Insect eggshells must meet various demands of developing embryos. These demands sometimes conflict with each other; therefore, there are tradeoffs between eggshell properties, such as robustness and permeability. To meet these conflicting demands, particular eggshell structures have evolved in diverse insect species. Here, we report a rare eggshell structure found in the eggshell of a cicada, Cryptotympana facialis. This species has a prolonged egg period with embryonic diapause and a trait of humidity-inducible hatching, which would impose severe demands on the eggshell. We found that in eggs of this species, unlike many other insect eggs, a dedicated cleavage site, known as a hatching line, was formed not in the chorion but in the serosal cuticle. The hatching line was composed of a fine furrow accompanied by ridges on both sides. This furrow-ridge structure formed in the terminal phase of embryogenesis through the partial degradation of an initially thick and nearly flat cuticle layer. We showed that the permeability of the eggshell was low in the diapause stage, when the cuticle was thick, and increased with degradation of the serosal cuticle. We also demonstrated that the force required to cleave the eggshell was reduced after the formation of the hatching line. These results suggest that the establishment of the hatching line on the serosal cuticle enables flexible modification of eggshell properties during embryogenesis, and we predict that it is an adaptation to maximize the protective role of the shell during the long egg period while reducing the barrier to emerging nymphs at the time of hatching.

中文翻译：

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浆膜角质层中的阴影线的形成赋予蝉卵壳多方面的适应性功能

虫卵壳必须满足发育胚胎的各种需求。这些需求有时会相互冲突。因此，在蛋壳性能（例如坚固性和渗透性）之间需要权衡取舍。为了满足这些相互矛盾的要求，特定的蛋壳结构已在多种昆虫中进化。在这里，我们报告了在蝉Cryptotympana facialis的蛋壳中发现的罕见蛋壳结构。该物种的卵期延长，有胚胎滞育，并且具有可诱导湿度的孵化特性，这将对蛋壳提出严格的要求。我们发现，与许多其他昆虫卵不同，在该物种的卵中，不是在绒毛膜中而是在浆膜角质层中形成了一个专门的卵裂位点，即孵化线。孵化线由细沟和两侧的山脊组成。这种沟脊结构在胚胎发生的末期通过最初较厚且几乎平坦的表皮层的部分降解而形成。我们显示，在滞育阶段，当表皮较厚时，蛋壳的渗透性较低，并且随着浆膜表皮的降解而增加。我们还证明，形成孵化线后，劈开蛋壳所需的力减小了。这些结果表明，在浆膜角质层上建立孵化线可以在胚胎发生过程中灵活地改变蛋壳的性质，并且我们预测这是一种适应性方法，可以在长卵期最大限度地发挥蛋壳的保护作用，同时减少对新兴蛋壳的屏障。孵化时的若虫。