HABITS OF WORMS—_continued_.
Manner in which worms seize objects—Their power of suction—The instinct
of plugging up the mouths of their burrows—Stones piled over the
burrows—The advantages thus gained—Intelligence shown by worms in their
manner of plugging up their burrows—Various kinds of leaves and other
objects thus used—Triangles of paper—Summary of reasons for believing
that worms exhibit some intelligence—Means by which they excavate their
burrows, by pushing away the earth and swallowing it—Earth also swallowed
for the nutritious matter which it contains—Depth to which worms burrow,
and the construction of their burrows—Burrows lined with castings, and in
the upper part with leaves—The lowest part paved with little stones or
seeds—Manner in which the castings are ejected—The collapse of old
burrows—Distribution of worms—Tower-like castings in Bengal—Gigantic
castings on the Nilgiri Mountains—Castings ejected in all countries.
IN the pots in which worms were kept, leaves were pinned down to the
soil, and at night the manner in which they were seized could be
observed. The worms always endeavoured to drag the leaves towards their
burrows; and they tore or sucked off small fragments, whenever the leaves
were sufficiently tender. They generally seized the thin edge of a leaf
with their mouths, between the projecting upper and lower lip; the thick
and strong pharynx being at the same time, as Perrier remarks, pushed
forward within their bodies, so as to afford a point of resistance for
the upper lip. In the case of broad flat objects they acted in a wholly
different manner. The pointed anterior extremity of the body, after
being brought into contact with an object of this kind, was drawn within
the adjoining rings, so that it appeared truncated and became as thick as
the rest of the body. This part could then be seen to swell a little;
and this, I believe, is due to the pharynx being pushed a little
forwards. Then by a slight withdrawal of the pharynx or by its
expansion, a vacuum was produced beneath the truncated slimy end of the
body whilst in contact with the object; and by this means the two adhered
firmly together. {53} That under these circumstances a vacuum was
produced was plainly seen on one occasion, when a large worm lying
beneath a flaccid cabbage leaf tried to drag it away; for the surface of
the leaf directly over the end of the worm’s body became deeply pitted.
On another occasion a worm suddenly lost its hold on a flat leaf; and the
anterior end of the body was momentarily seen to be cup-formed. Worms
can attach themselves to an object beneath water in the same manner; and
I saw one thus dragging away a submerged slice of an onion-bulb.
The edges of fresh or nearly fresh leaves affixed to the ground were
often nibbled by the worms; and sometimes the epidermis and all the
parenchyma on one side was gnawed completely away over a considerable
space; the epidermis alone on the opposite side being left quite clean.
The veins were never touched, and leaves were thus sometimes partly
converted into skeletons. As worms have no teeth and as their mouths
consist of very soft tissue, it may be presumed that they consume by
means of suction the edges and the parenchyma of fresh leaves, after they
have been softened by the digestive fluid. They cannot attack such
strong leaves as those of sea-kale or large and thick leaves of ivy;
though one of the latter after it had become rotten was reduced in parts
to the state of a skeleton.
Worms seize leaves and other objects, not only to serve as food, but for
plugging up the mouths of their burrows; and this is one of their
strongest instincts. They sometimes work so energetically that Mr. D. F.
Simpson, who has a small walled garden where worms abound in Bayswater,
informs me that on a calm damp evening he there heard so extraordinary a
rustling noise from under a tree from which many leaves had fallen, that
he went out with a light and discovered that the noise was caused by many
worms dragging the dry leaves and squeezing them into the burrows. Not
only leaves, but petioles of many kinds, some flower-peduncles, often
decayed twigs of trees, bits of paper, feathers, tufts of wool and
horse-hairs are dragged into their burrows for this purpose. I have seen
as many as seventeen petioles of a Clematis projecting from the mouth of
one burrow, and ten from the mouth of another. Some of these objects,
such as the petioles just named, feathers, &c., are never gnawed by
worms. In a gravel-walk in my garden I found many hundred leaves of a
pine-tree (_P. austriaca_ or _nigricans_) drawn by their bases into
burrows. The surfaces by which these leaves are articulated to the
branches are shaped in as peculiar a manner as is the joint between the
leg-bones of a quadruped; and if these surfaces had been in the least
gnawed, the fact would have been immediately visible, but there was no
trace of gnawing. Of ordinary dicotyledonous leaves, all those which are
dragged into burrows are not gnawed. I have seen as many as nine leaves
of the lime-tree drawn into the same burrow, and not nearly all of them
had been gnawed; but such leaves may serve as a store for future
consumption. Where fallen leaves are abundant, many more are sometimes
collected over the mouth of a burrow than can be used, so that a small
pile of unused leaves is left like a roof over those which have been
partly dragged in.
A leaf in being dragged a little way into a cylindrical burrow is
necessarily much folded or crumpled. When another leaf is drawn in, this
is done exteriorly to the first one, and so on with the succeeding
leaves; and finally all become closely folded and pressed together.
Sometimes the worm enlarges the mouth of its burrow, or makes a fresh one
close by, so as to draw in a still larger number of leaves. They often
or generally fill up the interstices between the drawn-in leaves with
moist viscid earth ejected from their bodies; and thus the mouths of the
burrows are securely plugged. Hundreds of such plugged burrows may be
seen in many places, especially during the autumnal and early winter
months. But, as will hereafter be shown, leaves are dragged into the
burrows not only for plugging them up and for food, but for the sake of
lining the upper part or mouth.
When worms cannot obtain leaves, petioles, sticks, &c., with which to
plug up the mouths of their burrows, they often protect them by little
heaps of stones; and such heaps of smooth rounded pebbles may frequently
be seen on gravel-walks. Here there can be no question about food. A
lady, who was interested in the habits of worms, removed the little heaps
of stones from the mouths of several burrows and cleared the surface of
the ground for some inches all round. She went out on the following
night with a lantern, and saw the worms with their tails fixed in their
burrows, dragging the stones inwards by the aid of their mouths, no doubt
by suction. “After two nights some of the holes had 8 or 9 small stones
over them; after four nights one had about 30, and another 34 stones.”
{58} One stone—which had been dragged over the gravel-walk to the mouth
of a burrow weighed two ounces; and this proves how strong worms are.
But they show greater strength in sometimes displacing stones in a
well-trodden gravel-walk; that they do so, may be inferred from the
cavities left by the displaced stones being exactly filled by those lying
over the mouths of adjoining burrows, as I have myself observed.
Work of this kind is usually performed during the night; but I have
occasionally known objects to be drawn into the burrows during the day.
What advantage the worms derive from plugging up the mouths of their
burrows with leaves, &c., or from piling stones over them, is doubtful.
They do not act in this manner at the times when they eject much earth
from their burrows; for their castings then serve to cover the mouths.
When gardeners wish to kill worms on a lawn, it is necessary first to
brush or rake away the castings from the surface, in order that the
lime-water may enter the burrows. {59a} It might be inferred from this
fact that the mouths are plugged up with leaves, &c., to prevent the
entrance of water during heavy rain; but it may be urged against this
view that a few, loose, well-rounded stones are ill-adapted to keep out
water. I have moreover seen many burrows in the perpendicularly cut
turf-edgings to gravel-walks, into which water could hardly flow, as well
plugged as burrows on a level surface. It is not probable that the plugs
or piles of stones serve to conceal the burrows from scolopendras, which,
according to Hoffmeister, {59b} are the bitterest enemies of worms, or
from the larger species of Carabus and Staphylinus which attack them
ferociously, for these animals are nocturnal, and the burrows are opened
at night. May not worms when the mouth of the burrow is protected be
able to remain with safety with their heads close to it, which we know
that they like to do, but which costs so many of them their lives? Or
may not the plugs check the free ingress of the lowest stratum of air,
when chilled by radiation at night, from the surrounding ground and
herbage? I am inclined to believe in this latter view: firstly, because
when worms were kept in pots in a room with a fire, in which case cold
air could not enter the burrows, they plugged them up in a slovenly
manner; and secondarily, because they often coat the upper part of their
burrows with leaves, apparently to prevent their bodies from coming into
close contact with the cold damp earth. Mr. E. Parfitt has suggested to
me that the mouths of the burrows are closed in order that the air within
them may be kept thoroughly damp, and this seems the most probable
explanation of the habit. But the plugging-up process may serve for all
the above purposes.
