A love dart (also known as a gypsobelum, shooting darts, or just as darts) is a sharp, calcareous or chitinous dart which some hermaphroditic land snails and slugs create. Love darts are both formed and stored internally in a dart sac. These darts are made in sexually mature animals only, and are used as part of the sequence of events during courtship, before actual mating takes place. Darts are quite large compared to the size of the animal: in the case of the semi-slug genus Parmarion, the length of a dart can be up to one fifth that of the semi-slug's foot. The process of using love darts in snails is a form of sexual selection. Prior to copulation, each of the two snails (or slugs) attempts to "shoot" one (or more) darts into the other snail (or slug). There is no organ to receive the dart; this action is more analogous to stabbing, or to being shot with an arrow or flechette. The dart does not fly through the air to reach its target, but is "fired" as a contact shot. The love dart is not a penial stylet (in other words, it is not an accessory organ for sperm transfer). The exchange of sperm between both of the two land snails is a completely separate part of the mating progression. Nevertheless, recent research shows that use of the dart can strongly favor the reproductive outcome for the snail that is able to lodge a dart in its partner. This is because mucus on the dart contains an allomone (pheromone-like) compound that promotes sperm preservation mechanisms. Love darts are shaped in many distinctive ways, which vary considerably between species. What all the shapes of love darts have in common is their harpoon-like or needle-like ability to pierce.
The mating dance
Mating begins with a courting ritual. For example, in land snails of the genus Helix, including the escargot Helix pomatia, and the common garden snail Helix aspersa (also known as Cornu aspersum and Cantareus aspersus), copulation is preceded by an elaborate tactile courtship. The two snails circle around each other for up to six hours, touching with their tentacles, and biting lips and the area of the genital pore, which shows some preliminary signs of the eversion of the penis. As the snails approach mating, hydraulic pressure builds up in the blood sinus surrounding the organ housing the dart. Each snail manoeuvres to get its genital pore in the best position, close to the other snail's body. Then, when the body of one snail touches the other snail's genital pore, it triggers the firing of the dart. The darting can sometimes be so forceful that the dart ends up buried in the internal organs. It can also happen that a dart will pierce the body or head entirely, and protrude on the other side. After both snails have fired their darts, the snails copulate and exchange sperm. A snail does not have a dart to fire the very first time it mates, because the first mating is necessary to trigger the process of dart formation. Once a snail has mated, it fires a dart before some, but not all, subsequent matings. A snail often mates without having a dart to use, because it takes time to create a replacement dart. In the case of the garden snail Cornu aspersum, it takes a week for a new dart to form. The dart is shot with some variation in force, and with considerable inaccuracy, such that one-third of the darts that are fired in Cornu aspersum either fail to penetrate the skin, or miss the target altogether. Snails have only very simple visual systems and cannot see well enough to use vision to help aim the darts.
Function
Although the existence and use of love darts in snails has been known for at least several centuries, until recently the actual function of love darts was not properly understood. It was long assumed that the darts had some sort of "stimulating" function, and served to make copulation more likely. It was also suggested that darts might be a "gift" of calcium. These theories have proved to be incorrect. Recent research has led to a new understanding of the function of love darts: manipulating the recipient's snail's sperm collection mechanism, thus increasing the chances of paternity for the sender. A close look into the behavior of Cornu aspersum shows that this is achieved not by the mechanical action of the dart as it penetrates the recipient's skin, but by the mucus that coats the dart: The mucus carries an allohormone that is transferred into the recipient's hemolymph when the dart is inserted, which reconfigures the recipient's reproductive system: the bursa copulatrix (sperm digestion organ) becomes closed off, and the copulatory canal (leading to the sperm storage) is opened. This reconfiguration allows more sperm to access the sperm storage area and fertilize eggs, rather than being digested, ultimately increasing the sender's chances of paternity.
Morphology of darts The love dart, also known as a "gypsobelum", is often made of calcium carbonate which is secreted by a specialized organ within the reproductive system of several families of air-breathing snails and slugs, mainly in terrestrial pulmonate gastropod mollusks within the clade Stylommatophora. Darts can range in size from about 30 millimetres (1.2 in) long in the larger snail species, down to about 1 millimetre (0.04 in) in the smallest snails that have darts. Typically most darts are less than 5 millimetres (0.20 in) long, but they are substantial compared with the size of the animal.
There is considerable variety in both the overall shape and the cross section of the love dart. The morphology (shape and form) of the dart is species-specific. For example, individual snails of the two rather similar helicid species Cepaea hortensis and Cepaea nemoralis can sometimes only be distinguished by examining the shape of the love dart and the vaginal mucous glands (which in the anatomical diagram are marked "MG" and are positioned off the structure marked "V".)
Anatomical context
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