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Even admitting some sense of touch at the end of the drill, one insuperable difficulty would always remain: that of finding the exact spot where the tiny speck lies in those spacious and mysterious regions. I go so far as to believe that the ovipositor tells the insect nothing, or at any rate very little, of the inside of the cell, whether propitious or not to the development of the germ.
I see them daily alighting on the envelopes and very busily exploring them, apprised of the contents by the gamy smell. Their incessant coming and going is a sign of intense cupidity; and yet none of them decides to lay on the bags. They do not even attempt to slide their ovipositor through the slits of the folds. The favorable season passes and not an egg is laid on the tempting wrappers.
This accommodating specimen had apparently just completed her egg-laying, or had perhaps just filled one nest; and while her attitude was precisely similar to that of her neighbor, I noticed a tiny ball of glistening froth at the tip of the ovipositor.
This is evidently intended as a weapon of defence against the attack of the ichneumon flies, that deposit their eggs in its soft body, for when the grub is pricked, either by the ovipositor of the ichneumon, or by any other sharp instrument, the horn is at once protruded, and struck upon the offending object with unerring aim.
It is homologous in situation with the middle pair of blades which composes the ovipositor of higher insects, and if it should prove to be used by the creature in laying its eggs, we should then have, with the spring, an additional point of resemblance to the Neuroptera and higher insects, and instead of this spring being an important differential character, separating the Thysanura from other insects, it binds them still closer, though still differing greatly in representing only a part of the ovipositor of the higher insects.
By means of these longitudinal movements of the inner trench inside the outer trench and of the sliding, one over the other, of the two portions of the former, the egg can be despatched to the end of the ovipositor notwithstanding the absence of any muscular contraction, which is impossible in a horny conduit.
"They are the work of a species of fly called Cynips, which is very apt to attack the oak. 'The female insect is armed with a sharp weapon called an ovipositor, which she plunges into a leaf and makes a wound. Here she lays her eggs; and when she has done so, she flies away and we hear no more of her. But the wound she has made disturbs the circulation of the sap.
The gravid female always retains an upright position at this time, as indeed she does at other times. She is so absorbed in her task that she may readily be watched, even through a magnifying glass. The ovipositor, which is about four-tenths of an inch in length, is plunged obliquely and up to the hilt into the twig. So perfect is the tool that the operation is by no means troublesome.
But natural selection can and does often produce structures for the direct injury of other species, as we see in the fang of the adder, and in the ovipositor of the ichneumon, by which its eggs are deposited in the living bodies of other insects.
Nothing tells us that this number was not exceeded in my absence. The little that I observed prevents me from fixing any limit. And now a momentous question arises: is the egg really laid each time that the probe enters a cell? I can see not the slightest excuse for supposing the contrary. The ovipositor, because of its horny nature, can have but a very dull sense of touch.
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