Whatever the motive may be, it appears that worms much dislike leaving
the mouths of their burrows open. Nevertheless they will reopen them at
night, whether or not they can afterwards close them. Numerous open
burrows may be seen on recently-dug ground, for in this case the worms
eject their castings in cavities left in the ground, or in the old
burrows instead of piling them over the mouths of their burrows, and they
cannot collect objects on the surface by which the mouths might be
protected. So again on a recently disinterred pavement of a Roman villa
at Abinger (hereafter to be described) the worms pertinaciously opened
their burrows almost every night, when these had been closed by being
trampled on, although they were rarely able to find a few minute stones
wherewith to protect them.
_Intelligence shown by worms in their manner of plugging up their
burrows_.—If a man had to plug up a small cylindrical hole, with such
objects as leaves, petioles or twigs, he would drag or push them in by
their pointed ends; but if these objects were very thin relatively to the
size of the hole, he would probably insert some by their thicker or
broader ends. The guide in his case would be intelligence. It seemed
therefore worth while to observe carefully how worms dragged leaves into
their burrows; whether by their tips or bases or middle parts. It seemed
more especially desirable to do this in the case of plants not natives to
our country; for although the habit of dragging leaves into their burrows
is undoubtedly instinctive with worms, yet instinct could not tell them
how to act in the case of leaves about which their progenitors knew
nothing. If, moreover, worms acted solely through instinct or an
unvarying inherited impulse, they would draw all kinds of leaves into
their burrows in the same manner. If they have no such definite
instinct, we might expect that chance would determine whether the tip,
base or middle was seized. If both these alternatives are excluded,
intelligence alone is left; unless the worm in each case first tries many
different methods, and follows that alone which proves possible or the
most easy; but to act in this manner and to try different methods makes a
near approach to intelligence.
In the first place 227 withered leaves of various kinds, mostly of
English plants, were pulled out of worm-burrows in several places. Of
these, 181 had been drawn into the burrows by or near their tips, so that
the foot-stalk projected nearly upright from the mouth of the burrow; 20
had been drawn in by their bases, and in this case the tips projected
from the burrows; and 26 had been seized near the middle, so that these
had been drawn in transversely and were much crumpled. Therefore 80 per
cent. (always using the nearest whole number) had been drawn in by the
tip, 9 per cent. by the base or foot-stalk, and 11 per cent. transversely
or by the middle. This alone is almost sufficient to show that chance
does not determine the manner in which leaves are dragged into the
burrows.
Of the above 227 leaves, 70 consisted of the fallen leaves of the common
lime-tree, which is almost certainly not a native of England. These
leaves are much acuminated towards the tip, and are very broad at the
base with a well-developed foot-stalk. They are thin and quite flexible
when half-withered. Of the 70, 79 per cent. had been drawn in by or near
the tip; 4 per cent. by or near the base; and 17 per cent. transversely
or by the middle. These proportions agree very closely, as far as the
tip is concerned, with those before given. But the percentage drawn in
by the base is smaller, which may be attributed to the breadth of the
basal part of the blade. We here, also, see that the presence of a
foot-stalk, which it might have been expected would have tempted the
worms as a convenient handle, has little or no influence in determining
the manner in which lime leaves are dragged into the burrows. The
considerable proportion, viz., 17 per cent., drawn in more or less
transversely depends no doubt on the flexibility of these half-decayed
leaves. The fact of so many having been drawn in by the middle, and of
some few having been drawn in by the base, renders it improbable that the
worms first tried to draw in most of the leaves by one or both of these
methods, and that they afterwards drew in 79 per cent. by their tips; for
it is clear that they would not have failed in drawing them in by the
base or middle.
The leaves of a foreign plant were next searched for, the blades of which
were not more pointed towards the apex than towards the base. This
proved to be the case with those of a laburnum (a hybrid between _Cytisus
alpinus_ and _laburnum_) for on doubling the terminal over the basal
half, they generally fitted exactly; and when there was any difference,
the basal half was a little the narrower. It might, therefore, have been
expected that an almost equal number of these leaves would have been
drawn in by the tip and base, or a slight excess in favour of the latter.
But of 73 leaves (not included in the first lot of 227) pulled out of
worm-burrows, 63 per cent. had been drawn in by the tip; 27 per cent. by
the base, and 10 per cent. transversely. We here see that a far larger
proportion, viz., 27 per cent. were drawn in by the base than in the case
of lime leaves, the blades of which are very broad at the base, and of
which only 4 per cent. had thus been drawn in. We may perhaps account
for the fact of a still larger proportion of the laburnum leaves not
having been drawn in by the base, by worms having acquired the habit of
generally drawing in leaves by their tips and thus avoiding the
foot-stalk. For the basal margin of the blade in many kinds of leaves
forms a large angle with the foot-stalk; and if such a leaf were drawn in
by the foot-stalk, the basal margin would come abruptly into contact with
the ground on each side of the burrow, and would render the drawing in of
the leaf very difficult.
Nevertheless worms break through their habit of avoiding the foot-stalk,
if this part offers them the most convenient means for drawing leaves
into their burrows. The leaves of the endless hybridised varieties of
the Rhododendron vary much in shape; some are narrowest towards the base
and others towards the apex. After they have fallen off, the blade on
each side of the midrib often becomes curled up while drying, sometimes
along the whole length, sometimes chiefly at the base, sometimes towards
the apex. Out of 28 fallen leaves on one bed of peat in my garden, no
less than 23 were narrower in the basal quarter than in the terminal
quarter of their length; and this narrowness was chiefly due to the
curling in of the margins. Out of 36 fallen leaves on another bed, in
which different varieties of the Rhododendron grew, only 17 were narrower
towards the base than towards the apex. My son William, who first called
my attention to this case, picked up 237 fallen leaves in his garden
(where the Rhododendron grows in the natural soil) and of these 65 per
cent. could have been drawn by worms into their burrows more easily by
the base or foot-stalk than by the tip; and this was partly due to the
shape of the leaf and in a less degree to the curling in of the margins:
27 per cent. could have been drawn in more easily by the tip than by the
base: and 8 per cent. with about equal ease by either end. The shape of
a fallen leaf ought to be judged of before one end has been drawn into a
burrow, for after this has happened, the free end, whether it be the base
or apex, will dry more quickly than the end imbedded in the damp ground;
and the exposed margins of the free end will consequently tend to become
more curled inwards than they were when the leaf was first seized by the
worm. My son found 91 leaves which had been dragged by worms into their
burrows, though not to a great depth; of these 66 per cent. had been
drawn in by the base or foot-stalk; and 34 per cent. by the tip. In this
case, therefore, the worms judged with a considerable degree of
correctness how best to draw the withered leaves of this foreign plant
into their burrows; notwithstanding that they had to depart from their
usual habit of avoiding the foot-stalk.
