Larval reproduction - larval reproduction

This is a form of parthenogenesis, in which reproductive cells mature and begin to develop inside the mother's body while the female's body is still in the larval stage. In the dipteran Tamaka, egg cells inside the body begin to develop during the larval stage, resulting in the production of 7-30 larvae. These larvae feed on the mother and grow, eventually eating through the parent's body wall to emerge. After repeating this process for several generations, they metamorphose into adults. The liver fluke, a flatworm trematode, also reproduces as a larva. The larvae, called miracidia, hatch from the eggs and enter the body of a Lymnaea stagnalis (an intermediate host), where they become larvae called sporocysts. The sporocysts produce numerous secondary larvae called regia through larval reproduction. These regia further produce third larvae called cercariae within the body (cercariae can also be produced from sporocysts). The cercariae swim out of the intermediate host, the shellfish, attach to waterside grass, and are then eaten by animals such as cattle, where they mature in their livers.

[Masayuki Uchibori]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

雌の体がまだ幼生であるのに、生殖細胞が成熟し母体内で発生を始めることで、単為生殖の一形態である。双翅(そうし)類のタマカでは幼生（幼虫）の時期に体内の卵細胞が発生を始め、7～30匹の幼虫が生じる。この幼虫は母体を食べて成長し、やがて親の体壁を食い破って外に出る。同じことを何代か繰り返したのち変態して成虫となる。扁形(へんけい)動物吸虫類の肝蛭(かんてつ)も幼生生殖を行う。卵から孵化(ふか)したミラシジウムという幼生がモノアラガイ（中間宿主）の体内に入り、スポロシストという幼生になる。スポロシストは幼生生殖によって多数のレジアという第二の幼生を生じる。このレジアはさらにその体内にセルカリアという第三の幼生を生じる（スポロシストからセルカリアが生じる場合もある）。セルカリアは中間宿主である貝から泳ぎ出して水辺の草に付着し、ウシなどに食べられてその肝臓内で成熟する。

［内堀雅行］

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