is waved between the follower's tentacles and occasionally makes contact, contributing to recognition between the slugs. The leading slug seemingly pauses to wait for the follower, tail waving, if the follower falls too far behind. Courtship Once a mate is found, land slugs undergo a prolonged courtship phase prior to copulation. Courtship can and usually does last for several hours. The two individuals get into position along the periphery of an imaginary circle with their heads towards the other's tail, and circle one another. The slugs crawl in a clockwise direction so that their right sides, containing the genital pores, continually face each other. This circling may serve to assess whether the other is ready to mate; the speed of circling generally decreases as courtship progresses. The slugs then lie close together, remaining in the antiparallel configuration. Each protrudes from its genital pore a sarcobelum, a highly manoeuvrable penile structure, with which it strokes its mate. Secretions, presumably from the glandular part of the penis wall, are probably transferred by the sarcobela during this mutual stroking. These secretions have several possible functions, including physiological stimulation and synchronisation, partner labelling or manipulation. Labelling may benefit the donor by helping it to avoid mating with individuals it has already mated with. Alternatively or additionally, labelling may serve as an indication to conspecifics that this individual has recently mated and therefore should be avoided, either because its sperm reserves are possibly depleted or, more likely, because of sperm competition. In the latter case, the secretion transferrer's risk of being subjected to sperm competition is also reduced. Besides labelling, another possibility is that the secretion is absorbed by the partner, in whom it acts as a manipulative allohormone. An allohormone is a substance that is transferred from one member of a species to another and that "induces a direct behavioural or physiological response". The secretion may then cause the recipient to refrain from mating with additional partners, to uptake more of the donor's sperm and to digest less of it, or to increase egg fertilisation and laying using the donated sperm. For more information on one use of this kind of hormone, see the article love dart. Species differ in the way stroking occurs and the duration of stroking, but all species get progressively closer during courtship and stroking intensity increases until it stops just before the start of mating. At this point, the