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Volume 19, 1886
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Art. XI.—On the Work of Earth-worms in New Zealand.

[Read before the Auckland Institute, 14th November, 1886.]

Although it has become generally known, since the publication of Mr. Darwin's researches, how rapidly surface débris is buried through the action of earth-worms, the result of a few observations may, nevertheless, be worth recording, as they afford means of comparison between the work done by the New Zealand and the British species. The comparative weight of their castings, and an estimate of the number of earth-worms usually found in some of the cultivated and uncultivated lands in Auckland District were given in a former paper, “On the Habits of Earth-worms in New Zealand.”*

A section was described, the result of the work of worms, chiefly the common Lumbricus campestris, but only an approximate estimate could be arrived at as to the length of time which it had taken for the surface - charred debris to sink to an average depth of about 5 ¾ inches. The section when first exposed, in 1875, showed an average of 4 ½ inches of black vegetable mould, free from stones, etc., and a horizontal layer, nearly 1 inch thick, of charred wood, burnt marl, fragments of jasper, and pumice, lying on the subsoil, a brownish-green arenaceous clay. In October, 1883, (i.e., in eight years), the depth of mould had increased 1 ¼ inches, giving an average depth of about 5 ¾ inches above the burnt layer; during the past three years there has been an even average increase to the superficial layer of nearly 1 inch, the total depth now being rather more than 6 ½ inches.

Considering the depth of vegetable mould above the charred layer, this certainly appears to be a considerable increase for so short a period, but apparently the estimate is correct: it is in all probability to be attributed, independently of the increase of worms since the land has been in grass, to the moisture and nature of the subsoil. Worms living in lands with a moist substratum continue to work at a low level, not only during the short intervals of dry weather that occur in the spring months, but as long as there is sufficient moisture; whereas in the upper lands, during these dry intervals, the worms cease to work, retire temporarily to their chambers, coil themselves up, and remain apparently in a dormant state until rain sets in again.

On the 15th October, 1883, a layer of charred wood and broken

[Footnote] * “Trans. N.Z. Inst.,” vol. xvi., 1883, p. 266.

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brick was strewed on the surface, above the former layer, and a similar strip of débris was also scattered in a grass paddock on the upper land, ordinary light clay soil, with a moderately hard subsoil. Although the spot selected was rather favourable for earth-worms to work in, the amount of castings ejected on the surface may be taken as a fair average for the whole field. Sheep were occasionally grazed in the field; but considering how soon small fragments disappear beneath the surface, probably their treading did not much influence the result.

Owing to there being no well-defined line between the dark mould and substratum, no estimate of any value can be given as to the probable amount of subsoil that has been brought to the surface during the past three years; but a record will be kept of any marked increase in the depth of the vegetable mould, the average depth now being about 5 ¾ inches. At the present time the section in the trench shows, the grass being shaved off close to the surface, an average of ⅞ths of an inch of brownish-black mould, free from coarse material, and a horizontal layer of charred wood and broken brick half an inch thick, forming an even line round the vertical sides. This means that in three years, or rather during the working months of that interval, through the agency of earth-worms, ⅞ths inch of mould has been added to the superficial layer—mould that has been enriched with vegetable and animal matter, passed the bodies of the worms, and ejected as castings on the surface.

This annual working of the superficial mould effects a remarkable change in the character of our fern-lands in the course of time—that is, after they have been cultivated, or, more especially, if left in permanent pasture, for 10–15 years, and have become fairly stocked with worms. There are, no doubt, other agencies that, under the circumstances, tend to improve the soil, but, more particularly in the case of permanent pastures, only minor agencies.

From the number of earth-worms that live in most of our old pasture-lands, it is evident—independently of the experiment given above—that, as Mr. Darwin has shown, the superficial mould must pass over and over again through their bodies, and be brought to the surface. It is hardly necessary to point out the value such work must be to the agriculturist, especially when taken in conjunction with the loosening of the subsoil by their burrows and chambers, which in time become more or less filled up with their castings and the dark viscid linings of the walls. These moist and nourishing galleries, penetrating, as a rule, to a depth of 6–15 inches beneath the surface, must tend to draw the roots of the vegetation to a depth that they would not otherwise attain.

