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For consistency's sake: the precise use of larva, nymph and naiad within Insecta

2015; Wiley; Volume: 40; Issue: 4 Linguagem: Inglês

10.1111/syen.12136

1365-3113

Seth Bybee, QUINN HANSEN, Sebastian Büsse, Haley M. Cahill Wightman, Marc Branham,

Species Distribution and Climate Change

Larva, nymph or naiad? Which term belongs to which insect groups? Entomology needs clear language – terminology – to ensure scientific integrity of the actual term that refers to each immature (juvenile) stage within Insecta. After observing students, amateurs and professional entomologists alike struggle with the appropriate use of the terms larva, nymph and naiad, the necessity of a revision of this terminology is evident. Established systematic entomologists have differing opinions as to which term is most appropriate for denoting the immature stage for each of the major groups of insects. Confusion over which terminology is best is perhaps most acute for Ephemeroptera (mayflies) and Odonata (dragonflies and damselflies), and perhaps to a lesser but still significant extent for Plecoptera (stoneflies), because the immature stage of these hemimetabolous insects differs significantly from the adult stage in terms of morphology, lifestyle and habitat (e.g. Corbet, 1999). Indeed, the immature stage of these insects appears more comparable with Endopterygota than hemimetabolous insects. Within the order Odonata, authors use all three terms interchangeably to describe the immature stage (larva, Maloeuf, 1935; Snodgrass, 1935; Asahina, 1954; Mueller et al., 2005; Fleck et al., 2008; naiad, Hussain & Ahmed, 2003; Perotti et al., 2006; Collier et al., 2010; nymph, Stav et al., 2007; Bybee & Tennessen, 2008; Touchon & Warkentin, 2008; Büsse et al., 2013; Büsse & Hörnschemeyer, 2013). The importance of unambiguous definitions for terminology used within science is paramount. Yet, there is a potential for these three terms to be denigrated to the same level as common names and suffer all of the same confusion associated with multiple common names for a given organism, among other problems (Morrison, 2014; Stevens et al., 2014; see also http://entomology.ifas.ufl.edu/frank/kiss/kiss8.htm). This fate for scientific terms should be avoided in order to keep terminology precise and restricted in definition so as to increase descriptive power. The goal of this paper is to clarify the origin and the validity of the terms larva, nymph and naiad to elucidate the proper use within different insect taxa. We therefore seek to revisit the definitions of these terms and argue for a consensus of careful usage that reflects postembryonic development: paurometabolous insects (i.e. gradual metamorphosis, as in Hemiptera, for example), hemimetabolous insects (i.e. incomplete metamorphosis with the immature stage differing greatly from the adult in habitat and form, as in Ephemeroptera and Odonata) and Endopterygota (i.e. complete metamorphosis, as in Lepidoptera and Coleoptera). The history of any word can be long and full of insight, which in turn illuminates the modern-day meanings. Words as old as 'larva' and 'nymph' provide an especially interesting history and view of what they can mean in modern entomology. While the original meanings for both nymph (Greek) and larva (Latin) are a part of modern discourse, they have now attained a new status in the entomological world as separate scientifically descriptive terms reflecting the immature stages of different insect groups and their life cycles. With the relatively recent introduction of the word 'naiad' to the debate, a clear history is required to restore a concrete lexical entry for each of these words within entomology. … cum mortuis non nisi larvas luctari. … that none but vain bugs & hobgoblins use to fight with the dead. I see no Reason but their Aureliae also may pretend to a specifick Difference from the Caterpillers and Butterflies, and so we shall have 300 Species more; therefore we exclude both these from the Degree of Species, making them to be the same Insect under a different Larva or Habit. (Ray, 1692, p. 6) The water-eft has not, that I can discern, the least appearance of any gills; for want of which it is continually rising to the surface of the water to take in fresh air. I opened a big-bellied one indeed, and found it full of spawn. Not that this circumstance at all invalidates the assertion that they are larvae: for the larvae of insects are full of eggs, which they exclude the instant they enter their last state. (White, 1789) It can be assumed that the term larva as applied to the immature form of insects was at this point well established in common scientific usage. According to Comstock (1918, 1920), in the early 19th century the term larva had become pervasive within entomology and was applied to all immature insects. For the present discussion, the word nymph should be considered in terms of its original meaning: young beings that are around water (Oxford English Dictionary). With a more pleasant meaning and connotation, nymph is derived from the Greek pantheon of gods. The nymphs were usually