Dynamic monitoring of vital functions and tissue re-organization in Saturnia pavonia (Lepidoptera, Saturniidae) during final metamorphosis by non-invasive MRI

Tim Laussmann,Hermann Aberle,Vera Flocke,Anton G Windfelder,Klaus Lunau,Ulrich Flögel,Paul Urspruch

doi:10.1038/s41598-022-05092-3

Abstract

Magnetic resonance imaging (MRI) is the key whole-body imaging technology for observing processes within a living object providing excellent resolution and contrast between soft tissues. In the present work, we exploited the non-destructive properties of MRI to track longitudinally the dynamic changes that take place in developing pupae of the Emperor Moth (Saturnia pavonia) during the last days before eclosion. While in diapause pupae, body fluid was almost homogeneously distributed over the internal compartments, as soon as wings, legs, flight muscles and the head region were fully developed, a significant redistribution of water levels occurred between thoracic and abdominal regions. During the last two days before eclosion, the developing moths transferred substantial amounts of liquid into the gut and the labial gland, and in case of females, into developing eggs. Concomitantly, the volume of the air sacs increased drastically and their expansion/compression became clearly visible in time-resolved MR images. Furthermore, besides ventilation of the tracheal system, air sacs are likely to serve as volume reservoir for liquid transfer during development of the moths inside their pupal case. In parallel, we were able to monitor noninvasively lipid consumption, cardiac activity and haemolymph circulation during final metamorphosis.

Highlights

The Emperor Moth (Saturnia pavonia) a member of the Saturniidae family is quite common in the Palaearctic area, occupying moorland and open country
We demonstrate day-to-day shifts in tissue properties of the different organs within the developing pupae and resolve dynamic alterations caused by air sac ventilation and cardiac activity during the late phase of metamorphosis
At an in-plane resolution of ~ 30 × 30 μm[2], both major organs and tiny structures of developing Saturnia pavonia pupae became clearly visible by non-invasive magnetic resonance imaging (MRI)

Summary

Introduction

The Emperor Moth (Saturnia pavonia) a member of the Saturniidae family is quite common in the Palaearctic area, occupying moorland and open country. In species with completely closed cocoons, this liquid, originating from the labial glands, contains a proteolytic enzyme known as c ocoonase[11,12] This enzyme is capable of digesting sericin, a protein glue between the silk fibers of the cocoon[12]. Technological improvements especially in X-ray micro-computed tomography (micro-CT) have led to a number of studies on insect metamorphosis, especially in flies[13] With this technique internal morphological alterations can be visualized at very high-resolution[14,15,16,17] and by time resolved X-ray imaging dynamic processes, such as movement of air trapped in an “air bubble” from inside the pupae to the space between the adult insect and the p uparium[18]. Given the substantial differences in life history, especially the immediate egg development and mating strategy of Saturnia pavonia, the present study about its final metamorphosis complements the understanding for development of different moth pupae species

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Discussion

Conclusion