The evolution of insect flight

BOOK

REVIEWS

Bog botany British Plant Communities, Vol. 4: Aquatic Communities, Swamps and Tall-herb Fens edited by J.S. Rodwell Cambridge University Press, 1995. £24.95/$64.95 hbk (xii + 283 pages) ISBN 0 521 39168 7 his is the fourth and penultimate volume of this outstanding series. The three T types of community gathered together in the present book have wetness in common, but precious little else. The aquatic communities are a classic example of the 'out of sight, out of mind' syndrome, and there are legions of otherwise competent naturalists who can't tell their Myriophyllum from their Ceratophyllum, or distinguish one species of Potamogeton from another. T]heswamp communities are hardly communiities at all; most of them are virtual monocultures of bulky emergent monocots like Phnzgmites, Typha, Glyceria or Carex. The tall-herb fens, on the other hand, are much more like the kinds of communities dealt with in Volumes 1-3, because they are amenable to the conventional, quadrat-based sampling techniques adopted by the UK National Vegetation Classification

(NVC). This was clearly not an easy volume to write. The difficulties of understanding the underwater structure of aquatic communities by shore-based grapnel sampling have been thrown into sharp focus by the small number of detailed studies where scuba divers have been employed to document sub-aquatic stratification and spatial patterning. Again, to quote the authors, 'swamps can be difficult and unpleasant to work in, and they reward the persiste:nt with a dismal poverty of species'. Heartfelt words, clearly. The tall-herb fens, while at least having the virtue of being semi-terrestrial, tend to exhibit a bewildering mosaic of small-scale heterogeneity in their dominant species; sensibly, they have been lumped into a small number of distinctive aggregates. The 24 aquatic communities include five water crowfoot (Ranunculus) communities, four dominated by Potamogeton species, two by water lilies (Nuphar and Nymphaea), two by duckweeds (Lemna), two by Ceratophyllum, two by Hydrocharis and a ragbag of seven communities dominated by Juncus

bulbosus, Isoetes, Litorella, Callitriche stagnails, EIodea spp., Polygonum amphibium and Myriophyllum alterniflomm, respectively. Apparently, one of the sampling techniques for this kind of community is called 'walkabout'; I had never previously thought of Crocodile Dundee as an NVC surveyor. The authors admit that, compared to the other kinds of vegetation included in the survey, 302

'coverage here is rather poor and patchy'. This is reflected in the fact that the usual dot distribution maps have not been provided for the aquatic communities. One of the things that the floating plant communities do provide, however, is a wonderful system for testing ideas about species coexistence. For example, last September 1 discovered that a single handful, scooped from the continuous floating mat covering the surface of the Lee Navigation in east London, contained all five British species of duckweeds (three native Lemna spp., Spirodela polyrhiza plus the tiny alien L. minuta). It would be wonderful to know where each of these species finds its refuge from competitive exclusion. The swamps are split up into 23 communities, dominated by Phragmites, Glyceria (3), Carex (8), Cladium, Scirpus (3), Equisetum fluviatile, Typha (2), Sparganium and Sagittaria. Communities dominated by alien species are few and far between in Britain (in marked contrast to countries at lower latitudes), but one of the swamp communities ($15) has this distinction; $15 is Acorus calamus swamp, and this can often contain other aliens like Mimulus guttatus and Crassula helmsii; the community is found in lowland England from Lancashire southwards to the London Basin. The NVC does not cover Ireland, but this Acorus vegetation is perhaps most common around Lough Neagh and in the Lagan Canal. While the coverage within Britain appears to be reasonably comprehensive, no mention is made of the swamp community containing Lysichiton americanus, whose chrome-yellow spathes are so conspicuous in early spring in lowlying wet places near to the sea in northwestern Britain. As Rodwell freely admits, communities containing alien species are under-represented in most of the NVC surveys. The tall-herb fens are grouped into just five communities, four of which are dominated byPhragmitesand the fifth is dominated by Carex rostrata and Potentilla palustris. This latter community, $27, is one of the few to conjure up a real sense of place. The description is redolent of Malham Tarn, with glowering clouds above the smooth grey cliffs of Great Scar Limestone, where the adventurous explorer of the Fen can, by vigorous flexing of the knees, cause the whole bouncy, floating mat of vegetation to wobble disconcertingly for several metres around. The Norfolk Broads are also well documented in Volume 4. Their most characteristic community is $24, the Phragmites australisPeucedanum palustre tall-herb fen, which supports such rarities as Carexdiandra, Lathyrus palustris and Thelypteris palustris and is found outside the Broads only in scattered locations in Somerset and the East Riding. Like the Mires of Volume 2, this book is a gem for connoisseurs of sedges and bryophytes. On reflection, I suppose that it is no

