Charles Darwin’s last major work, published when he was 72
years old, was on earthworms and the way they transform vegetable matter and
soil profiles. He understood that earthworms ingested as they burrowed and the
main beneficial effect was provided by their faecal material, as well as the
soil aeration that resulted from their burrows. The faecal material is
sometimes egested on the surface to provide casts, but it is mostly retained
within soil profiles and earthworms also bring organic matter from the surface
to be eaten, and then egested, below ground.
There was no doubting Darwin’s
enthusiasm for his subject, and the hours of observation that he devoted to it.
In the Conclusion of The Formation of
Vegetable Mould, he writes [1]:
Worms have played a more important
part in the history of the world than most persons would at first suppose..
..Worms prepare the ground in an excellent manner for the growth of
fibrous-rooted plants and for seedlings of all kinds. They periodically expose
the [vegetable] mould to the air, and sift it so that no stones larger than the
particles which they can swallow are left in it. They mingle the whole
intimately together, like a gardener who prepares fine soil for his choicest
plants.. ..They allow the air to penetrate deeply into the ground.. ..The
plough is one of the most ancient and most valuable on man’s inventions; but
long before he existed the land was in fact regularly ploughed, and still continues
to be thus ploughed by earth-worms. It may be doubted whether there are many
other animals which have played so important a part in the history of the
world, as have these lowly organised creatures.
It is impossible not to disagree with Darwin’s comment on
the significance of earthworms, although we now recognise that their efforts
are aided by bacteria and fungi that release nutrients during the re-packaging activities
of the worms. During their feeding, worms
only assimilate a fraction of the organic matter that they ingest, so their
role in transformation is a major one, as they are numerous and feed continuously
as they burrow. Any gardener making a compost heap is familiar with the changes
to organic matter brought about by worms, aided by a range of other invertebrates
that are similarly ingesting, assimilating little, and egesting compacted
faecal material. This compaction ensures that micro-organisms that break down
complex organic molecules to release nutrients are packed close to potential
substrates.In soil, everything becomes mixed with mineral particles.
Although there are at least 6000 species of earthworms, the
majority conform to the same body plan and we are hard pressed to tell one
species from another. When we look closely at earthworms, we see that they are
segmented and this is not just an external feature, as the body is separated
into segments internally, with sheets of tissue called septa separating each segment
from that preceding and that following. The body wall of the earthworm is made
up largely of muscle and a resistant covering that protects the worm from
abrasion, together with an epidermis containing secretory cells. The long
tubular gut runs along the length of the worm, passing through the septa, and
the space between the gut and the muscles is filled with coelomic fluid to form
a hydrostatic skeleton. This fluid is the secret of the earthworm’s
characteristic method of locomotion, together with the use of small spines
called chaetae (or setae) that are projected through the body wall by muscles.
Movement, and burrow formation, is achieved by means of the
alternate contraction of the circular and longitudinal muscles in each segment
(see below, taken from [2]). When the circular muscles contract, the segment
becomes narrower and the pressure on the coelomic fluid causes it to lengthen. Conversely,
contraction of the longitudinal muscles shortens the segment and makes it
broader. By having a sequence of contractions of each muscle set in succeeding
segments, the result is a wave of contraction and expansion that passes along
the worm's body. The most anterior segments act as a "battering ram"
during circular muscle contraction and the worm’s body is anchored by means of
the segments where longitudinal muscles are contracting, the pressure exerted
on the burrow walls being enhanced by the pushing out of the chaetae that act
like crampons. A summary of the sequence is shown in the second diagram below, and the
worms use a similar method to move over surfaces, on the few occasions when
they are not burrowing. This use of hydraulic pressure provides an elegant solution
to moving without the use of limbs, and lubrication is aided by mucus secreted
over the body surface from glands in the epidermis. It's another wonder of evolution.
As mentioned earlier, we take the soil moving, and
composting, activities of earthworms for granted and they are indirectly of
great importance to us; but there are other ways in which earthworms are useful to
humans. For example, they provide bait for fishing and earthworm oil is used to entice fish
to attack plastic lures [3]. Further uses of earthworms come in medicine, where
extracts have been shown to enhance the healing of skin wounds [4] and in the
reduction of cell death in heart muscles after infection [5].
One way of
exploiting earthworms that has been little explored in Western culture is as food
for humans, despite worms being an important part of the diet of several other
mammals and some birds. The cross-sectional view of a body segment (shown
above) reveals the size of the two sets of muscles used in locomotion and it is
a surprise that this source of animal protein has not been investigated more thoroughly. There
are recipes available, and a few can be given here, but a warning is required
before trying them, as the gut contents of earthworms may be gritty and it is
probably best to ensure the worms have egested as much mineral material as
possible before using them in cooking. Experiments with feeding worms on sage
plants, other culinary herbs, and grains such as corn meal, might be worthwhile
before preparing the worms for cooking: there are also interesting
possibilities for flavour combinations from such experiments, with the worms
providing their own stuffing. Here are some links to dishes you might like to
try:
Earthworm chow: http://www.cooks.com/recipe/46m94rl/earthworm-chow.html
Earthworm sauté
(and other recipes): http://www.eattheweeds.com/cooking-with-earthworms-2/
Quiche Lorraine
avec Ver de Terre: https://recipeland.com/recipe/v/quiche-lorraine-avec-ver-de-ter-24860
- or eat them just as they are (the preferred method of
young children....)
Darwin was right to emphasise the importance of earthworms
in the history of the world and that includes the extensive period of pre-human
history. It’s a pity they aren't more recognised – and utilised.
[1] Charles
Darwin (1881) The Formation of Vegetable Mould, through the action of worms,
with observations on their habits. London, John Murray.
[4] M. Grdiša,
M. Popović and T. Hrženjak (2004) Stimulation of growth
factor synthesis in skin wounds using tissue extract (G-90) from the earthworm Eissenia foetida. Cell Biochemistry and Function 22:373-378.
[5] Ping-Chun Li and 9 authors (2015) Impact of LPS-induced
cardiomyoblast cell apoptosis inhibited by earthworm extracts. Cardiovascular Toxicology 15:172-179.
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