Slugs
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| Missing image Unknown_slug_on_rhubarb.jpg Red Slug (Arion rufus) Red Slug (Arion rufus) - red color form | ||||||||||||||
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Slugs are gastropods without shells or with very small shells, in contrast with snails from which they evolved, which have a prominent shell. Although they undergo torsion (twisting) during development, their bodies are streamlined and worm-like, and so show little external evidence of it. This same basic design developed independently in several different groups, the largest being the sea slugs or nudibranchs. Other slugs are found on land, but their soft, slimy bodies are prone to desiccation, so they are confined to moist environments. Among the various species are the grey field slug, Deroceras reticulatus; the garden slug, Arion hortensis; and the banana slug, Ariolimax columbianus.
Like snails, slugs have two pairs of 'feelers' or tentacles on their head. The upper pair--optical tentacles--are light sensors; the lower pair provides the sense of smell. Both pairs are retractable and can be regrown if lost. On top of the slug, behind the head, is the saddle-shaped mantle, and under this are the genital opening and anus. The mantle also has a hole, the pneumostome, for respiration. The slug moves by rythmic muscular action of its foot.
Most slugs eat leaves, fungus, and decaying vegetable material, but some are predators and most also eat carrion including dead of their own kind. Slugs eat using a radula, a rough, tongue-like organ with many tiny tooth-like denticles.
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Parts of a slug
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Mucus
Slugs produce two types of mucus: one which is thin and watery, and another which is thick and sticky. Both are hygroscopic. The thin mucus is spread out from the centre of the foot to the edges,the thick mucus spreads out from front to back.
Mucus is very important to slugs as it helps them move around, and contains fibres which prevent the slug from sliding down vertical surfaces. Mucus also provides protection against predators and helps retain moisture. Some species use slime cords to lower themselves on the ground.
Reproduction and life cycle
Slugs are hermaphroditic: having both female and male reproductive organs. Once a slug has located a mate they encircle each other and sperm is exchanged through their protruding genitalia. A few days later hundreds of eggs are laid in holes in the ground. Although some species hibernate over the winter in temperate climates, in most species the adults die in the autumn.
Various species of slug can also reproduce via tiny "darts" of sperm which they fling in the direction of their mate's genitalia.
Predation, defense and pest contol
Frogs, toads, hedgehogs, and some birds and beetles are natural slug predators. Slugs, when attacked, can contract their body, making themselves harder and more compact and thus more difficult for many animals to get a hold. The unpleasant taste of the mucus is also a deterrent.
Some slugs are notable garden pests and there are various methods of controlling them (see Pest control of slugs) including, slug pellets, beer traps, salt, physical bariers and biological pest controls.