accident that such a large fraction of Britain's surviving semi-natural vegetation should be dominated by these soggy plants, given that they are so utterly unrewarding under human exploitation. Volume 4 maintains the exceptionally high standard set by the first three. While it is certainly true that we know a lot less about the ecology of these wetland communities than we do about terrestrial vegetation, nevertheless, what we do know is presented with the usual clarity, precision and circumspection. As with the other books in the series, even the most cursory inspection provides enough intriguing questions to keep an entire generation of plant ecologists busy. Roll on Volume 5, when a quantitative comparison of the attributes of the full range of British Plant Communities can be attempted.

Mlck Crawley Dept of Biology, Imperial College at Silwood Park, Ascot, Berkshire, UK SL5 7PY

Morphological treasure The Evolution

of Insect Flight

by Andrei K. Brodsky Oxford University Press, 1994. £55.00 hbk (xiv + 229 pages) ISBN 0 19 854681 5 solating barriers, as every good population geneticist knows, can result in exciting deIvelopments. For more than two decades, Andrei Brodsky of St Petersburg University has led the only major group working on insect flight mechanics in what was formerly the Eastern Bloc, publishing almost exclusively in Russian, with little opportunity to attend international conferences. Partly, perhaps, in consequence, his approach to the subject has developed very differently from those of workers in Western Europe and the USA. With a few exceptions, the latter have tended to investigate in depth the flight physiology, kinematics and aerodynamics of a few easily cultured or readily available species - locusts, Drosophila, tobacco hornworm, Bombus. Professor Brodsky's group has taken a far more comparative approach, investigating a wide range of insects, including phylogenetically interesting minor orders like Ephemeroptera, Piecoptera and Neuroptera, and an array of differing Lepidoptera. Typically they have combined detailed studies of wing design with investigation of the kinematics and aerodynamics of insects in tethered flight, in particular developing elegant methods of examining the form of the vortex wakes that their insects generate. Brodsky's T R E E v o l . 10, n o .

7 July 1995

BOOK laser-scanned pictures of the wakes behind peacock butterflies 1 are certainly the best and most beautiful yet published. In 1988 Professor Brodsky published a book on the evolution of flight in insects and the evolution of the wing apparatus, and immediately began negotiations for an English edition, with the heartfelt support of some of us who had struggled for years with the primary literature. Here it is; but it has been so extensively revised, updated and reordered that it is virtually a new book. The Evolution of lnsect Flight has been well worth waiting for. It is a remarkable work, quite outstanding in its scope and breadth of vision. Here, as anticipated, are the kinematic and wing studies, and the wake vortex work, clearly explained. But what has surprised and delighted me is the magnificent functional treatment of the morphology of the flight system as a whole. Contrary to general belief, morphology is a difficult subject. Good morphologists are rare; morphologists who can communicate are rarer; and good functional morphologists, capable of understanding and conveying the dynamics of complex, three-dimensional mechanisms are so uncommon that they should be treated as international treasures. This book is nothing less than the first comparative functional account of the morphology of the insect pterothorax, and this on its own fully justifies its publication. It is also the fullest account of pterothoracic morphology since Matsuda's massive and wholly un-functional monograph 2, and it is clearer, better illustrated, and infinitely more interesting. But the main thrust of the book is evolutionary, and the second half is devoted to an ambitious, fascinating discussion of the evolution of insect flight mechanisms and techniques. Russia has a distinguished tradition in insect evolutionary studies dating back to Martynov in the 1920s and 1930s. The Palaeontological Institute in Moscow has the world's largest collection of fossil insects, and largest group of palaeoentomoiogists. The author has conferred extensively with them, and his arguments take good account

