AS AERIAL PREDATORS, adult dragonflies have few peers. Their extraordinary agility is unequalled among animals, except perhaps some of the smaller birds of prey, bats and bee-eaters; and they have an outstanding ability to detect the movement of small flying objects.
Dragonflies are robust insects comprising the order Odonata (‘toothed ones’). Almost all species are aquatic in the larval (growth) stage and aerial as adults. In all but the first larval stage they are obligate predators, using powerful mandibles to masticate their prey. All odonate larvae catch their prey by explosive extension of a specialised lower lip (labium) (Fig. 60, p.115) derived from the fusion of the second maxillae. Among the senses used for prey detection, the compound eyes steadily acquire primacy during larval development. Concealment, immobility and occasionally rapid movement are among the attributes that make larvae effective predators of other, usually smaller, animals. Adults catch prey while on the wing and likewise masticate it using the mandibles. The compound eyes are well developed, especially in the adult.
In common with other morphologically generalised insects, dragonflies lack a pupal stage between larva and adult; the larva, though possessing rudimentary wings, otherwise broadly resembles the adult.
Odonata derive, with little structural change, from the Protodonata, an extinct order that flourished in the Upper Carboniferous, more than 300 million years ago. Those early dragonflies broadly resembled extant species but were much larger, having a wingspan of about 70 centimetres.1 Some authorities attribute this gigantism to the higher concentrations of oxygen in the atmosphere at the time.2 Together with their ‘sister’ group, the mayflies, or Ephemeroptera, Odonata, by direct lineage, are among the most ancient of flying insects surviving to the present day. Mayflies and dragonflies (together comprising the Palaeoptera) are believed to have shared a common ancestor that separated from the progenitors of all other winged insects (the Neoptera) early in the evolution of insect flight. Larvae of the extant Palaeoptera are almost all aquatic. Unfortunately, no fossil larvae of the early Palaeoptera have been found, so it is not known when larvae of Odonata became aquatic, although this may well have taken place during the Lower Permian.3
Today Odonata are to be found in every continent except Antarctica. In the northern hemisphere they occur from the equator to latitudinal tree line. About 6,000 species have been described, although the world fauna probably exceeds this number by several hundred. New species are being described at a steady rate, especially from tropical rainforest, where the diversity of species and suitable habitats is greatest. Because this ecosystem is being rapidly lost due to human impact, odonate taxonomists are aware that they are working against time to describe many species before they become extinct. The extent to which species and families of Odonata are endangered on a global scale is better known than for most other insect orders.4
We subscribe to the view that there are two extant suborders of Odonata: Zygoptera and Anisoptera. Some odonatologists recognise a third suborder, Anisozygoptera, but we share the opinion5 that members of this third suborder occurred only during the Triassic, Jurassic and Cretaceous periods and so have been long extinct.6 Extant Zygoptera and Anisoptera are each represented by about 3,000 described species.
Adult Zygoptera (‘similar wings’) (Fig. 8, p.25), known as damselflies, are typically small and slender; the fore and hind wings are similar in shape and (except in Calopterygidae) are stalked (petiolate) at the base; the wing-loading and flight speed are low, the Zygoptera being ‘forward-thrust’ fliers.7 The larvae are slender and usually bear three conspicuous leaflike or sac-like appendages at the tip of the abdomen (Fig. 20, p.35). Inside the abdomen they lack recognisable gills and a muscular diaphragm. They swim by lateral movements of the abdomen, wriggling like a fish.
Adult Anisoptera (‘dissimilar wings’) (Fig. 30, p.50) are typically large and robust. Their wing-loading is high7 and they can fly rapidly and powerfully. In some, the base of the hind wing is much expanded, facilitating gliding and soaring, a modification found especially in species that habitually migrate. The larvae (Fig. 28, p.46) are robust and within the abdomen possess gills and a muscular diaphragm. The diaphragm plays a triple role: it controls pumping movements that generate inhalant and exhalant currents that ventilate the gills lining the rectum; it applies this capability in a more vigorous mode to enable a larva to swim by jet propulsion, as a means of escape; and it regulates the increase in blood pressure that enables the labium to project forwards suddenly when prey is being captured (Fig. 60, p.115). Larvae of Zygoptera can also project the labium abruptly, but do not employ a muscular diaphragm to do so.
