On the function of body piercing during copulation in earthworms

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Abstract
In many organisms, including hermaphroditic species, received sperm can be either digested or stored. If they do not get digested, the sperm are often stored for prolonged periods of time during which copulations with several partners occur. Due to these processes, sexual selection favours tactics that increase the chances of the donated sperm fertilizing the partner's eggs. One way to achieve this is by transferring a bioactive substance (an allohormone) that affects the reproductive processes of the mating partner. The hermaphroditic earthworm Lumbricus terrestris uses 40 needle-like setae (chitinous hairs) to inject a substance into the mating partner. During mating, these copulatory setae are pierced through the partner's skin. Compared to the normal (crawling) setae, these setae are longer and grooved; they are situated ventrally on segments 10, 26, and 31 to 38 (the clitellar segments). This body piercing results in the injection of the product from the setal glands which accompany these setae. We propose that the setal glands may produce an allohormone that manipulates the reproductive physiology of the mating partner. The damage and possible loss of control over fertilization as a result of body piercing hint at the existence of a sexual conflict in simultaneous hermaphrodites.