surface. However, there is much intraspecific variation. An extraordinary example of external sperm transfer is given by the great grey slug (Limax maximus). A pair of slugs climb a tree or wall to an overhang and suspend together head down from a 10–25 cm long mucus strand before everting their penises and exchanging sperm in mid air. Evolutionary explanations for this behaviour are unclear. Internal sperm transfer In other cases, sperm transfer is internal and takes place by inserting the everted penis into the partner slug. The sperm is conveyed via sperm packets or spermatophores into the recipient's spermatheca duct. and then released from the tail of the spermatophore. Only a very small percentage escape to higher regions of the female reproductive system for storage (in the spermatheca) and later fertilisation. Intromission is simultaneously reciprocal in some species such as Ariolimax dolichophallus and unilateral in others such as Ariolimax californicus Apophallation After sperm transfer a process termed apophallation, or penis amputation, may take place. This is a lengthy process whereby the penis of one or both slugs is chewed off using the radula. In the case where one penis is chewed off, this is usually done by the other slug. A replacement penis does not grow back. Apophallation is most widely found in Ariolimax species, which have internal sperm exchange. In these slugs, apophallation is only an occasional occurrence which follows a period of struggle and pulling by the owner. This suggests it is a ‘last resort’ if the penis becomes stuck in the genital tract of the partner slug. Once a penis is severed, it is usually eaten by the partner. The reason the penis gets eaten is unclear, although it could simply be a source of nutrients. Apophallation may be made necessary in order for separation to occur by the partner tightly gripping the penis via a special muscle of the female reproductive tract. Apophallation may benefit the amputator because it prevents the amputee from mating again in the near future, which would result in sperm competition. The amputee is at least prevented from mating in the future as a male, which could divert resources from producing eggs with the amputator's sperm. In addition to Ariolimax species, autoapophallation (self-amputation) has been documented in Deroceras laeve. Here, it is a regular part of mating, although the penis does not get trapped in the other slug (as there is external