Figure 1

Classification scheme to identify sperm-transfer modes for internally fertilizating species. The labels a1, a2, b1 and b2 refer to the labels used in the text. The dashed arrows represent the introduction of sperm into the female system; the solid arrows represent the injection of Acps via the body wall. The sperm within the drop-shape represent semen (i.e., sperm plus non-sperm components); the sperm in the oval shape represent sperm encased in a spermatophore. Underneath each category, we have indicated several animal species that are known to use this mode of sperm and Acps transfer: a1, pond snails and fruit flies; a2, sea slugs and earthworms; b1, butterflies and springtails; b2, salamanders and land snails.