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The test in the lower land (mentioned above), a black arenaceous loam, was submitted to rather severe conditions—that is, the coarse wood-ashes and broken brick were spread over a thick sward of cocksfoot grass, from which stock is always excluded. Last September, when the strong grass was shaved off close to the surface and a trench opened up, the section, in the vertical sides of the walls showed that all the débris was buried beneath an average depth of rather more than ⅜ths of an inch of mould; the layer forms a horizontal line ⅞ths of an inch in thickness, but most of the débris is more or less entangled amongst the roots, consequently does not represent a compact and well-defined line; some of the fragments of brick, about 1 ½ inches square, weighing over loz., have sunk about ½ inches beneath the surface. This irregularity is to be attributed to the fragments having remained entangled amongst the grass for a more or less length of time, consequently buried at various intervals. It will be observed that there is a difference of about half an inch between the increase to the superficial layer in the upper and lower lands, which is easily accounted for: it was an oversight on my part that, when the débris was scattered over the latter, a portion of the grass was not skimmed off; this would have given the worms a fair chance of more rapidly burying the material thrown over it; however, this error will be remedied before next winter.

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The larger of the two stones mentioned in the former paper, which was laid on the turf in May, 1876, when raised on the 15th October, 1883, left a cast 2 inches in depth; when again examined on the 26th September, 1886, it was firmly embedded, and required some force to raise it; the most protuberant point was 3 inches beneath the level of the surface, the raised margins being removed; the flatter portion had sunk 2 6/10 inches; worm burrows were numerous beneath. The smaller stone, which was placed near the former in September, 1882, sank in thirteen months 1 inch; on the 26th September of the present year it had attained a depth of 2 inches below the general level of the ground; its convex margins were partially covered with worm castings and grass; it will, if left undisturbed, soon become entirely buried beneath the surface. It is well, perhaps, to bear in mind that under favourable conditions, through the agency of earth-worms, it is possible for a stone 6 ½ inches long, 3 ⅜ inches broad, and 3 ⅜ inches in thickness, to disappear below the surface of the ground in about seven or eight years.

The experimental stones laid on the turf on October 15th, 1883, when recently examined gave the following results:—

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No. 1, triangular block of trachyte, 6 inches high, when raised left an impression 6 inches long, 4 4/10 inches wide, depth below general level of the ground 1 2/10 inches.

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No. 2, 4 inches in thickness: cast, long, 6 8/10 inches; wide, 6 inches; slopes from surface to a depth of 1 2/10 inches.

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No. 3, average thickness 4 inches, bottom irregular: cast, long, 8 inches; greatest width, 5 5/10 inches; average depth, 1 4/10 inches.

These stones were surrounded with the usual margin of fine vegetable mould, were rather firmly embedded, and had not been raised since they were first laid down; beneath were numerous worm-burrows.

The researches which I have carried out during the last three winters, with the rather vague hope of affording some evidence that earth-worms possessed, to a limited extent, a sense of direction (for it may be assumed that there is such a sense), have, owing chiefly to the unnatural conditions to which they had to be submitted, not been by any means conclusive.

Although it was very improbable that, when worms left their burrows and wandered about for considerable distances after more or less heavy rain, they ever returned to them, as it was possible that on ordinary nights, when they sometimes only wander for a short distance from their burrows in search of food, that they might intentionally return to them, I thought it worth while to endeavour to determine if such was the case, as earth-worms certainly appear not to be devoid of a low form of intelligence. In the centre of boxes, 2 feet square, a small hole was pierced, a shallow vessel was placed beneath the orifice, filled with moist siliceous sand, with the hope that it would be the means of compelling the worms to come to the surface in search of food. Four or five of our large Lumbricus uliginosus were placed within each box—the boards having been well damped. As is almost invariably the case when worms are placed in confinement, in the first instance they travelled round the margins (when placed in pots containing well-pressed earth they finally force their way down the sides, rarely towards the centre); after attempting for 15–20 minutes to burrow into the boards, or scale the walls, one or two of the more vigorous, possibly more intelligent, struck out across the centre; after a few traverses they finally, as a rule in about 30 minutes, discovered and entered the artificial burrow. About as often as not, all the worms had found out the entrance before the morning; but in some instances, part or all, after crawling for a time round the sides of the box, finally huddled up into a corner, and would have died there, had they not been removed. Although tempting baits consisting of their favourite food, onion bulb, etc., were placed close to the mouth of the burrow, the worms rarely came to the surface; when they did, and left the burrow to wander about, apparently in most instances they never

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returned to it, but remained alongside the walls. In hopes of affording them greater inducement to return, tufts of grass were placed in the centre; but the result was not satisfactory, as they appeared never to leave them, probably owing to there being no inducement for them to do so. Although the fact that some of the earth-worms after coming to the surface and wandering about returned to the burrow, by no means proves that the act was intentional, the fact that the majority of them never did so, hardly disproves the possibility of their possessing sufficient intelligence to do so. Considering the unfavourable conditions to which they were submitted, and that they must necessarily have been weakened through want of food, it could hardly be expected that they would under the circumstances have acted with the same freedom and intelligence as when in their natural haunts.