TREE vol. 10, no. 7 J u l y 1 9 9 5

of fossil material. He briefly discusses the origin of flight and interprets the probable modes of flight in the archaic Palaeozoic orders, mainly from wing and axillary structure. His account of evolutionary trends in modern orders and supra-ordinal groups draws on evidence from fossils, and from the functional morphology of extant forms, but also makes full use of his extensive knowledge of flight kinematics, wake form, aerodynamic principles and flight behaviour in the field. The arguments are plausible and internally consistent, and generally fit well with orthodox phylogenetic views. Interestingly, he concludes that stoneflies probably have the most archaic flight system of all surviving insects - particularly topical in view of Marden and Kramer's reference to nemouroid 'skimming' in their new theory of the origin of insect flight 3. The book is very much a personal view, based above all on the work of the author and his group, though there are plenty of references to research elsewhere. The style is politely didactic: he generally avoids controversy simply by not discussing alternative views. He makes no attempt to confront Kukalova-Peck on the interpretation of structures 4, or myself on wing-functioning 5, or Ellington on unsteady aerodynamic mechanisms; indeed he pays us rather little attention. The treatment of the origin of flight takes no account of the important experimental and theoretical developments since 1985 (Ref. 6). Plenty of his views are controversial. He is convinced that mayflies and dragonflies have secondarily lost the ability to fold their wings, a view long held by the Moscow school but unpopular elsewhere. Some important conclusions on the early evolution of flight are based on his own reinterpretation of other workers' illustrations of fossils, without reference to the original material - a dangerous procedure in every sense. In evaluating his views on the evolution of wake forms, one needs to bear in mind that these are based on tethered insects, not necessarily flying at their preferred airspeeds, or supporting all their own

REVIEWS

weight. High-speed film of freely flying insects amply demonstrates the huge limitations that tethering imposes on flight performance and techniques. In using flight data, there is an inevitable tendency to overgeneralize from the few species that have actually been studied. We are, for example, told that Diptera do not perform a dorsal wing-clap (Drosophila, Bibio, Calliphora, Tipula and various muscids all do - when it suits themT, s) and that Hymenoptera only perform a near-clap - when the full clap, as he has already stated, was first described in a chalcid wasp. Presumably many small Hymenoptera fly similarly. None of this matters too much. The work is a stimulating presentation of the opinions and conclusions, based on a solid body of research, of an exceptionally wellinformed, innovative entomologist. The outstanding functional morphology component will make it an invaluable reference book for years to come. The aerodynamic sections are a challenge to other workers to test how far his generalizations apply to other insects, and to unimpeded flight. And the book as a whole fulfils all expectations in making the fruits of 25 years research by an extremely important team available to a wide, Englishspeaking readership. Robin J. Wootton Dept of Biological Science, University of Exeter, Exeter, Devon, UK EX4 4PS

References 1 Brodsky, A.K. (1991)J. Exp. Biol. 161, 77-96 2 Matsuda, R. (1970) Mere. EntomoL Soc. Can. 76, 1-431 3 Marden, J.H. and Kramer, M.G. (1984)Science 266, 427-430 4 Kukalova-Peck,J. (1990) in The Insects of Australia (Naumann, T.D., ed.), pp. 12-17, CSIRO 5 Wootton, R.J. (1992)Annu. Rev. Entomol. 37, 113-140 6 Kingsolver, J.G. and Koehl, M.A.R. (1994) Annu. Rev. Entomol. 39, 425-451 7 Ellington, C.P. (1984) Philos. Trans. R. Soc. London Ser. B 305, 41-75 8 Ennos, A.R. (1989) J. Exp. Biol. 142, 49-85

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