On the gravel-walks in my garden a very large number of leaves of three
species of Pinus (_P. austriaca_, _nigricans_ and _sylvestris_) are
regularly drawn into the mouths of worm burrows. These leaves consist of
two so-called needles, which are of considerable length in the two first
and short in the last named species, and are united to a common base; and
it is by this part that they are almost invariably drawn into the
burrows. I have seen only two or at most three exceptions to this rule
with worms in a state of nature. As the sharply pointed needles diverge
a little, and as several leaves are drawn into the same burrow, each tuft
forms a perfect _chevaux de frise_. On two occasions many of these tufts
were pulled up in the evening, but by the following morning fresh leaves
had been pulled in, and the burrows were again well protected. These
leaves could not be dragged into the burrows to any depth, except by
their bases, as a worm cannot seize hold of the two needles at the same
time, and if one alone were seized by the apex, the other would be
pressed against the ground and would resist the entry of the seized one.
This was manifest in the above mentioned two or three exceptional cases.
In order, therefore, that worms should do their work well, they must drag
pine-leaves into their burrows by their bases, where the two needles are
conjoined. But how they are guided in this work is a perplexing
question.
This difficulty led my son Francis and myself to observe worms in
confinement during several nights by the aid of a dim light, while they
dragged the leaves of the above named pines into their burrows. They
moved the anterior extremities of their bodies about the leaves, and on
several occasions when they touched the sharp end of a needle they
withdrew suddenly as if pricked. But I doubt whether they were hurt, for
they are indifferent to very sharp objects, and will swallow even
rose-thorns and small splinters of glass. It may also be doubted,
whether the sharp ends of the needles serve to tell them that this is the
wrong end to seize; for the points were cut off many leaves for a length
of about one inch, and fifty-seven of them thus treated were drawn into
the burrows by their bases, and not one by the cut-off ends. The worms
in confinement often seized the needles near the middle and drew them
towards the mouths of their burrows; and one worm tried in a senseless
manner to drag them into the burrow by bending them. They sometimes
collected many more leaves over the mouths of their burrows (as in the
case formerly mentioned of lime-leaves) than could enter them. On other
occasions, however, they behaved very differently; for as soon as they
touched the base of a pine-leaf, this was seized, being sometimes
completely engulfed in their mouths, or a point very near the base was
seized, and the leaf was then quickly dragged or rather jerked into their
burrows. It appeared both to my son and myself as if the worms instantly
perceived as soon as they had seized a leaf in the proper manner. Nine
such cases were observed, but in one of them the worm failed to drag the
leaf into its burrow, as it was entangled by other leaves lying near. In
another case a leaf stood nearly upright with the points of the needles
partly inserted into a burrow, but how placed there was not seen; and
then the worm reared itself up and seized the base, which was dragged
into the mouth of the burrow by bowing the whole leaf. On the other
hand, after a worm had seized the base of a leaf, this was on two
occasions relinquished from some unknown motive.
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 16 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 now 72 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 and 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 2½ to 4½ 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.
The fallen petioles of our native ash-tree were next observed, and the
rule with most objects, viz., that a large majority are dragged into the
burrows by the more pointed end, had not here been followed; and this
fact much surprised me at first. These petioles vary in length from 5 to
8½ inches; they are thick and fleshy towards the base, whence they taper
gently towards the apex, which is a little enlarged and truncated where
the terminal leaflet had been originally attached. Under some ash-trees
growing in a grass-field, 229 petioles were pulled out of worm burrows
early in January, and of these 51.5 per cent. had been drawn in by the
base, and 48.5 per cent. by the apex. This anomaly was however readily
explained as soon as the thick basal part was examined; for in 78 out of
103 petioles, this part had been gnawed by worms, just above the
horse-shoe shaped articulation. In most cases there could be no mistake
about the gnawing; for ungnawed petioles which were examined after being
exposed to the weather for eight additional weeks had not become more
disintegrated or decayed near the base than elsewhere. It is thus
evident that the thick basal end of the petiole is drawn in not solely
for the sake of plugging up the mouths of the burrows, but as food. Even
the narrow truncated tips of some few petioles had been gnawed; and this
was the case in 6 out of 37 which were examined for this purpose. Worms,
after having drawn in and gnawed the basal end, often push the petioles
out of their burrows; and then drag in fresh ones, either by the base for
food, or by the apex for plugging up the mouth more effectually. Thus,
out of 37 petioles inserted by their tips, 5 had been previously drawn in
by the base, for this part had been gnawed. Again, I collected a handful
of petioles lying loose on the ground close to some plugged-up burrows,
where the surface was thickly strewed with other petioles which
apparently had never been touched by worms; and 14 out of 47 (_i.e._
nearly one-third), after having had their bases gnawed had been pushed
out of the burrows and were now lying on the ground. From these several
facts we may conclude that worms draw in some petioles of the ash by the
base to serve as food, and others by the tip to plug up the mouths of
their burrows in the most efficient manner.
The petioles of _Robinia pseudo-acacia_ vary from 4 or 5 to nearly 12
inches in length; they are thick close to the base before the softer
parts have rotted off, and taper much towards the upper end. They are so
flexible that I have seen some few doubled up and thus drawn into the
burrows of worms. Unfortunately these petioles were not examined until
February, by which time the softer parts had completely rotted off, so
that it was impossible to ascertain whether worms had gnawed the bases,
though this is in itself probable. Out of 121 petioles extracted from
burrows early in February, 68 were imbedded by the base, and 53 by the
apex. On February 5 all the petioles which had been drawn into the
burrows beneath a Robinia, were pulled up; and after an interval of
eleven days, 35 petioles had been again dragged in, 19 by the base, and
16 by the apex. Taking these two lots together, 56 per cent. were drawn
in by the base, and 44 per cent. by the apex. As all the softer parts
had long ago rotted off, we may feel sure, especially in the latter case,
that none had been drawn in as food. At this season, therefore, worms
drag these petioles into their burrows indifferently by either end, a
slight preference being given to the base. This latter fact may be
accounted for by the difficulty of plugging up a burrow with objects so
extremely thin as are the upper ends. In support of this view, it may be
stated that out of the 16 petioles which had been drawn in by their upper
ends, the more attenuated terminal portion of 7 had been previously
broken off by some accident.
_Triangles of paper_.—Elongated triangles were cut out of moderately
stiff writing-paper, which was rubbed with raw fat on both sides, so as
to prevent their becoming excessively limp when exposed at night to rain
and dew. The sides of all the triangles were three inches in length,
with the bases of 120 one inch, and of the other 183 half an inch in
length. These latter triangles were very narrow or much acuminated. {79}
As a check on the observations presently to be given, similar triangles
in a damp state were seized by a very narrow pair of pincers at different
points and at all inclinations with reference to the margins, and were
then drawn into a short tube of the diameter of a worm-burrow. If seized
by the apex, the triangle was drawn straight into the tube, with its
margins infolded; if seized at some little distance from the apex, for
instance at half an inch, this much was doubled back within the tube. So
it was with the base and basal angles, though in this case the triangles
offered, as might have been expected, much more resistance to being drawn
in. If seized near the middle the triangle was doubled up, with the apex
and base left sticking out of the tube. As the sides of the triangles
were three inches in length, the result of their being drawn into a tube
or into a burrow in different ways, may be conveniently divided into
three groups: those drawn in by the apex or within an inch of it; those
drawn in by the base or within an inch of it; and those drawn in by any
point in the middle inch.
