Snails and slugs possess a head which bears unjointed tentacles, and
also eyes, which, in many cases, are borne on the tips of these. The
foot serves for creeping, and is flattened in a sole-like way on its
under side. The mantle is generally small, and limited to the dorsal
side of the animal. The usually spiral shell covers therefore only a
small part of the body, but the rest of the body can also be mostly or
entirely drawn back into it. Snails possess jaws, but the chief organ
used in attacking the plant structures is the swollen tongue, covered by
a rasping plate (radula), on the surface of which regular rows of teeth
are arranged. The rest of the tongue consists of muscle, and two small
cartilages are also found within it. The teeth stick up when the tongue
is protruded, and the whole apparatus forms a kind of rasp, by the
backward and forward movements of which the food is reduced to small
fragments. Marine snails breathe by gills, which are found in the
mantle-cavity, while most of the freshwater and all the land forms
(“Pulmonata”) breathe by means of the inner wall of the mantle-cavity,
which thus serves as a lung. Some members of the group are
hermaphrodite, while the sexes are distinct in others; the first is the
case in all terrestrial forms. Such snails pair, however, mutually
fertilizing each other. They are not able to fertilize themselves. It is
only among land snails and slugs that injurious kinds are found. Species
of the genus _Helix_, _e.g._ the =Edible= or =Roman Snail= (_H.
pomatia_) and the =Field Snail= (_H. nemoralis_), may do harm in
fruit-culture; =Slugs= are very injurious to agriculture (_Limax_ and
_Arion_); in Limax, the respiratory opening is situated behind the
middle of the right margin of the mantle, while the genital opening lies
behind the right feeler; in Arion, the respiratory opening is situated
on the front edge of the mantle, and the genital opening immediately in
front of it. Any kind of snail or slug may be harmful, especially if it
attacks young plants. Some kinds, however, live chiefly in woods,
feeding upon toadstools and dung, perhaps also on bark and weeds (_e.g._
dandelion). This is especially true of the larger kinds, but these too,
if they multiply very rapidly, may effect much damage in gardens and
fields. The commonest slugs are—
The =Common Black Slug= (_Arion ater_).—About four inches long, and
generally black.
The =Grey Field Slug= (_Limax agrestis_, Fig. 144).—Four-fifths of an
inch long, small and smooth, with dark tentacles, grey back, often a
reddish sheen, and bright grey or white under side.
The =Large Slug= (_Limax maximus_).—Up to six inches long, with black
flecks on the hump-like mantle, and black streaks on the rest of the
body.
In a large majority of cases where slugs appear to an injurious extent,
the grey field slug is the offender, and I will therefore deal with it
in greater detail. What is said regarding the small species, however,
may be taken to apply to all the other kinds, should they become a pest.
The =Grey Field Slug= (_Limax agrestis_).
[Illustration:
FIG. 144.—The Grey Field Slug (_Limax agrestis_).
]
The life of the grey field slug is very dependent on moisture. During
dry weather it rolls itself up and hides in the ground, under a stone,
or in some such place, where it is protected as much as possible from
completely drying up. At such times it only comes to the surface to feed
at night, but does so during the day as well when the weather is wet.
The field slug is very destructive in fields and gardens, but this is
only the case in those years the great dampness of which is specially
favourable to its continuance and multiplication. The glassy eggs are
laid in heaps containing from six to fifteen, in damp concealed spots in
the ground, under moss, grass, etc., during August, September, and even
October. A single slug may lay as many as four hundred. The eggs are
able to withstand extreme cold and extreme dryness, and moisture makes
them swell out again if they have shrivelled up. In warm, damp weather
the young slugs (which are one-twelfth of an inch long in the
contracted, and two-fifths of an inch in the extended condition) hatch
out in three or four weeks. Cold weather retards the development. If the
weather is favourable, and suitable food is not wanting, they are often
almost full-grown in six weeks’ time. It is clear, from the foregoing,
that in damp years the slugs may appear in very large numbers,
especially in autumn; but it may also happen that most of the young
slugs hatch out the following spring, in which case the greatest amount
of damage will be done then. A snail may live several years, spending
the dry days and the whole of the cold season in the ground or some
other sheltered place. These slugs devour by preference delicate leaves
and stems; young grain, young clover plants, leaves of rape, cabbage,
turnips, and lettuce are their favourite food, also fallen fruit,
strawberries, cucumbers, and vegetable marrows. _Preventive Measures_:
Protection of their natural enemies: pigs, moles, shrews, ducks, fowls,
pigeons, rooks, starlings, toads. Draining the soil, as slugs flourish
most where it is moist. Sowing the winter grain as soon as possible, as
the young plants will then probably have attained a fair size when the
slugs appear in force. _Remedies_: Collection. The slugs readily crawl
on to bits of cucumber or turnip which are thrown down, and can then
easily be collected, or killed with quicklime. All substances which
readily absorb water kill slugs by taking from them the moisture which
is necessary for their existence. Cooking-salt, unslaked and slaked lime
may be employed for this purpose. It seems to me most convenient to use
freshly slaked lime, ten to twelve bushels per acre. Since the lime can
only kill the slugs when it comes into contact with their skins, it must
always be strewn on the land at a time when they are on the crawl, not
later than 8 a.m., and only in dry weather. As soon as the slugs are
touched by the lime they begin to give out a great deal of slime, and
many of them, especially the younger ones, are killed and become black.
Very many of the older slugs, however, escape death, since the tenacious
slime they give out, when the lime touches them, forms a continuous coat
like a cocoon, out of which the slug creeps, now much poorer in water.
It is therefore a good plan to strew powdered lime over a slug-infested
field twice in succession, leaving an interval of some ten or fifteen
minutes. The second strewing brings the lime into contact with those
slugs which have just crawled out of their slimy investment in a
desiccated condition. They are unable to form a second coating of slime,
and if touched for this second time with even the smallest particles of
lime they at once contract and die. If a double strewing is repeated
very early on the following morning, no slugs will be left alive on the
field so treated.