The Zygoptera comprises 18 extant families and the Anisoptera comprises 12, one of which, the Epiophlebiidae, is assigned by some odonatologists to the suborder Anisozygoptera (see above).
Odonatology is the study of dragonflies with the explicit aim of learning about their biology and making the information so acquired generally available, in a formal way, through recognised channels. Practised for at least 250 years, odonatology has yielded a treasure house of fascinating information, many items of which have been acquired, serendipitously, by skilled observers. Such disparate facts, together with the products of observations and field experiments, provide the tesserae of a rich mosaic that await assembly and interpretation. Present and future odonatologists face the challenge of arranging and interpreting these tesserae so as to construct a coherent picture of dragonfly biology, and preferably one that will throw light on the paths along which evolution by natural selection has moulded the patterns of behaviour and ecology found in the order today. We hope that our treatment of this legacy of information constitutes a modest step towards this goal, especially as regards the British species.
The odonatologists, past and present, to whom we owe this legacy of facts (and hypotheses) have come from many walks of life (Chapter 10). Despite their varied backgrounds, they share certain attributes that have enabled them to contribute importantly to odonatology: enthusiasm and affection for Odonata; willingness to apply scientific rigour to their methods of study; and willingness to undergo the self-imposed discipline of communicating their findings to other odonatologists. We, their beneficiaries, are much in their debt, and are under obligation to maintain the rigour that is a feature of their work.
All odonatologists share a common, implicit, overriding objective: to increase the likelihood of the long-term survival of vigorous populations of dragonflies. Virtually all that is known about dragonflies, including the contents of this book, derives from the science of odonatology, practised with the rigour that underpins all branches of biology. We have recently detected trends in human behaviour that may threaten the integrity and reputation of odonatology as a respected branch of science. Some involve the perceived propriety of collecting specimens for scientific study, a stance that can sometimes give rise to conflict. Odonatologists today have a responsibility to ensure that they prove worthy custodians of the legacy they have inherited from those practitioners who, by their dedication and skill, have made this science a respected and influential branch of biology. As practising odonatologists, we see ourselves as under obligation to try to improve the prospects for odonatology in Britain. Here we address two aspects of odonatology that could benefit from attention and reform. They relate, respectively, to nomenclature and collecting specimens.
Every species of dragonfly known to science has been assigned a scientific name according to the rules laid down by the International Commission for Zoological Nomenclature (Box 1). The official, required name of any dragonfly consists of four elements: genus, species, name of author of first published description and date of that description. To comply with the rules, the first (definitive) description should designate a type specimen (i.e. the specimen on which the description was based) and say where it has been deposited. The latter provision is mandatory and necessary because comparison with the type specimen is often the only way in which a future taxonomist can verify that a specimen of a species suspected of being undescribed is indeed new to science.
Following Carolus Linnaeus, who devised and established this binomial system of nomenclature for animals in 1758,8 all names of species follow a Latinate form which is used by zoologists internationally when referring to species.
In Britain, naturalists have long shown a tendency to assign English names to some of the more conspicuous and handsome insects (such as butterflies, moths and dragonflies). Unlike butterflies and moths, dragonflies have been given English names relatively recently, so some of the names appear contrived. Cynthia Longfield, in the first (1937) edition of her book,9 and in a successful attempt to make dragonflies more accessible to the non-specialist, gave each species of British dragonfly an English name. She was apparently unaware of the English names that had already been assigned to British dragonflies and which were revealed by Richard Gabb’s examination of a collection in the Grosvenor Museum in Poynton, Cheshire.10 Her approach was to retain the scientific
SYSTEMATIC POSITION OF ANAX IMPERATOR LEACH, 1815, THE EMPEROR DRAGONFLY
Taxon Name Distinguishing attributes Phylum Arthropoda Jointed, chitinous exoskeleton; jointed limbs. Class Insecta Tripartite, segmented body; thorax bearing a pair of legs on each of its three segments. Subclass Exopterygota Incomplete metamorphosis (i.e. no pupal stage). Order Odonata Two pairs of long, membranous wings; larva typically aquatic; biting mouthparts in larva and adult; larva with specialised, protractible labium. Suborder Anisoptera Fore and hind wings differ in shape; larva has muscular diaphragm and gills inside abdomen. Family Aeshnidae Diagnostic features of wing venation and shape. Genus Anax Diagnostic features of body, especially colour pattern, and shape of abdominal appendages and genitalia. Species imperator Diagnostic features of body, especially colour pattern, and shape of abdominal appendages and genitalia. Authority Leach The authority who first described and named the species and designated the type specimen. Date 1815 The year in which Leach’s description of the species was first published, giving the location of the type specimen (which in this case has been lost).