In order to see how the triangles would be seized by worms, some in a
damp state were given to worms kept in confinement. They were seized in
three different manners in the case of both the narrow and broad
triangles: viz., by the margin; by one of the three angles, which was
often completely engulfed in their mouths; and lastly, by suction applied
to any part of the flat surface. If lines parallel to the base and an
inch apart, are drawn across a triangle with the sides three inches in
length, it will be divided into three parts of equal length. Now if
worms seized indifferently by chance any part, they would assuredly seize
on the basal part or division far oftener than on either of the two other
divisions. For the area of the basal to the apical part is as 5 to 1, so
that the chance of the former being drawn into a burrow by suction, will
be as 5 to 1, compared with the apical part. The base offers two angles
and the apex only one, so that the former would have twice as good a
chance (independently of the size of the angles) of being engulfed in a
worm’s mouth, as would the apex. It should, however, be stated that the
apical angle is not often seized by worms; the margin at a little
distance on either side being preferred. I judge of this from having
found in 40 out of 46 cases in which triangles had been drawn into
burrows by their apical ends, that the tip had been doubled back within
the burrow for a length of between 1/20 of an inch and 1 inch. Lastly,
the proportion between the margins of the basal and apical parts is as 3
to 2 for the broad, and 2½ to 2 for the narrow triangles. From these
several considerations it might certainly have been expected, supposing
that worms seized hold of the triangles by chance, that a considerably
larger proportion would have been dragged into the burrows by the basal
than by the apical part; but we shall immediately see how different was
the result.
Triangles of the above specified sizes were scattered on the ground in
many places and on many successive nights near worm-burrows, from which
the leaves, petioles, twigs, &c., with which they had been plugged, were
removed. Altogether 303 triangles were drawn by worms into their
burrows: 12 others were drawn in by both ends, but as it was impossible
to judge by which end they had been first seized, these are excluded. Of
the 303, 62 per cent. had been drawn in by the apex (using this term for
all drawn in by the apical part, one inch in length); 15 per cent. by the
middle; and 23 per cent. by the basal part. If they had been drawn
indifferently by any point, the proportion for the apical, middle and
basal parts would have been 33.3 per cent. for each; but, as we have just
seen, it might have been expected that a much larger proportion would
have been drawn in by the basal than by any other part. As the case
stands, nearly three times as many were drawn in by the apex as by the
base. If we consider the broad triangles by themselves, 59 per cent.
were drawn in by the apex, 25 per cent. by the middle, and 16 per cent.
by the base. Of the narrow triangles, 65 per cent. were drawn in by the
apex, 14 per cent, by the middle, and 21 per cent. by the base; so that
here those drawn in by the apex were more than 3 times as many as those
drawn in by the base. We may therefore conclude that the manner in which
the triangles are drawn into the burrows is not a matter of chance.
In eight cases, two triangles had been drawn into the same burrow, and in
seven of these cases, one had been drawn in by the apex and the other by
the base. This again indicates that the result is not determined by
chance. Worms appear sometimes to revolve in the act of drawing in the
triangles, for five out of the whole lot had been wound into an irregular
spire round the inside of the burrow. Worms kept in a warm room drew 63
triangles into their burrows; but, as in the case of the pine-leaves,
they worked in a rather careless manner, for only 44 per cent. were drawn
in by the apex, 22 per cent. by the middle, and 33 per cent. by the base.
In five cases, two triangles were drawn into the same burrow.
It may be suggested with much apparent probability that so large a
proportion of the triangles were drawn in by the apex, not from the worms
having selected this end as the most convenient for the purpose, but from
having first tried in other ways and failed. This notion was
countenanced by the manner in which worms in confinement were seen to
drag about and drop the triangles; but then they were working carelessly.
I did not at first perceive the importance of this subject, but merely
noticed that the bases of those triangles which had been drawn in by the
apex, were generally clean and not crumpled. The subject was afterwards
attended to carefully. In the first place several triangles which had
been drawn in by the basal angles, or by the base, or a little above the
base, and which were thus much crumpled and dirtied, were left for some
hours in water and were then well shaken while immersed; but neither the
dirt nor the creases were thus removed. Only slight creases could be
obliterated, even by pulling the wet triangles several times through my
fingers. Owing to the slime from the worms’ bodies, the dirt was not
easily washed off. We may therefore conclude that if a triangle, before
being dragged in by the apex, had been dragged into a burrow by its base
with even a slight degree of force, the basal part would long retain its
creases and remain dirty. The condition of 89 triangles (65 narrow and
24 broad ones), which had been drawn in by the apex, was observed; and
the bases of only 7 of them were at all creased, being at the same time
generally dirty. Of the 82 uncreased triangles, 14 were dirty at the
base; but it does not follow from this fact that these had first been
dragged towards the burrows by their bases; for the worms sometimes
covered large portions of the triangles with slime, and these when
dragged by the apex over the ground would be dirtied; and during rainy
weather, the triangles were often dirtied over one whole side or over
both sides. If the worms had dragged the triangles to the mouths of
their burrows by their bases, as often as by their apices, and had then
perceived, without actually trying to draw them into the burrow, that the
broader end was not well adapted for this purpose—even in this case a
large proportion would probably have had their basal ends dirtied. We
may therefore infer—improbable as is the inference—that worms are able by
some means to judge which is the best end by which to draw triangles of
paper into their burrows.
The percentage results of the foregoing observations on the manner in
which worms draw various kinds of objects into the mouths of their
burrows may be abridged as follows:—
Nature of Drawn into the Drawn in, by or Drawn in, by or
Object. burrows, by or near the near the base.
near the apex. middle.
Leaves of 80 11 9
various kinds
—of the Lime, 79 17 4
basal margin of
blade broad,
apex acuminated
—of a Laburnum, 63 10 27
basal part of
blade as narrow
as, or
sometimes
little narrower
than the apical
part
—of the 34 ... 66
Rhododendron,
basal part of
blade often
narrower than
the apical part
—of Pine-trees, ... ... 100
consisting of
two needles
arising from a
common base
Petioles of a 76 ... 24
Clematis,
somewhat
pointed at the
apex, and blunt
at the base
—of the Ash, 48.5 ... 51.5
the thick basal
end often drawn
in to serve as
food
—of Robinia, 44 ... 56
extremely thin,
especially
towards the
apex, so as to
be ill-fitted
for plugging up
the burrows
Triangles of 62 15 23
paper, of the
two sizes
—of the broad 59 25 16
ones alone
—of the narrow 65 14 21
ones alone
If we consider these several cases, we can hardly escape from the
conclusion that worms show some degree of intelligence in their manner of
plugging up their burrows. Each particular object is seized in too
uniform a manner, and from causes which we can generally understand, for
the result to be attributed to mere chance. That every object has not
been drawn in by its pointed end, may be accounted for by labour having
been saved through some being inserted by their broader or thicker ends.
No doubt worms are led by instinct to plug up their burrows; and it might
have been expected that they would have been led by instinct how best to
act in each particular case, independently of intelligence. We see how
difficult it is to judge whether intelligence comes into play, for even
plants might sometimes be thought to be thus directed; for instance when
displaced leaves re-direct their upper surfaces towards the light by
extremely complicated movements and by the shortest course. With
animals, actions appearing due to intelligence may be performed through
inherited habit without any intelligence, although aboriginally thus
acquired. Or the habit may have been acquired through the preservation
and inheritance of beneficial variations of some other habit; and in this
case the new habit will have been acquired independently of intelligence
throughout the whole course of its development. There is no _à priori_
improbability in worms having acquired special instincts through either
of these two latter means. Nevertheless it is incredible that instincts
should have been developed in reference to objects, such as the leaves of
petioles of foreign plants, wholly unknown to the progenitors of the
worms which act in the described manner. Nor are their actions so
unvarying or inevitable as are most true instincts.
