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

Abstract

The late 5 th 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 temporallydefined 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.

The Filippi’s Glands of Giant Silk Moths: To Be or Not to Be?

“…In the course of our previous study [23], we had noticed the absence of FGs in the larvae of the fifth instar of the cecropia silk moth, H. cecropia. To determine whether the absence of FGs is rather an exception in the superfamily Bombycoidae, we studied the morphology, anatomy, and development of FGs in selected silk moth and hawk moth species.…”

The Role of Filippi’s Glands in the Silk Moths Cocoon Construction

“…Thus, the presence or absence of FGs was clearly correlated with the type of cocoon spun. To further support this conclusion, we also compared all published results demonstrating the presence or absence of FGs in giant silkworms [3,16] with their respective cocoon architecture [18,19,21]. For the following species, Saturnia pavoniella, Antherina suraka, and Aglia tau, where the complete absence of FGs has been demonstrated [16] and whose cocoon structure has not yet been described in the literature, we used SEM to show that they form compact cocoons (Supplementary Materials Figure S1).…”

“…It is known that giant silk moths can spin cocoons that consist of multiple layers of silk. Based on their overall morphology, cocoons can be divided into two distinct types: (1) compact cocoons, in which all silk layers are glued together and form a single cocoon shell; (2) loose cocoons in which the silk layers form three distinct compartments: the outer shell, the intermediate silk scaffolding, and the inner shell [17][18][19][20][21].…”

Dimorphic cocoons of the robin moth, Hyalophora cecropia, reflect the existence of two distinct architectural syndromes

“…Moreover, the amount of silk used for the baggy or compact morphs is similar (Waldbauer et al, 1982;Guerra and Reppert, 2017), with the primary difference between the two cocoon morphs being how the silk is woven to construct cocoons with different architectural structures and properties. The structure of the silk produced by Hyalophora, the material used for building cocoons, consists of parallel dual silk filaments (Sehadova et al, 2020) that are similar to those of the domesticated silk moth Bombyx mori in tensile properties but different in thickness (e.g. Hyalophora fibers are much finer) and amino acid composition (e.g.…”