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 MEAL-WORM and its ravages-Weevils-The PALM-WEEVIL of Jamaica-Its development and uses as an edible-Its Cocoons-The WASP-BEETLE, its shape, colours, and tunnelling 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 YoungThe POPPY BEE-The PITH-BORING BEES and their Habits-Structure of the cells and escape of the Young-Economy of labour-Shell-nests of BeesWonderful adaptation to circumstances-How the Bee burrows-The HoopSHAVER-BEE-Gilbert White's description of its habits--The SIREX and its Burrow-Its ravages among fig-trees-Formidable aspect of the insectThe two British species-CARPENTER BEE-Mode of making its burrow-Methodical labour-Food of the Young-How to make a ceiling-Number of cells in each burrow-The Carpenter Bee of Australia-The PELOPEUS as a Wood-borer-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 cannot 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.

not aware that it has a popular name that will distinguish it from other small beetles which bore into wood.

If he

The accompanying illustration will probably call to the mind of the reader, the insect which now comes before our notice. 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 firearms, 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:

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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 outwards.

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 exogenous 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 labour 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 skilful 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.

There is hardly a grove or a park in the neighbourhood of London where the ravages of the Scolytus are not painfully apparent, and in Greenwich Park especially, some of the finest trees are riddled with the cylindrical tunnels of this destructive insect. There are several species of Scolytus, each affecting certain trees, so that there is scarcely any tree that can hope to escape from the jaws of some member of this family.

THE well-known worm-eaten appearance of furniture is caused by certain beetles belonging to another family. As may be seen from the dimensions of the turnels, the insects are very smali. and their bodies are nearly cylindrical. The ravages which these beetles cause are fatal to all who happen to possess old furniture, but Mr. Westwood mentions that one common species, Ptilinus pectinicornis, completely destroyed a new bedpost, in the short

space of three years. There is but one known method of killing the insects which have already taken possession, and of preventing others from following their example, namely, by injecting a solution of corrosive sublimate into the holes, and then treating the whole of the surface with the same poisonous liquid. I need perhaps scarcely mention, that insects which are popularly called Death-watches, belong to this family. Not only do furniture and timber suffer from the attacks of the Ptilinus, but articles of dress and food are also injured by them. Specimens of natural history are often spoiled by the holes which are drilled through them by the beetles; and stationers sometimes suffer from the voracious insects, which bore holes through their wafers, fix them together, and there undergo their transformations within them. One species is obnoxious to wholesale druggists, on account of the damage which it does to the ginger. In some cases, half the ginger is drilled with holes, and rendered quite unsaleable. It is not, however, lost entirely, because it is reserved for the mill, and is then sold as ground ginger, the insects and their grubs being reduced to powder together with the ginger which they have not consumed. Such specimens are of course not exhibited to the general gaze, as the public would be very cautious of purchasing ground ginger if they knew what it contained. In the British Museum, however, may be seen several pieces of ginger completely eaten away by the beetle, and numerous examples of the insect itself are placed in the same tray. The little beetles which eat cork, and are so mischievous in the cellar, belong to the genus Mycetophagus. They will eat rotten wood or fungi, but always prefer cork, and in some cases have not only caused much expense by forcing the proprietor to recork all his bottles, but have sometimes destroyed the cork so completely, that the wine has escaped.

The reader may remember that a so-called "petrified man was brought from Australia, and exhibited in London during 1862. Having very great doubts about the petrifaction of a human being, I went to see it, and at a glance perceived that it was no petrifaction at all, but simply a moderately-good example of a desiccated body, such as are common enough in museums, and sometimes occur even in this country. The exhibitor stoutly asserted that it was a petrifaction, but as I noticed the tunnels of sundry Ptilini in various parts of the head, body, and limbs,