As worms are not guided by special instincts in each particular case,
though possessing a general instinct to plug up their burrows, and as
chance is excluded, the next most probable conclusion seems to be that
they try in many different ways to draw in objects, and at last succeed
in some one way. But it is surprising that an animal so low in the scale
as a worm should have the capacity for acting in this manner, as many
higher animals have no such capacity. For instance, ants may be seen
vainly trying to drag an object transversely to their course, which could
be easily drawn longitudinally; though after a time they generally act in
a wiser manner, M. Fabre states {89a} that a Sphex—an insect belonging to
the same highly-endowed order with ants—stocks its nest with paralysed
grass-hoppers, which are invariably dragged into the burrow by their
antennæ. When these were cut off close to the head, the Sphex seized the
palpi; but when these were likewise cut off, the attempt to drag its prey
into the burrow was given up in despair. The Sphex had not intelligence
enough to seize one of the six legs or the ovipositor of the grasshopper,
which, as M. Fabre remarks, would have served equally well. So again, if
the paralysed prey with an egg attached to it be taken out of the cell,
the Sphex after entering and finding the cell empty, nevertheless closes
it up in the usual elaborate manner. Bees will try to escape and go on
buzzing for hours on a window, one half of which has been left open.
Even a pike continued during three months to dash and bruise itself
against the glass sides of an aquarium, in the vain attempt to seize
minnows on the opposite side. {89b} A cobra-snake was seen by Mr. Layard
{90} to act much more wisely than either the pike or the Sphex; it had
swallowed a toad lying within a hole, and could not withdraw its head;
the toad was disgorged, and began to crawl away; it was again swallowed
and again disgorged; and now the snake had learnt by experience, for it
seized the toad by one of its legs and drew it out of the hole. The
instincts of even the higher animals are often followed in a senseless or
purposeless manner: the weaver-bird will perseveringly wind threads
through the bars of its cage, as if building a nest: a squirrel will pat
nuts on a wooden floor, as if he had just buried them in the ground: a
beaver will cut up logs of wood and drag them about, though there is no
water to dam up; and so in many other cases.
Mr. Romanes, who has specially studied the minds of animals, believes
that we can safely infer intelligence, only when we see an individual
profiting by its own experience. By this test the cobra showed some
intelligence; but this would have been much plainer if on a second
occasion he had drawn a toad out of a hole by its leg. The Sphex failed
signally in this respect. Now if worms try to drag objects into their
burrows first in one way and then in another, until they at last succeed,
they profit, at least in each particular instance, by experience.
But evidence has been advanced showing that worms do not habitually try
to draw objects into their burrows in many different ways. Thus
half-decayed lime-leaves from their flexibility could have been drawn in
by their middle or basal parts, and were thus drawn into the burrows in
considerable numbers; yet a large majority were drawn in by or near the
apex. The petioles of the Clematis could certainly have been drawn in
with equal ease by the base and apex; yet three times and in certain
cases five times as many were drawn in by the apex as by the base. It
might have been thought that the foot-stalks of leaves would have tempted
the worms as a convenient handle; yet they are not largely used, except
when the base of the blade is narrower than the apex. A large number of
the petioles of the ash are drawn in by the base; but this part serves
the worms as food. In the case of pine-leaves worms plainly show that
they at least do not seize the leaf by chance; but their choice does not
appear to be determined by the divergence of the two needles, and the
consequent advantage or necessity of drawing them into their burrows by
the base. With respect to the triangles of paper, those which had been
drawn in by the apex rarely had their bases creased or dirty; and this
shows that the worms had not often first tried to drag them in by this
end.
If worms are able to judge, either before drawing or after having drawn
an object close to the mouths of their burrows, how best to drag it in,
they must acquire some notion of its general shape. This they probably
acquire by touching it in many places with the anterior extremity of
their bodies, which serves as a tactile organ. It may be well to
remember how perfect the sense of touch becomes in a man when born blind
and deaf, as are worms. If worms have the power of acquiring some
notion, however rude, of the shape of an object and of their burrows, as
seems to be the case, they deserve to be called intelligent; for they
then act in nearly the same manner as would a man under similar
circumstances.
To sum up, as chance does not determine the manner in which objects are
drawn into the burrows, and as the existence of specialized instincts for
each particular case cannot be admitted, the first and most natural
supposition is that worms try all methods until they at last succeed; but
many appearances are opposed to such a supposition. One alternative
alone is left, namely, that worms, although standing low in the scale of
organization, possess some degree of intelligence. This will strike
every one as very improbable; but it may be doubted whether we know
enough about the nervous system of the lower animals to justify our
natural distrust of such a conclusion. With respect to the small size of
the cerebral ganglia, we should remember what a mass of inherited
knowledge, with some power of adapting means to an end, is crowded into
the minute brain of a worker-ant.
_Means by which worms excavate their burrows_.—This is effected in two
ways; by pushing away the earth on all sides, and by swallowing it. In
the former case, the worm inserts the stretched out and attenuated
anterior extremity of its body into any little crevice, or hole; and
then, as Perrier remarks, {93} the pharynx is pushed forwards into this
part, which consequently swells and pushes away the earth on all sides.
The anterior extremity thus serves as a wedge. It also serves, as we
have before seen, for prehension and suction, and as a tactile organ. A
worm was placed on loose mould, and it buried itself in between two and
three minutes. On another occasion four worms disappeared in 15 minutes
between the sides of the pot and the earth, which had been moderately
pressed down. On a third occasion three large worms and a small one were
placed on loose mould well mixed with fine sand and firmly pressed down,
and they all disappeared, except the tail of one, in 35 minutes. On a
fourth occasion six large worms were placed on argillaceous mud mixed
with sand firmly pressed down, and they disappeared, except the extreme
tips of the tails of two of them, in 40 minutes. In none of these cases,
did the worms swallow, as far as could be seen, any earth. They
generally entered the ground close to the sides of the pot.
A pot was next filled with very fine ferruginous sand, which was pressed
down, well watered, and thus rendered extremely compact. A large worm
left on the surface did not succeed in penetrating it for some hours, and
did not bury itself completely until 25 hrs. 40 min. had elapsed. This
was effected by the sand being swallowed, as was evident by the large
quantity ejected from the vent, long before the whole body had
disappeared. Castings of a similar nature continued to be ejected from
the burrow during the whole of the following day.
As doubts have been expressed by some writers whether worms ever swallow
earth solely for the sake of making their burrows, some additional cases
may be given. A mass of fine reddish sand, 23 inches in thickness, left
on the ground for nearly two years, had been penetrated in many places by
worms; and their castings consisted partly of the reddish sand and partly
of black earth brought up from beneath the mass. This sand had been dug
up from a considerable depth, and was of so poor a nature that weeds
could not grow on it. It is therefore highly improbable that it should
have been swallowed by the worms as food. Again in a field near my house
the castings frequently consist of almost pure chalk, which lies at only
a little depth beneath the surface; and here again it is very improbable
that the chalk should have been swallowed for the sake of the very little
organic matter which could have percolated into it from the poor
overlying pasture. Lastly, a casting thrown up through the concrete and
decayed mortar between the tiles, with which the now ruined aisle of
Beaulieu Abbey had formerly been paved, was washed, so that the coarser
matter alone was left. This consisted of grains of quartz, micaceous
slate, other rocks, and bricks or tiles, many of them from 1/20 to 1/10
inch in diameter. No one will suppose that these grains were swallowed
as food, yet they formed more than half of the casting, for they weighed
19 grains, the whole casting having weighed 33 grains. Whenever a worm
burrows to a depth of some feet in undisturbed compact ground, it must
form its passage by swallowing the earth; for it is incredible that the
ground could yield on all sides to the pressure of the pharynx when
pushed forwards within the worm’s body.
