Department of Biology, Taiyuan Normal University, Jinzhong, 030619, China.
Department of Entomology, China Agricultural University, Beijing, China.
BMC Evol Biol. 2020 Jun 29;20(1):79. doi: 10.1186/s12862-020-01643-2.
Metamorphosis remains one of the most complicated and poorly understood processes in insects. This is particularly so for the very dynamic transformations that take place within the pupal sheath of holometabolous insects. Only few studies address these transformations especially with regard to cranial structures of those holometabolous species where the larval and adult forms have a similar diet. It thus remains unclear to what extent the internal structures undergo histolysis and rebuilding. Here, the development of the brain and skeleto-muscular system of the head of Chrysopa pallens (Rambur, 1838) is studied. This species is a predator of aphids in the larval and adult stage.
We used micro-computed-tomography (μ-CT) to study the transformations of the larval, prepupal and pupal head within the cocoon. We first assessed the morphological differences and similarities between the stages. We then determined the point in time when the compound eyes appear and describe the re-orientation of the head capsule which transforms the prognathous larva into a hypognathous adult. The internal head muscles are distinctly more slender in larvae than adults. In addition, the adults have a significantly larger brain which is likely needed for the processing of the signals obtained by the adults vastly expanded sensory organs that are presumably needed for dispersal and mating. Our study shows that the histolysis and modification of the inner muscles and skeletal elements take place within the prepupa. The central nervous system persists throughout metamorphosis but its morphology changes significantly.
Our study reveals that not only the inner structures, but also the outer morphology continues to change after the final larval moult. The adult cuticle and internal structures form gradually within the cocoon. The histolysis and rebuilding begin with the skeletal elements and is followed by changes in the central nervous system before it concludes with modifications of the musculature. This order of events is likely ancestral for Holometabola because it is also known from Hymenoptera, Diptera, Mecoptera, and Coleoptera.
变态仍然是昆虫中最复杂和最不被理解的过程之一。对于完全变态昆虫的蛹鞘内发生的非常动态的变态尤其如此。只有少数研究涉及这些变态，特别是对于幼虫和成虫形式具有相似饮食的那些完全变态物种的颅结构。因此，内部结构经历组织溶解和重建的程度尚不清楚。在这里，研究了 Chrysopa pallens（Rambur，1838）幼虫、预蛹和蛹头部的大脑和骨骼肌肉系统的发育。该物种在幼虫和成虫阶段是蚜虫的捕食者。
我们使用微计算机断层扫描（μ-CT）来研究茧内幼虫、预蛹和蛹头部的变态。我们首先评估了这些阶段之间的形态差异和相似性。然后，我们确定了复眼出现的时间，并描述了头壳的重新定向，该头壳将前口幼虫转变为后口成虫。内部头部肌肉在幼虫中明显比成虫更细长。此外，成虫的大脑明显更大，这可能是成虫处理通过其大大扩展的感觉器官获得的信号所必需的，这些感觉器官可能是为了分散和交配而需要的。我们的研究表明，组织溶解和内部肌肉和骨骼元素的修饰发生在预蛹期内。中枢神经系统在变态过程中持续存在，但形态发生了显著变化。
我们的研究表明，不仅内部结构，而且外部形态在最后一次幼虫蜕皮后仍继续变化。成虫的外骨骼和内部结构在茧内逐渐形成。组织溶解和重建首先从骨骼元素开始，然后是中枢神经系统的变化，最后是肌肉的变化。这种事件顺序可能是对完全变态的祖先，因为它也已知来自膜翅目、双翅目、长翅目和鞘翅目。