generic name, so leaving no doubt about the genus to which the English name applied. Thus Aeshna juncea became the Common Aeshna, and Sympetrum striolatum became the Common Sympetrum. Since then the nomenclature has undergone several changes. In 1977 Cyril Hammond gave all generic names English equivalents.11 As a result, Aeshna juncea became the Common Hawker and Sympetrum striolatum became the Common Darter. When some writers began to show originality in their use of English names, it became desirable to standardise the nomenclature and, beginning in 1991, the British Dragonfly Society (BDS) has listed in each issue of its journal the scientific name and its approved English equivalent for all species found in Britain. All these names, except that for Gomphus vulgatissimus, conform to those in Hammond’s book and are accepted as definitive in English usage today, at least in Great Britain.
There have recently been two significant developments.
First, in their book on the dragonflies of Ireland, Brian Nelson and Robert Thompson introduced English names different from those in the BDS list for eight genera and 20 species, retaining the same names for only 14 species.12 In the Irish list, Calopteryx becomes Jewelwing (instead of Demoiselle) and Lestes becomes Spreadwing (instead of Emerald Damselfly). These two generic names (and some others in the Irish list) conform with those approved for North American dragonflies in 1996 by the Dragonfly Society of the Americas (DSA),13 although the English names used for four genera (Anax, Ischnura, Leucorrhinia and Sympetrum) do not.
Second, in response to a perceived need, coupled with range expansions of European dragonflies into Britain (see Chapter 10), English names have been generated for Odonata of continental Europe for 29 species not on the British list.14 The names chosen have in general conformed with the system already in use by the BDS.
Two long-term goals among naturalists and odonatologists who wish to use English names must be to achieve quick recognition and to avoid ambiguity. As long as naturalists adhere to the names in the BDS list, it does not matter whether the English name or the scientific name is used, at least among odonatologists in Britain. However, it makes life much easier for non-British readers if odonatologists use the scientific names in their published work. Indeed it was to remove ambiguity deriving from the use of vernacular names that Linnaeus developed his system of nomenclature in the first place! We hope that before long unanimity can be reached so that a list of vernacular equivalents can be agreed upon that applies to both Britain and Ireland. Such a list could with benefit conform closely to North American usage. In this book we use only scientific names throughout the text for the species of British dragonflies. In Appendix 1 we list the scientific names of all species found in Britain, together with their vernacular equivalents in Britain (excluding Ireland).
The position is less straightforward regarding the names of the suborders of Odonata. At some time, probably early in the nineteenth century, the English term ‘damselfly’ was adopted to denote a member of the suborder Zygoptera. This was unfortunate, because a corresponding English term for the suborder Anisoptera was not introduced at the same time. The precedent for the word ‘damselfly’ was probably the French ‘demoiselle’ which, according to Réaumur,15 was the vernacular name applied throughout France to all Odonata, not merely Zygoptera. By restricting this term to Zygoptera, English-speaking odonatologists laid the foundation for the ambiguity we now address. After Réaumur, Fabricius (1745-1808), a pupil of Linnaeus, separated dragonflies from the Neuroptera (of Linnaeus), assigning them to the order Odonata in 1793.16 Much later, in 1853, Selys recognised and defined the suborders Zygoptera and Anisoptera.17 Because the term ‘dragon fly’ or ‘dragonfly’ had already been pre-empted to mean a member of the order Odonata, some English-speaking authors decided subsequently that an English term for Anisoptera was needed, but unfortunately a valuable opportunity was missed. Having decided to call Zygoptera ‘damselflies’, authors in North America18 and Britain9 chose to meet this need in several ways: by calling Anisoptera ‘dragonflies proper’; by hyphenating the word ‘dragonflies’;19 or by using a lower-case initial letter (for ‘dragonfly’) to denote the suborder, and a capital initial letter to denote the order.20,21 Such suggestions have proved unworkable, partly because the initiators themselves sometimes failed to conform with the remedies they suggested! So the existing situation perpetuates an absurdity and stands in urgent need of reform.