That worms swallow a larger quantity of earth for the sake of extracting
any nutritious matter which it may contain than for making their burrows,
appears to me certain. But as this old belief has been doubted by so
high an authority as Claparède, evidence in its favour must be given in
some detail. There is no _à priori_ improbability in such a belief, for
besides other annelids, especially the _Arenicola marina_, which throws
up such a profusion of castings on our tidal sands, and which it is
believed thus subsists, there are animals belonging to the most distinct
classes, which do not burrow, but habitually swallow large quantities of
sand; namely, the molluscan Onchidium and many Echinoderms. {97}
If earth were swallowed only when worms deepened their burrows or made
new ones, castings would be thrown up only occasionally; but in many
places fresh castings may be seen every morning, and the amount of earth
ejected from the same burrow on successive days is large. Yet worms do
not burrow to a great depth, except when the weather is very dry or
intensely cold. On my lawn the black vegetable mould or humus is only
about 5 inches in thickness, and overlies light-coloured or reddish
clayey soil: now when castings are thrown up in the greatest profusion,
only a small proportion are light coloured, and it is incredible that the
worms should daily make fresh burrows in every direction in the thin
superficial layer of dark-coloured mould, unless they obtained nutriment
of some kind from it. I have observed a strictly analogous case in a
field near my house where bright red clay lay close beneath the surface.
Again on one part of the Downs near Winchester the vegetable mould
overlying the chalk was found to be only from 3 to 4 inches in thickness;
and the many castings here ejected were as black as ink and did not
effervesce with acids; so that the worms must have confined themselves to
this thin superficial layer of mould, of which large quantities were
daily swallowed. In another place at no great distance the castings were
white; and why the worms should have burrowed into the chalk in some
places and not in others, I am unable to conjecture.
Two great piles of leaves had been left to decay in my grounds, and
months after their removal, the bare surface, several yards in diameter,
was so thickly covered during several months with castings that they
formed an almost continuous layer; and the large number of worms which
lived here must have subsisted during these months on nutritious matter
contained in the black earth.
The lowest layer from another pile of decayed leaves mixed with some
earth was examined under a high power, and the number of spores of
various shapes and sizes which it contained was astonishingly great; and
these crushed in the gizzards of worms may largely aid in supporting
them. Whenever castings are thrown up in the greatest number, few or no
leaves are drawn into the burrows; for instance the turf along a
hedgerow, about 200 yards in length, was daily observed in the autumn
during several weeks, and every morning many fresh castings were seen;
but not a single leaf was drawn into these burrows. These castings from
their blackness and from the nature of the subsoil could not have been
brought up from a greater depth than 6 or 8 inches. On what could these
worms have subsisted during this whole time, if not on matter contained
in the black earth? On the other hand, whenever a large number of leaves
are drawn into the burrows, the worms seem to subsist chiefly on them,
for few earth-castings are then ejected on the surface. This difference
in the behaviour of worms at different times, perhaps explains a
statement by Claparède, namely, that triturated leaves and earth are
always found in distinct parts of their intestines.
Worms sometimes abound in places where they can rarely or never obtain
dead or living leaves; for instance, beneath the pavement in well-swept
courtyards, into which leaves are only occasionally blown. My son Horace
examined a house, one corner of which had subsided; and he found here in
the cellar, which was extremely damp, many small worm-castings thrown up
between the stones with which the cellar was paved; and in this case it
is improbable that the worms could ever have obtained leaves. Mr. A. C.
Horner confirms this account, as he has seen castings in the cellars of
his house, which is an old one at Tonbridge.
But the best evidence, known to me, of worms subsisting for at least
considerable periods of time solely on the organic matter contained in
earth, is afforded by some facts communicated to me by Dr. King. Near
Nice large castings abound in extraordinary numbers, so that 5 or 6 were
often found within the space of a square foot. They consist of fine,
pale-coloured earth, containing calcareous matter, which after having
passed through the bodies of worms and being dried, coheres with
considerable force. I have reason to believe that these castings had
been formed by species of Perichæta, which have been naturalized here
from the East. {101} They rise like towers, with their summits often a
little broader than their bases, sometimes to a height of above 3 and
often to a height of 2½ inches. The tallest of those which were measured
was 3.3 inches in height and 1 inch in diameter. A small cylindrical
passage runs up the centre of each tower, through which the worm ascends
to eject the earth which it has swallowed, and thus to add to its height.
A structure of this kind would not allow leaves being easily dragged from
the surrounding ground into the burrows; and Dr. King, who looked
carefully, never saw even a fragment of a leaf thus drawn in. Nor could
any trace be discovered of the worms having crawled down the exterior
surfaces of the towers in search of leaves; and had they done so, tracks
would almost certainly have been left on the upper part whilst it
remained soft. It does not, however, follow that these worms do not draw
leaves into their burrows during some other season of the year, at which
time they would not build up their towers.
From the several foregoing cases, it can hardly be doubted that worms
swallow earth, not only for the sake of making their burrows, but for
obtaining food. Hensen, however, concludes from his analyses of mould
that worms probably could not live on ordinary vegetable mould, though he
admits that they might be nourished to some extent by leaf-mould. {102}
But we have seen that worms eagerly devour raw meat, fat, and dead worms;
and ordinary mould can hardly fail to contain many ova, larvæ, and small
living or dead creatures, spores of cryptogamic plants, and micrococci,
such as those which give rise to saltpetre. These various organisms,
together with some cellulose from any leaves and roots not utterly
decayed, might well account for such large quantities of mould being
swallowed by worms. It may be worth while here to recall the fact that
certain species of Utricularia, which grow in damp places in the tropics,
possess bladders beautifully constructed for catching minute subterranean
animals; and these traps would not have been developed unless many small
animals inhabited such soil.
_The depth to which worms penetrate_, _and the construction of their
burrows_.—Although worms usually live near the surface, yet they burrow
to a considerable depth during long-continued dry weather and severe
cold. In Scandinavia, according to Eisen, and in Scotland, according to
Mr. Lindsay Carnagie, the burrows run down to a depth of from 7 to 8
feet; in North Germany, according to Hoffmeister, from 6 to 8 feet, but
Hensen says, from 3 to 6 feet. This latter observer has seen worms
frozen at a depth of 1½ feet beneath the surface. I have not myself had
many opportunities for observation, but I have often met with worms at
depths of 3 to 4 feet. In a bed of fine sand overlying the chalk, which
had never been disturbed, a worm was cut into two at 55 inches, and
another was found here at Down in December at the bottom of its burrow,
at 61 inches beneath the surface. Lastly, in earth near an old Roman
Villa, which had not been disturbed for many centuries, a worm was met
with at a depth of 66 inches; and this was in the middle of August.
The burrows run down perpendicularly, or more commonly a little
obliquely. They are said sometimes to branch, but as far as I have seen
this does not occur, except in recently dug ground and near the surface.
