The sheep liver fluke, a digenetic trematode, is a parasite whose life-cycle takes it through three - count three! - hosts. This particular fluke, Fasciola hepatica, reaches sexual maturity in the liver of sheep, laying eggs which are passed in the fecal matter to snails. Asexual reproduction occurs in the snail, accompanied by irritation of the snail's salivary glands, and the asexually produced offspring (cercariae) are secreted in salivic slime-balls which ants apparently find very tasty. Upon ingestion by an ant, most, but not all, of the cercariae migrate into the blood-filled hemocoel of the ant where they remain in temporary hibernation. Those cercariae not capable of reaching sexual maturity, however, migrate into the esophageal region of the ant. A temperature based ant behavior-modification mechanism takes over, resulting in evening lock-jaw. If all goes well, this evening lock-jaw occurs while the ant is happily feasting upon a blade of grass. The ant remains stuck until the morning sun warms him up, making him easy prey for grazing sheep. The remaining cercariae in the ingested ant turn into little worms which travel through the intenstine, into the liver, and finally to their home in the sheep's bile ducts where they reach sexual maturity and repeat the process. This fascinating scientific excursion was prompted by the pseusdo-copulatory behavior of Cnemidophorus uniparens, an all-female species of lizard that has no (obvious) need for bumping genitalia, as recounted in The Golem: What You Should Know about Science by Collins and Pinch. (The relation? One benefit of sexual reproduction is parasite resistance.)
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