German-speaking odonatologists have tackled this difficulty by calling Zygoptera ‘Kleinlibellen’ and Anisoptera ‘Grosslibellen’. But for English-speaking odonatologists no such solution has been sought. Until a corresponding English name is adopted to denote Anisoptera, English-speaking odonatologists will continue to experience embarrassment, either by having repeatedly to explain in which sense they are using the term ‘dragonfly’ or by tolerating an ambiguity; and poor Linnaeus (one may suppose) will continue to turn in his grave. We believe that correction of this anomaly is long overdue and accordingly take this opportunity to recommend the following terminology for those who feel compelled to use an English name for Anisoptera. Thus we would have:
Odonata: dragonflies
Zygoptera: damselflies
Anisoptera: warriorflies.
We use this terminology in Appendix 1 but have no need to do so elsewhere in the text, where the terms ‘Zygoptera’ and ‘Anisoptera’ suffice.
There is a second respect in which the conduct of odonatology needs urgent attention.
As odonatology (or at least dragonfly watching) has grown in popularity for field naturalists and photographers, the ranks of dragonfly watchers have been enlarged (and enriched) by new enthusiasts, many of whom have come to dragonflies after seeing them while out watching birds. Birdwatchers typically claim that they do not need to capture a specimen to identify it reliably. Such an option is often unavailable to entomologists, including odonatologists, despite the fact that dragonflies have been nicknamed ‘the birdwatcher’s insect’.22 It is clearly understood among odonatologists that, to pursue their science with rigour, they sometimes need to capture and preserve a specimen and that, when this need arises, they alone (as odonatologists) should decide whether or not a specimen needs to be collected. The conduct of odonatology can be severely compromised if non-odonatologists try to prevent odonatologists from collecting specimens. It has happened recently that an odonatologist trying to collect a specimen for deposition in The Natural History Museum was obstructed, and subsequently abused, by self-appointed vigilantes who had chosen to intervene in the field.23 We cannot emphasise strongly enough that such behaviour constitutes a severe threat to the future viability of odonatology – as a science – and so should be promptly and unequivocally denounced.24
Odonatology is a science. Much of the information in this book would not exist had specimens of dragonflies not been collected and preserved for study. In the first place, as explained above, a species cannot be validly described and named unless represented by a designated type specimen. Furthermore, voucher specimens preserved for later study are essential for the pursuit of several branches of biology as well as for some aspects of conservation management and habitat protection. Appendix 2 explains further why voucher specimens are sometimes needed for the pursuit of odonatology.
No one, especially an odonatologist, likes to deprive a dragonfly of life. On the contrary, many odonatologists are deeply committed to conserving habitats on which the survival of dragonfly populations depends. Aware of their responsibility for the conduct of odonatology, and for its image, some odonatological societies have drawn up codes of conduct that apply to collecting. The most balanced, useful and comprehensive code known to us is included in Appendix 2.
The first book on dragonflies in the New Naturalist series, by Philip Corbet, Cynthia Longfield and Norman Moore, was published in 1960.25 It was reprinted in 1985, in a paperback edition, but is now very difficult to obtain. It broke new ground in focusing on behaviour and ecology, but the information it contains has long been superseded. Since 1960 there have been massive advances in our knowledge and understanding of dragonfly biology. These have been reviewed, from a global perspective, in 1962, 1980 and 1999.26
By far the most significant of these advances has been the discovery of sperm displacement.27 This entails the dual function of the penis of the male dragonfly during copulation – as an organ for transferring sperm to the female and also as a device for removing or repositioning the sperm of rival males already in the female’s body. Sperm displacement has pervasive implications for almost every facet of dragonfly biology, and its discovery has revolutionised our perception of the evolutionary implications of reproductive behaviour of both sexes.28 Almost every action associated with reproduction can now be understood better, and interpreted, in terms of the struggle between males and males, and between males and females, to secure parentage of offspring that themselves will compete successfully to leave vigorous descendants. An important part of this increased understanding is our interpretation of intramale contests during territorial activity. Noteworthy advances have been made in other fields of dragonfly biology, especially in larval ecology and behaviour; and we now have a much clearer understanding of the ways in which life cycles are regulated and are adapted to seasonal changes in the environment.