They are generally, or as I believe invariably, lined with a thin layer
of fine, dark-coloured earth voided by the worms; so that they must at
first be made a little wider than their ultimate diameter. I have seen
several burrows in undisturbed sand thus lined at a depth of 4 ft. 6 in.;
and others close to the surface thus lined in recently dug ground. The
walls of fresh burrows are often dotted with little globular pellets of
voided earth, still soft and viscid; and these, as it appears, are spread
out on all sides by the worm as it travels up or down its burrow. The
lining thus formed becomes very compact and smooth when nearly dry, and
closely fits the worm’s body. The minute reflexed bristles which project
in rows on all sides from the body, thus have excellent points of
support; and the burrow is rendered well adapted for the rapid movement
of the animal. The lining appears also to strengthen the walls, and
perhaps saves the worm’s body from being scratched. I think so because
several burrows which passed through a layer of sifted coal-cinders,
spread over turf to a thickness of 1½ inch, had been thus lined to an
unusual thickness. In this case the worms, judging from the castings,
had pushed the cinders away on all sides and had not swallowed any of
them. In another place, burrows similarly lined, passed through a layer
of coarse coal-cinders, 3½ inches in thickness. We thus see that the
burrows are not mere excavations, but may rather be compared with tunnels
lined with cement.
The mouths of the burrow are in addition often lined with leaves; and
this is an instinct distinct from that of plugging them up, and does not
appear to have been hitherto noticed. Many leaves of the Scotch-fir or
pine (_Pinus sylvestris_) were given to worms kept in confinement in two
pots; and when after several weeks the earth was carefully broken up, the
upper parts of three oblique burrows were found surrounded for lengths of
7, 4, and 3½ inches with pine-leaves, together with fragments of other
leaves which had been given the worms as food. Glass beads and bits of
tile, which had been strewed on the surface of the soil, were stuck into
the interstices between the pine-leaves; and these interstices were
likewise plastered with the viscid castings voided by the worms. The
structures thus formed cohered so well, that I succeeded in removing one
with only a little earth adhering to it. It consisted of a slightly
curved cylindrical case, the interior of which could be seen through
holes in the sides and at either end. The pine-leaves had all been drawn
in by their bases; and the sharp points of the needles had been pressed
into the lining of voided earth. Had this not been effectually done, the
sharp points would have prevented the retreat of the worms into their
burrows; and these structures would have resembled traps armed with
converging points of wire, rendering the ingress of an animal easy and
its egress difficult or impossible. The skill shown by these worms is
noteworthy and is the more remarkable, as the Scotch pine is not a native
of this district.
After having examined these burrows made by worms in confinement, I
looked at those in a flower-bed near some Scotch pines. These had all
been plugged up in the ordinary manner with the leaves of this tree,
drawn in for a length of from 1 to 1½ inch; but the mouths of many of
them were likewise lined with them, mingled with fragments of other kinds
of leaves, drawn in to a depth of 4 or 5 inches. Worms often remain, as
formerly stated, for a long time close to the mouths of their burrows,
apparently for warmth; and the basket-like structures formed of leaves
would keep their bodies from coming into close contact with the cold damp
earth. That they habitually rested on the pine-leaves, was rendered
probable by their clean and almost polished surfaces.
The burrows which run far down into the ground, generally, or at least
often, terminate in a little enlargement or chamber. Here, according to
Hoffmeister, one or several worms pass the winter rolled up into a ball.
Mr. Lindsay Carnagie informed me (1838) that he had examined many burrows
over a stone-quarry in Scotland, where the overlying boulder-clay and
mould had recently been cleared away, and a little vertical cliff thus
left. In several cases the same burrow was a little enlarged at two or
three points one beneath the other; and all the burrows terminated in a
rather large chamber, at a depth of 7 or 8 feet from the surface. These
chambers contained many small sharp bits of stone and husks of
flax-seeds. They must also have contained living seeds, for on the
following spring Mr. Carnagie saw grass-plants sprouting out of some of
the intersected chambers. I found at Abinger in Surrey two burrows
terminating in similar chambers at a depth of 36 and 41 inches, and these
were lined or paved with little pebbles, about as large as mustard seeds;
and in one of the chambers there was a decayed oat-grain, with its husk.
Hensen likewise states that the bottoms of the burrows are lined with
little stones; and where these could not be procured, seeds, apparently
of the pear, had been used, as many as fifteen having been carried down
into a single burrow, one of which had germinated. {108} We thus see how
easily a botanist might be deceived who wished to learn how long deeply
buried seeds remained alive, if he were to collect earth from a
considerable depth, on the supposition that it could contain only seeds
which had long lain buried. It is probable that the little stones, as
well as the seeds, are carried down from the surface by being swallowed;
for a surprising number of glass beads, bits of tile and of glass were
certainly thus carried down by worms kept in pots; but some may have been
carried down within their mouths. The sole conjecture which I can form
why worms line their winter-quarters with little stones and seeds, is to
prevent their closely coiled-up bodies from coming into close contact
with the surrounding cold soil; and such contact would perhaps interfere
with their respiration which is effected by the skin alone.
A worm after swallowing earth, whether for making its burrow or for food,
soon comes to the surface to empty its body. The ejected earth is
thoroughly mingled with the intestinal secretions, and is thus rendered
viscid. After being dried it sets hard. I have watched worms during the
act of ejection, and when the earth was in a very liquid state it was
ejected in little spurts, and by a slow peristaltic movement when not so
liquid. It is not cast indifferently on any side, but with some care,
first on one and then on another side; the tail being used almost like a
trowel. When a worm comes to the surface to eject earth, the tail
protrudes, but when it collects leaves its head must protrude. Worms
therefore must have the power of turning round in their closely-fitting
burrows; and this, as it appears to us, would be a difficult feat. As
soon as a little heap has been formed, the worm apparently avoids, for
the sake of safety, protruding its tail; and the earthy matter is forced
up through the previously deposited soft mass. The mouth of the same
burrow is used for this purpose for a considerable time. In the case of
the tower-like castings (see Fig. 2) near Nice, and of the similar but
still taller towers from Bengal (hereafter to be described and figured),
a considerable degree of skill is exhibited in their construction. Dr.
King also observed that the passage up these towers hardly ever ran in
the same exact line with the underlying burrow, so that a thin
cylindrical object such as a haulm of grass, could not be passed down the
tower into the burrow; and this change of direction probably serves in
some manner as a protection.
Worms do not always eject their castings on the surface of the ground.
When they can find any cavity, as when burrowing in newly turned-up
earth, or between the stems of banked-up plants, they deposit their
castings in such places. So again any hollow beneath a large stone lying
on the surface of the ground, is soon filled up with their castings.
According to Hensen, old burrows are habitually used for this purpose;
but as far as my experience serves, this is not the case, excepting with
those near the surface in recently dug ground. I think that Hensen may
have been deceived by the walls of old burrows, lined with black earth,
having sunk in or collapsed; for black streaks are thus left, and these
are conspicuous when passing through light-coloured soil, and might be
mistaken for completely filled-up burrows.
It is certain that old burrows collapse in the course of time; for as we
shall see in the next chapter, the fine earth voided by worms, if spread
out uniformly, would form in many places in the course of a year a layer
0.2 of an inch in thickness; so that at any rate this large amount is not
deposited within the old unused burrows. If the burrows did not
collapse, the whole ground would be first thickly riddled with holes to a
depth of about ten inches, and in fifty years a hollow unsupported space,
ten inches in depth, would be left. The holes left by the decay of
successively formed roots of trees and plants must likewise collapse in
the course of time.
