第16章

As already remarked, the habit of plugging up the mouths of the burrows with various objects, is no doubt instinctive in worms; and a very young one, born in one of my pots, dragged for some little distance a Scotch-fir leaf, one needle of which was as long and almost as thick as its own body.No species of pine is endemic in this part of England, it is therefore incredible that the proper manner of dragging pine-leaves into the burrows can be instinctive with our worms.But as the worms on which the above observations were made, were dug up beneath or near some pines, which had been planted there about forty years, it was desirable to prove that their actions were not instinctive.Accordingly, pine-leaves were scattered on the ground in places far removed from any pine-tree, and 90 of them were drawn into the burrows by their bases.Only two were drawn in by the tips of the needles, and these were not real exceptions, as one was drawn in for a very short distance, and the two needles of the other cohered.Other pine-leaves were given to worms kept in pots in a warm room, and here the result was different; for out of 42 leaves drawn into the burrows, no less than i6 were drawn in by the tips of the needles.These worms, however, worked in a careless or slovenly manner; for the leaves were often drawn in to only a small depth; sometimes they were merely heaped over the mouths of the burrows, and sometimes none were drawn in.I believe that this carelessness may be accounted for either by the warmth of the air, or by its dampness, as the pots were covered by glass plates; the worms consequently did not care about plugging up their holes effectually.Pots tenanted by worms and covered with a net which allowed the free entrance of air, were left out of doors for several nights, and now72 leaves were all properly drawn in by their bases.

It might perhaps be inferred from the facts as yet given, that worms somehow gain a general notion of the shape or structure of pine-leaves, and perceive that it is necessary for them to seize the base where the two needles are conjoined.But the following cases make this more than doubtful.The tips of a large number of needles of P.austriaca were cemented together with shell-lac dissolved in alcohol, and were kept for some days, until, as I believe, all odour or taste had been lost; and they were then scattered on the ground where no pine-trees grew, near burrows from which the plugging had been removed.Such leaves could have been drawn into the burrows with equal ease by either end; and judging from analogy and more especially from the case presently to be given of the petioles of Clematis montana, I expected that the apex would have been preferred.But the result was that out of 121 leaves with the tips cemented, which were drawn into burrows, 108 were drawn in by their bases, and only 13 by their tips.Thinking that the worms might possibly perceive and dislike the smell or taste of the shell-lac, though this was very improbable, especially after the leaves had been left out during several nights, the tips of the needles of many leaves were tied together with fine thread.Of leaves thus treated 150 were drawn into burrows--123 by the base and 27 by the tied tips; so that between four land five times as many were drawn in by the base as by the tip.It is possible that the short cut-off ends of the thread with which they were tied, may have tempted the worms to drag in a larger proportional number by the tips than when cement was used.Of the leaves with tied and cemented tips taken together (271 in number) 85 per cent.were drawn in by the base and 15 per cent.by the tips.We may therefore infer that it is not the divergence of the two needles which leads worms in a state of nature almost invariably to drag pine-leaves into their burrows by the base.Nor can it be the sharpness of the points of the needles which determines them; for, as we have seen, many leaves with the points cut off were drawn in by their bases.We are thus led to conclude, that with pine-leaves there must be something attractive to worms in the base, notwithstanding that few ordinary leaves are drawn in by the base or foot-stalk.

Petioles.--We will now turn to the petioles or foot-stalks of compound leaves, after the leaflets have fallen off.Those from Clematis montana, which grew over a verandah, were dragged early in January in large numbers into the burrows on an adjoining gravel- walk, lawn, and flower- bed.These petioles vary from 2.5 to 4.5 inches in length, are rigid and of nearly uniform thickness, except close to the base where they thicken rather abruptly, being here about twice as thick as in any other part.The apex is somewhat pointed, but soon withers and is then easily broken off.Of these petioles, 314 were pulled out of burrows in the above specified sites; and it was found that 76 per cent.had been drawn in by their tips, and 24 per cent by their bases; so that those drawn in by the tip were a little more than thrice as many as those drawn in by the base.Some of those extracted from the well-beaten gravel- walk were kept separate from the others; and of these (59 in number) nearly five times as many had been drawn in by the tip as by the base; whereas of those extracted from the lawn and flower- bed, where from the soil yielding more easily, less care would be necessary in plugging up the burrows, the proportion of those drawn in by the tip (130) to those drawn in by the base (48) was rather less than three to one.That these petioles had been dragged into the burrows for plugging them up, and not for food, was manifest, as neither end, as far as I could see, had been gnawed.As several petioles are used to plug up the same burrow, in one case as many as 10, and in another case as many as 15, the worms may perhaps at first draw in a few by the thicker end so as to save labour; but afterwards a large majority are drawn in by the pointed end, in order to plug up the hole securely.