This book is about the natural history of dragonflies that inhabit Britain. Readers interested in the natural history of British species should be aware of several excellent books on western European dragonflies in French29 and German.30
After briefly describing dragonflies, as animals and insects, in this chapter, we introduce the British species in Chapter 2; and in Chapters 3 to 9 we explore each stage of the life history in detail, giving weight to behaviour and ecology. Although a great deal is known about the biology of dragonflies, many questions remain to be tackled, and we conclude each of Chapters 3 to 9 with suggestions for investigations that enthusiasts may wish to undertake. We recommend that, to save time and effort, would-be investigators consult the relevant literature before embarking on a project. To facilitate this we provide a Bibliography on pages 395-442 listing details of the literature sources cited under each topic in the text. In Chapter 10 we trace the development of odonatology in Britain, including the history and status of conservation – of dragonflies and their habitats – and the relevant, anticipated effects of prospective climate change. We do not provide scale bars on any of the photographs that appear in this book. Readers are referred to the books by Hammond31 and Brooks32 where measurements are given for adults and larvae of all British species. Certain terms italicised in the text are used in a specialised sense and are defined in the Glossary on pages 373-381.
1) a checklist of species occurring in Britain, giving scientific names and their English (largely BDS) equivalents;
2) an introduction to the practice and philosophy of collecting specimens and advice for photographing dragonflies in the field;
3) the odonatological criteria according to which a site can qualify for designation as a Site of Special Scientific Interest (SSSI) in Britain; and
4) maps showing the distribution in Britain of British dragonflies, categorised according to date.
Until 1960, when the predecessor of this volume appeared,25 books and monographs on British dragonflies were concerned primarily with identification – of adults, by Lucas33 and Longfield34 and of larvae by Lucas35 and Gardner36. The identification of larvae was placed on a firmer footing by Eric Gardner who reared and described many British species from egg to adult. The keys he produced36 were reprinted in the New Naturalist book Dragonflies, published in 1960,25 and in the book by Hammond in 1977 and 1983.31
Useful manuals exist for the identification of adults and larvae, for the Odonata of Britain (by Hammond, McGeeney, Miller and Brooks)37 and Europe (by Askew),38 and all except the book by Miller contain information about distribution. Miller’s book also contains quality information about biology, especially behaviour. A publication focusing on distribution and conservation status and including summary information about the biology of each British species is the Atlas by Merritt, Moore and Eversham.39
The lay reader interested in dragonflies on a global scale will be well served by recent books by Silsby40 and Brooks.41 Both titles are liberally furnished with colour images and the former is unique in illustrating an example of an adult representing each of the 73 subfamilies of Odonata. Specialised information about the behaviour and ecology of Odonata as an order can be found in two books and an article by Corbet.26 The classic book The Biology of Dragonflies, by R.J. Tillyard, one of the giants of odonatology,42 was published in 1917.43 It emphasised morphology and systematics; it is now long out of print and has become a collector’s item. It is packed with information not easily found in modern publications. Fortunately, thanks to the initiative and industry of Richard Rowe, it is now available on the worldwide web.44
The BDS, established in 1983, collates information about British Odonata, maintains a vigorous education programme and publishes the Journal of the British Dragonfly Society, featuring research reports about British species, as well as a bulletin, Dragonfly News, which gives details of field and indoor meetings. The BDS supports a standing committee, the Dragonfly Conservation Group, which inter alia advises statutory bodies on the conservation needs of dragonflies. There are now several books describing the dragonflies of different regions or counties in Britain (Box 2, p.14).
Two bodies serve odonatologists internationally: the Foundation Societas Internationalis Odonatologica publishes Odonatologica and Notulae Odonatologicae, and The Worldwide Dragonfly Association publishes The International Journal of Odonatology and the newsletter Agrion.