A re-evaluation of silk measurement by the cecropia caterpillar (Hyalophora cecropia) during cocoon construction reveals use of a silk odometer that is temporally regulated

The late 5th instar caterpillar of the cecropia silk moth (Hyalophora cecropia) spins a silken cocoon with a distinct, multilayered architecture. The cocoon construction program, first described by the seminal work of Van der Kloot and Williams, consists of a highly ordered sequence of events. We perform behavioral experiments to re-evaluate the original cecropia work, which hypothesized that the length of silk that passes through the spinneret controls the orderly execution of each of the discrete events of cocoon spinning. We confirm and extend by three-dimensional scanning and quantitative measurements of silk weights that if cocoon construction is interrupted, upon re-spinning, the caterpillar continues the cocoon program from where it left off. We also confirm and extend by quantitative measurements of silk weights that cecropia caterpillars will not bypass any of the sections of the cocoon during the construction process, even if presented with a pre-spun section of a cocoon spun by another caterpillar. Blocking silk output inhibits caterpillars from performing normal spinning behaviors used for cocoon construction. Surprisingly, unblocking silk output 24-hr later did not restart the cocoon construction program, suggesting the involvement of a temporally-defined interval timer. We confirm with surgical reductions of the silk glands that it is the length of silk itself that matters, rather than the total amount of silk extracted by individuals. We used scanning electron microscopy to directly show that either mono- or dual-filament silk (i.e., equal silk lengths but which vary in their total amount of silk extracted) can be used to construct equivalent cocoons of normal size and that contain the relevant layers. We propose that our findings, taken together with the results of prior studies, strongly support the hypothesis that the caterpillar uses a silk “odometer” to measure the length of silk extracted during cocoon construction but does so in a temporally regulated manner. We further postulate that our examination of the anatomy of the silk spinning apparatus and ablating spinneret sensory output provides evidence that silk length measurement occurs upstream of output from the spinneret.

桕蚕蛾（cecropia silk moth，学名Hyalophora cecropia）的5龄末期幼虫会织造具有独特多层结构的丝质茧。由Van der Kloot与Williams的开创性研究首次阐明的茧织造程序，包含一系列高度有序的行为事件。本研究通过行为学实验重新评估了最初针对该物种的相关研究，该研究提出假说：经由吐丝器（spinneret）喷出的丝长，可调控茧织造过程中每一项独立行为事件的有序执行。本研究通过三维扫描与丝重定量测量，验证并拓展了相关结论：当茧织造过程被中断后，幼虫在重新吐丝时会从之前中断的节点处继续执行茧织造程序。本研究还通过丝重定量测量验证并拓展了相关结论：桕蚕蛾幼虫在织造茧的过程中，不会跳过任何一段茧体结构，即便被提供了其他幼虫预先织好的茧段。阻断丝液输出会抑制幼虫进行正常的茧织造吐丝行为。令人意外的是，在24小时后恢复丝液输出，却无法重新启动茧织造程序，这表明该过程存在一个受时间调控的间隔计时器。本研究通过对丝腺进行外科缩减操作验证：真正起调控作用的是丝本身的长度，而非单条幼虫所抽取的总丝量。本研究利用扫描电子显微镜（scanning electron microscopy）直接证实：单丝或双丝纤维（即丝长一致但总抽取丝量存在差异的丝纤维）均可用于织造尺寸正常、结构完整的同类型茧体。结合已有研究结果与本研究的发现，我们提出强有力的证据支持下述假说：桕蚕蛾幼虫会利用丝“里程计”来计量茧织造过程中抽取的丝长，且该计量过程受时间调控。我们进一步推测：本研究对吐丝器解剖结构的解析，以及对吐丝器感觉输出的消融实验，为“丝长计量发生于吐丝器输出之前”这一论断提供了证据。