Asexual freshwater planarians reproduce by tearing themselves into two pieces by a process called binary fission. The resulting head and tail pieces regenerate within about a week, forming two new worms. Understanding this process of ripping oneself into two parts poses a challenging biomechanical problem. Because planarians stop “doing it” at the slightest disturbance, this remained a centuries-old puzzle. We focus on Dugesia japonica fission and show that it proceeds in three stages: a local constriction (“waist formation”), pulsation—which increases waist longitudinal stresses—and transverse rupture. We developed a linear mechanical model with a planarian represented by a thin shell. The model fully captures the pulsation dynamics leading to rupture and reproduces empirical time scales and stresses. It asserts that fission execution is a mechanical process. Furthermore, we show that the location of waist formation, and thus fission, is determined by physical constraints. Together, our results demonstrate that where and how a planarian rips itself apart during asexual reproduction can be fully explained through biomechanics.

中文翻译：

---

力学决定了淡水涡虫在何处以及如何裂变

无性淡水涡虫通过称为二元裂变的过程将自己撕成两片而繁殖。产生的头和尾巴碎片在约一周内再生，形成两个新的蠕虫。了解将自己分为两部分的过程提出了一个具有挑战性的生物力学问题。由于平面工人在丝毫不安的情况下停止“这样做”，因此这仍然是一个世纪以来的难题。我们专注于日本落叶松裂变，并表明它分三个阶段进行：局部收缩（“腰部形成”），搏动（增加腰部纵向应力）和横向破裂。我们开发了一个以薄壳为代表的平面力学的线性力学模型。该模型完全捕获了导致破裂的脉动动力学，并再现了经验时间尺度和应力。它断言裂变执行是一个机械过程。此外，我们表明腰部形成的位置以及裂变的位置是由身体约束决定的。在一起，我们的结果表明，可以通过生物力学充分解释在无性生殖过程中腓肠肌在哪里以及如何撕裂。