To watch these frolics in the tops of the trees is hardly possible; let us try to watch them in captivity. I collect four couples in the morning and place them in a roomy cage, with a few twigs of pine. The spectacle hardly comes up to my expectations. This is because they are deprived of the power of flight. At most, from time to time, a male approaches his coveted bride; he spreads the leaves of his antennæ and shakes them with a slight quiver, perhaps to discover if he is welcome; he shows off, exhibiting his antlered beauty. It is a useless display: the female does not budge, as though insensible to these demonstrations. Captivity has sorrows that are hard to overcome. More than this I could not see. Pairing, it seems, must take place during the later hours of the night, so that I have missed the propitious moment.
One detail in particular interested me. The Pine-chafer possesses a musical instrument. Male and female are similarly [[202]]gifted. Does the suitor make use of his faculty as a means of seduction and appeal? Does the other answer her lover’s strophe with a similar strophe? That this happens under normal conditions, amidst the branches, is highly probable; but I should not care to say so for certain, having never heard anything of the kind among the pine-trees or in the cage.
The sound is produced by the tip of the abdomen, which, with a gentle movement, alternately rises and falls, rubbing its rear-most segments against the hinder edge of the wing-cases, which are held motionless. There is no special appliance on the rubbing surface nor on the surface rubbed. The magnifying-glass searches in vain for minute ridges such as might produce a note. On either hand all is smooth. How then is the sound produced?
Moisten the tip of a finger and run it over a strip of glass, over a window-pane: you will obtain a fairly well-sustained sound, not unlike that emitted by the Cockchafer. Better still: use a bit of india-rubber to rub the glass with and you will obtain a pretty faithful reproduction of the noise made by the insect. If the musical rhythm is well [[203]]preserved, the imitation might deceive anybody.
Well, in the Cockchafer’s apparatus, the pad of the finger-tip and the bit of india-rubber are represented by the softness of the moving abdomen and the window-pane by the plate of the wing-cases, a thin, rigid plate eminently capable of vibration. The Cockchafer’s musical instrument is thus one of the simplest.
A small number of other Beetles are endowed with the same privilege. These include the Spanish Copris and the truffle-eating Bolboceras.[5] Both make a sound by means of slight oscillations of the abdomen, which gently grazes the hinder edge of the wing-cases.
The Cerambyx-beetles have another method, likewise based on friction. The Great Capricorn, for instance, moves his corselet over its junction with the thorax. There is here a large cylindrical projection which fits tightly into the cavity of the corselet and forms a joint which is at the same time powerful and mobile. This projection is surmounted by a convex surface, [[204]]shaped like an heraldic scutcheon, perfectly smooth and absolutely devoid of any sort of fluting. This is the musical-box.
The edge of the corselet, itself smooth inside, rubs over this surface, passing to and fro with a rhythmical movement and thus creating a sound which is once more like that of a window-pane rubbed with a moistened finger. Still, I am unable to make the dead insect’s apparatus sound by moving the corselet myself. Though I hear nothing, I at least feel with my moving fingers the shrill vibration of the surfaces rubbed. A little more and the sound would be audible. What is lacking? The stroke of the bow which the live insect alone is able to supply.
We find the same mechanism in the small Capricorn, Cerambyx cerdo,[6] and in the denizen of the willows, the Rose-scented Aromia, A. moschata.[7] On the other hand, the Ægosoma and Ergates, mighty Longicorns both, are without the projection fitting into the corselet, or rather possess of it only as much as is strictly necessary to join [[205]]the two parts together. Consequently the two big night-insects are dumb.
Though we are acquainted with the simple mechanism of the Cockchafer’s instrument, its employment none the less remains a riddle. Does the insect use it as a means of nuptial appeal? This is likely. Nevertheless, I have not heard the slightest grating on the pines, in spite of all my attention at propitious hours. I have heard nothing either in the cages, where distance formed no obstacle to the hearing.