Abstract
Metamorphosis of cyclorrhaphous flies takes place inside a barrel‐like puparium, formed by the shrinking, hardening and darkening of the third‐instar larval cuticle. The opacity of this structure hampers the visualization of the morphological changes occurring inside and therefore a full understanding of the metamorphosis process. Here, we use micro‐computed tomography (micro‐CT) to describe the internal morphological changes that occur during metamorphosis of the blow fly, Calliphora vicina Robineau‐Desvoidy 1830 (Diptera: Calliphoridae) at a greater temporal resolution than anything hitherto published. The morphological changes were documented at 10% intervals of the total intra‐puparial period, and down to 2.5% intervals during the first 20% interval, when the most dramatic morphological changes occur. Moreover, the development of an internal gas bubble, which plays an essential role during early metamorphosis, was further investigated with X‐ray images and micro‐CT virtual sections. The origin of this gas bubble has been largely unknown, but micro‐CT virtual sections show that it is connected to one of the main tracheal trunks. Micro‐CT virtual sections also provided enough resolution for determining the completion of the larval‐pupal and pupal‐adult apolyses, thus enabling an accurate timing of the different intra‐puparial life stages. The prepupal, pupal, and pharate adult stages last for 7.5%, 22.5%, and 70% of the total intra‐puparial development, respectively. Furthermore, we provide for the first time quantitative data on the development of two organ systems of the blow fly: the alimentary canal and the indirect flight muscles. There is a significant and negative correlation between the volume of the indirect flight muscles and the pre‐helicoidal region of the midgut during metamorphosis. The latter occupies a large portion of the thorax during the pupal stage but narrows progressively as the indirect flight muscles increase in volume during the development of the pharate adult.
Highlights
The morphological changes shown by some insects during metamorphosis has always been both puzzling and captivating for scientists (Erezyilmaz, 2006)
At 192 hr after pupariation, part of the yellow body has been absorbed and the waste products have started to be transformed into the meconium, which is visualized in the virtual sections as an overstained body in the rectum (Figure 12c)
The first signs of the degeneration of larval tissues are reported to appear immediately after pupariation (Levy & Bautz, 1985). This is supported by Cepeda-Palacios and Scholl (2000) who showed how larval-pupal apolysis begins within 3 hr of pupariation in the sheep bot fly Oestrus ovis L. (Oestridae)
Summary
The morphological changes shown by some insects during metamorphosis has always been both puzzling and captivating for scientists (Erezyilmaz, 2006). This group roughly corresponds to the cyclorrhaphous dipterans, where the pupal stage and the following development of the adult certainly take place inside a barrellike puparium, formed from the hardening and darkening of the thirdinstar larval cuticle (Fraenkel & Bhaskaran, 1973; Martín-Vega, Hall, & Simonsen, 2016). This feature, found in only a few other insects (e.g., Strepsiptera, some Hemiptera), allows an extensive and complete histolysis of most larval tissues, as the insect lies within the rigid, protective puparium. Recent studies of morphology have been few and the studies of Robertson (1936) and Bainbridge and Bownes (1981) still stand as two of the most detailed morphological analyses on the metamorphosis of D. melanogaster, as research on this topic in recent years has been mostly focussed on molecular aspects (e.g., Thummel, 1996; Takashima, Mkrtchyan, Younossi-Hartenstein, Merriam, & Hartenstein, 2008; Rewitz, Yamanaka, & O’Connor, 2010; Rifkin, Kim, & White, 2003)
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