young goddesses who resided near Mount Olympus (MacDougall, 1975). … the other common sort, when they begin to have fashion are called nimphes. (Googe, 1577) … at length they grow to be nymphes, i.e. little bees without wings. (Rowland, 1667, p. 898) When the time of the last change of this insect draws nigh, it leaves the tree, and descending to the earth, makes its way into it in a proper place, and then becomes a nymph, out of which, at a proper time, issues a four winged fly. This is also the last state of ail the bastard-caterpillars that have yet been traced through their several changes. (Chambers, 1753) … the term larva was restricted to the young of insects with a complete metamorphosis and the term nymph, which had been formerly used as a synonym of pupa, was applied to the young of insects with an incomplete metamorphosis. (Comstock, 1918) Modern entomologists no longer use nymph as a synonym to pupa, and indeed many may be surprised to learn its original definition among early entomologists. Naiad is the most recent of the three words added to the English lexicon and biological terminology to refer to insects. The term naiad began much like nymph, in Greek mythology, and they share a similar history. In Greek mythology a naiad is one of the three main classes of nymphs, with naiads being bound to fresh water lakes, rivers, springs and fountains (Fisher, 2005). In a biological context, naiad has long been used as a common name to refer to water plants, such as those from the genus Najas (e.g. Richardson, 2008) and other living creatures found in water, such as bivalves of the genera Unio and Mytilus (e.g. Jenkinson, 2014; Prié et al., 2014). … it can be seen that the immature stages of the Plecoptera, the Odonata, and the Ephemerida differ fundamentally from those of insects with a gradual metamorphosis on the one hand and from those of insects with a complete metamorphosis on the other; and that the reasons that make desirable the restriction of the use of the term larva to designate the young of insects with a complete metamorphosis make desirable a distinctive term for the sideways developed young of those with an incomplete metamorphosis. (Comstock, 1918) The use of the terms 'larva, nymph and naiad' in modern entomological circles is quite varied, even within groups of specialists focusing on a single insect taxon. There appears to be a greater consistency of use concerning the term larva than for either nymph or naiad, with the greatest confusion concerning the correct use of naiad. Perhaps this is because the groups designated by Comstock as 'naiads' do not form a monophyletic group. Though the concept of monophyly did not exist at the time, insect classification has been greatly in flux since and confusion between terms existed well before Comstock's (1918) paper and persists today. Further, there is not a requirement for scientific terms to reflect homology (e.g. insect wing venation and genitalia). Thus, the use of each term and descriptive scientific terms in general is not restricted to any idea of monophyly, homology (as ideal as that might be), classification (classical or modern) or zoological nomenclature. Instead, we argue that these terms should be used to define both general habitat and postembryonic developmental similarities, characteristics that are unlikely to change without millions of years of evolution. For example, dragonflies, damselflies, mayflies and stoneflies all inhabit freshwater ecosystems as immatures, have gills that are subsequently lost upon eclosion to adulthood without passing through a pupal stage, and have significant morphological differences from the adult. Groups such as aquatic hemipterans that also do not pass through a pupal stage are still considered nymphs because they do not have gills as immatures, but instead perform respiration through a snorkel, siphon or plastron (e.g. Nepidae). Further, hemipterans have gradual development to adulthood and resemble the adult form upon leaving the egg and throughout their entire development (i.e. gradual or paurometabolous metamorphosis). Larva: a young insect which quits the egg in an early stage of morphological development and differs fundamentally in form from the adult … … feeding and growing stage of holometabolous [Endopterygota] insects that undergo complete metamorphosis. Nymph: an immature stage of paurometabolous insects, e.g. Heteroptera (Hemiptera). Naiad: the aquatic nymph of hemimetabolous insects, i.e. Odonata, Ephemeroptera, and Plecoptera. Our hope is that this paper will serve to encourage the careful application of these terms within entomology and the larger biological community. At the very least, we hope to educate nymphal entomologists in the accurate and intended usage of these terms. We would like to thank Drs. Kelly Miller, Kenneth Tennessen, Jennifer Zaspel and Michael May, and two anonymous reviewers for thorough and thoughtful comments that have greatly improved this paper. We would also like to thank BYU for generously funding H.W., the National Science Foundation for supporting S.B. (DEB- 1265714) and the University of Florida for supporting Q.H.