—Wood-boring Bees.—Willow Bee, its Tunnel, and Mode of making the Cells.—Food of the Young.—The Poppy Bee.—The Pith-boring Bees and their Habits.—Structure of the Cells and Escape of the Young.—Economy of Labor.—Shell-nests of Bees.—Wonderful adaptation to Circumstances.—How the Bee burrows.—The Hoop-shaver Bee.—Gilbert White's Description of its Habits.—The Sirex and its Burrow.—Its Ravages among Fig-trees.—Formidable Aspect of the Insect.—The two British Species.—Carpenter Bee.—Mode of making its Burrow.—Methodical Labor.—Food of the Young.—How to make a Ceiling.—Number of Cells in each Burrow.—The Carpenter Bee of Australia.—The Pelopæus as a Woodborer.—Its Tunnel, and Mode of making Cells.—The Saperda.—Damage caused to Aspen and other Trees.—A useful Parasite.—The Goat Moth.—Wood-leopard Moth.—Clear-wings and Honey-comb Moths.

WE now leave the earth-burrowers, and proceed to those insects which tunnel into wood and other substances. The Hymenoptera are again the best burrowers in wood as they are in the earth, but, as some of the beetles are notable wood-borers, and we shall only mention a few of them, we will take them first in order.

BEETLES generally burrow while in their larval state, though there are some that do so when they have attained their perfect form, and are able to bore their way through wood or into the ground with wonderful ease. All the boring beetles are formed in such a manner that an entomologist can at once detect their habits from their shape. The combination of the cylinder and the sharp-edged screw is well known to be the best form of boring-tool, whether under the name of auger, gimlet, or centre-bit, and it will be found that the harder the substance into which the insect burrows, the more cylindrical is its shape. The dors, clocks, and other earth-boring beetles, depart from that form, but when we come to look at the scolytus, the ptinus, and other wood-borers, we can not but notice how very cylindrical they are in their shape.

Perhaps there is no wood-boring beetle which is known so well as the little insect which is called Scolytus destructor. I am not aware that it has a popular name that will distinguish it from other small beetles which bore into wood.

The accompanying illustration will probably call to the mind of the reader the insect which now comes before our notice. If he should have examined the bark of certain trees, particularly that of the elm, he will often have seen that it is perforated with circular holes, very like those which are drilled into worm-eaten furniture, but of rather larger diameter. When I was a very little boy and first saw these holes, I thought that they had been made by shot, and in trying to pick out the shot with my knife, made the discovery that the holes were not due to fire-arms, but to insects. The pleasure of the discovery nearly compensated for the disappointment concerning the shot, the possession of which seemed to my boyish mind to be a manly trait of character, and calculated to raise me in the eyes of my playfellows.

If the bark be cut through, and then raised with the knife, the curious radiating system of tunnels will be exposed to view, and the observer will notice that, however these tunnels may vary in size and direction, they all agree in these points; firstly, that they radiate nearly at right angles from a single cylindrical tunnel; and secondly, that they are very small at their base, and gradually increase to their termination. The cause of this formation is as follows:

The mother insect enters the bark in search of food, and burrows deeply into the tree, sometimes boring into the substance of the wood itself, but generally cutting a tunnel between the wood and the bark. She then deposits her eggs regularly along the cylindrical tunnels, and in most cases retreats to the entrance, and there dies, her body forming a natural stopper. In due time the eggs are hatched, producing a number of very minute white grubs, which immediately begin to feed, the substance of the tree being the only diet of this insect in every stage of existence. Urged by a wonderful instinct, each grub arranges its body at a right angle with the burrow in which it was hatched, and so eats its way steadily outward.

When the grubs have made some progress, the wisdom of this arrangement becomes evident. As they increase in size, the burrows necessarily increase with them, so that if they had all started parallel with each other, the tunnels would coalesce and the grubs be unable to procure their proper amount of food. As, however, the tunnels radiate like the spokes of a wheel, they very seldom interfere with each other, their radiation more than keeping pace with their increasing size. It will easily be seen by reference to the illustration, that if a number of these beetles attack a tree, the bark is gradually separated from the woody portion, and that, as in all exogeneous trees the nourishment is derived from the bark, the tree must die as soon as the functions of the bark are suspended.

Settlers in any new colony are well aware of this fact, and when they want to kill a tree, they do so by simply removing a rather wide ring of bark from the trunk, and thus cutting off the supply of nourishment. The tree is thus starved to death, and in the following year, a fire applied to the trunk is able to burn it through, and bring down the tree with scarcely any expenditure of labor by the settlers. This mode of killing a tree is technically called "girdling" it. In proportion, therefore, to the amount of bark removed, the tree sickens, from defective nourishment, and if once the bark be separated all round the trunk, the tree will instantly die.

The reader may probably be aware that some of our most skillful naturalists have thought that the Scolytus is not so culpable an insect as is generally supposed, and that it does not attack trees until their race is run, and they have begun to show symptoms of decay. There is great truth in this conjecture, for it is beyond a doubt that if a tree be seriously injured, and begin to droop, the Scolytus is sure to make a lodgment before very long. Girdled trees, for example, are almost always attacked by this beetle as soon as the effects of the injury are apparent. But, though the female may not lay her eggs in healthy trees, there is little doubt but that she and her mate have aided, in no small degree, in bringing the tree to so diseased a condition. For, as has already been mentioned, the food of the adult, as well as of the imperfect insect, consists of the bark and wood, and in boring the tree for the purpose of feeding, the numerous Scolyti can but enfeeble its constitution, and so bring it to that state of ill health which renders it a fit cradle for the immature beetles.

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