The burrows of worms run down perpendicularly or a little obliquely, and
where the soil is at all argillaceous, there is no difficulty in
believing that the walls would slowly flow or slide inwards during very
wet weather. When, however, the soil is sandy or mingled with many small
stones, it can hardly be viscous enough to flow inwards during even the
wettest weather; but another agency may here come into play. After much
rain the ground swells, and as it cannot expand laterally, the surface
rises; during dry weather it sinks again. For instance, a large flat
stone laid on the surface of a field sank 3.33 mm. whilst the weather was
dry between May 9th and June 13th, and rose 1.91 mm, between September
7th and 19th of the same year, much rain having fallen during the latter
part of this time. During frosts and thaws the movements were twice as
great. These observations were made by my son Horace, who will hereafter
publish an account of the movements of this stone during successive wet
and dry seasons, and of the effects of its being undermined by worms.
Now when the ground swells, if it be penetrated by cylindrical holes,
such as worm-burrows, their walls will tend to yield and be pressed
inwards; and the yielding will be greater in the deeper parts (supposing
the whole to be equally moistened) from the greater weight of the
superincumbent soil which has to be raised, than in the parts near the
surface. When the ground dries, the walls will shrink a little and the
burrows will be a little enlarged. Their enlargement, however, through
the lateral contraction of the ground, will not be favoured, but rather
opposed, by the weight of the superincumbent soil.
_Distribution of Worms_.—Earth-worms are found in all parts of the world,
and some of the genera have an enormous range. {113} They inhabit the
most isolated islands; they abound in Iceland, and are known to exist in
the West Indies, St. Helena, Madagascar, New Caledonia and Tahiti. In
the Antarctic regions, worms from Kerguelen Land have been described by
Ray Lankester; and I found them in the Falkland Islands. How they reach
such isolated islands is at present quite unknown. They are easily
killed by salt-water, and it does not appear probable that young worms or
their egg-capsules could be carried in earth adhering to the feet or
beaks of land-birds. Moreover Kerguelen Land is not now inhabited by any
land-bird.
In this volume we are chiefly concerned with the earth cast up by worms,
and I have gleaned a few facts on this subject with respect to distant
lands. Worms throw up plenty of castings in the United States. In
Venezuela, castings, probably ejected by species of Urochæta, are common
in the gardens and fields, but not in the forests, as I hear from Dr.
Ernst of Caracas. He collected 156 castings from the court-yard of his
house, having an area of 200 square yards. They varied in bulk from half
a cubic centimeter to five cubic centimeters, and were on an average
three cubic centimeters. They were, therefore, of small size in
comparison with those often found in England; for six large castings from
a field near my house averaged 16 cubic centimeters. Several species of
earth-worms are common in St. Catharina in South Brazil, and Fritz Müller
informs me “that in most parts of the forests and pasture-lands, the
whole soil, to a depth of a quarter of a metre, looks as if it had passed
repeatedly through the intestines of earth-worms, even where hardly any
castings are to be seen on the surface.” A gigantic but very rare
species is found there, the burrows of which are sometimes even two
centimeters or nearly 0.8 of an inch in diameter, and which apparently
penetrate the ground to a great depth.
In the dry climate of New South Wales, I hardly expected that worms would
be common; but Dr. G. Krefft of Sydney, to whom I applied, after making
inquiries from gardeners and others, and from his own observations,
informs me that their castings abound. He sent me some collected after
heavy rain, and they consisted of little pellets, about 0.15 inch in
diameter; and the blackened sandy earth of which they were formed still
cohered with considerable tenacity.
The late Mr. John Scott of the Botanic Gardens near Calcutta made many
observations for me on worms living under the hot and humid climate of
Bengal. The castings abound almost everywhere, in jungles and in the
open ground, to a greater degree, as he thinks, than in England. After
the water has subsided from the flooded rice-fields, the whole surface
very soon becomes studded with castings—a fact which much surprised Mr.
Scott, as he did not know how long worms could survive beneath water.
They cause much trouble in the Botanic garden, “for some of the finest of
our lawns can be kept in anything like order only by being almost daily
rolled; if left undisturbed for a few days they become studded with large
castings.” These closely resemble those described as abounding near
Nice; and they are probably the work of a species of Perichæta. They
stand up like towers, with an open passage in the centre.
[Picture: Fig. 3: A tower-like casting. Fig. 4: A casting from the
Nilgiri Mountains]
A figure of one of these castings from a photograph is here given (Fig.
3). The largest received by me was 3½ inches in height and 1.35 inch in
diameter; another was only ¾ inch in diameter and 2¾ in height. In the
following year, Mr. Scott measured several of the largest; one was 6
inches in height and nearly 1½ in diameter: two others were 5 inches in
height and respectively 2 and rather more than 2½ inches in diameter.
The average weight of the 22 castings sent to me was 35 grammes (1¼ oz.);
and one of them weighed 44.8 grammes (or 2 oz.). All these castings were
thrown up either in one night or in two. Where the ground in Bengal is
dry, as under large trees, castings of a different kind are found in vast
numbers: these consist of little oval or conical bodies, from about the
1/20 to rather above 1/10 of an inch in length. They are obviously
voided by a distinct species of worms.
The period during which worms near Calcutta display such extraordinary
activity lasts for only a little over two months, namely, during the cool
season after the rains. At this time they are generally found within
about 10 inches beneath the surface. During the hot season they burrow
to a greater depth, and are then found coiled up and apparently
hybernating. Mr. Scott has never seen them at a greater depth than 2½
feet, but has heard of their having been found at 4 feet. Within the
forests, fresh castings may be found even during the hot season. The
worms in the Botanic garden, during the cool and dry season, draw many
leaves and little sticks into the mouths of their burrows, like our
English worms; but they rarely act in this manner during the rainy
season.
Mr. Scott saw worm-castings on the lofty mountains of Sikkim in North
India. In South India Dr. King found in one place, on the plateau of the
Nilgiris, at an elevation of 7000 feet, “a good many castings,” which are
interesting for their great size. The worms which eject them are seen
only during the wet season, and are reported to be from 12 to 15 inches
in length, and as thick as a man’s little finger. These castings were
collected by Dr. King after a period of 110 days without any rain; and
they must have been ejected either during the north-east or more probably
during the previous south-west monsoon; for their surfaces had suffered
some disintegration and they were penetrated by many fine roots. A
drawing is here given (Fig. 4) of one which seems to have best retained
its original size and appearance. Notwithstanding some loss from
disintegration, five of the largest of these castings (after having been
well sun-dried) weighed each on an average 89.5 grammes, or above 3 oz.;
and the largest weighed 123.14 grammes, or 4⅓ oz.,—that is, above a
quarter of a pound! The largest convolutions were rather more than one
inch in diameter; but it is probable that they had subsided a little
whilst soft, and that their diameters had thus been increased. Some had
flowed so much that they now consisted of a pile of almost flat confluent
cakes. All were formed of fine, rather light-coloured earth, and were
surprisingly hard and compact, owing no doubt to the animal matter by
which the particles of earth had been cemented together. They did not
disintegrate, even when left for some hours in water. Although they had
been cast up on the surface of gravelly soil, they contained extremely
few bits of rock, the largest of which was only 0.15 inch in diameter.
Dr. King saw in Ceylon a worm about 2 feet in length and ½ inch in
diameter; and he was told that it was a very common species during the
wet season. These worms must throw up castings at least as large as
those on the Nilgiri Mountains; but Dr. King saw none during his short
visit to Ceylon.
Sufficient facts have now been given, showing that worms do much work in
bringing up fine earth to the surface in most or all parts of the world,
and under the most different climates.