Comprehensive comparative morphology and developmental staging of final instar larvae toward metamorphosis in the insect order Odonata

Genta Okude,Ryo Futahashi,Takema Fukatsu

doi:10.1038/s41598-021-84639-2

Abstract

The order Odonata (dragonflies and damselflies) is among the most ancestral groups of winged insects with drastic morphological changes upon metamorphosis, and thus important for understanding evo-devo aspects of insects. However, basic developmental descriptions of Odonata have been scarce. In an attempt to establish the foundation of developmental and experimental biology of Odonata, we present an unprecedentedly comprehensive survey of dragonflies and damselflies, in total 158 larvae representing 49 species and 14 families, wherein morphological changes of all the final and/or penultimate instar larvae were photographed and monitored everyday. Although their morphology and development were diverse, we consistently identified two visually recognizable morphogenetic events in the final larval instar, namely start of wing expansion and onset of melanization on the wing sheaths, thereby categorizing the final instar into three stages. While the duration of the first stage ranged 4–66 days across diverse Odonata species, the second or third stages exhibited relatively small variation ranging 3–22 days or 1–8 days, respectively, probably reflecting the steady and irreversible metamorphosis process after stage 2. We also described other characteristic morphological changes during the larval development, although they were observed only in some Odonata species and lineages.

Highlights

Dragonflies and damselflies are among the most ancestral winged insects[1,2] and radically change their morphology, habitat, and behavior from aquatic larvae to terrestrial adults without pupal stage[3,4], which entail drastic changes of gene expression patterns
It should be noted that two species of the genus Lestes (Lestidae) did not exhibit black dots on the wings but could be classified as stage 3 by melanization at the base of the wing sheaths (Tables 1, S1; Fig. S1) While a previous study partly mentioned these criteria for developmental staging of Pyrrhosoma nymphula[11], here we demonstrate that these developmental criteria are applicable to most of the diverse Odonata species (Table 1)
For all the 22 F−1 instar larvae representing 12 species (Tables 1, S1), the compound eyes gradually tilted during F−1 instar, and the wing sheaths began to expand a few days before ecdysis to F−0 instar (Figs. 2C, S1)

Summary

Introduction

Dragonflies and damselflies (order Odonata) are among the most ancestral winged insects[1,2] and radically change their morphology, habitat, and behavior from aquatic larvae to terrestrial adults without pupal stage[3,4], which entail drastic changes of gene expression patterns (e.g. opsin genes in compound eyes[5,6]). Senegalensis13), Aeshnidae (Anax imperator[14]; Aeshna cyanea[15]; Aeshna viridis16), Gomphidae (Asiagomphus pryeri17) and Libellulidae (Pachydiplax longipennis[18]; Pseudothemis zonata[19]; Urothemis assignata[20]) In these studies, the final larval instar was categorized into 3 to 8 developmental stages defined by morphological traits. From the enormous amount of morphological data based on approximately 5000 photos, the following two morphological changes, namely “the start of wing expansion” and “the onset of melanization on the wing sheaths”, emerged as and commonly recognizable developmental indices across the diverse Odonata species Based on these indicators, we propose that the final larval instar of Odonata could be generally classified into three developmental stages. Other morphological changes in the compound eyes, heads, larval labia, and wing sheaths were described in detail, they were observed in more specific Odonata lineages and species

Methods

Results

Conclusion