Tasmanian Mayflies: Identification, Ecology, Behaviour and Imitation

By Ron Thresher

Mayflies are one of the world’s most diverse, abundant and important aquatic insects. Famous for their brief adult life spans, mayflies play a key role in the ecology of aquatic and associated terrestrial ecosystems, and are critical bioindicators of ecosystem health.

Sitting at the southern limit of Australia’s temperate zone, Tasmania is home to a diverse array of mayflies and renowned fisheries based on them. The state’s storied ‘Lambda Dun’ hatches bring fishers from all over Australia to try their luck each summer on its rivers and ponds. Yet little is known about their behaviour and ecology, and more than half of the mayflies in Tasmania have never been described.

This extensively illustrated book is the first synthesis of the biology of south-east Australia’s mayflies, with a focus on those in Tasmania. It combines information gleaned from scientific literature as well as more than 30 years of the author’s studies and flyfishing experiences. It explores the biology, identification, conservation, ecology and behaviour of mayflies, and also includes fishing strategies and fly patterns.

Tasmanian Mayflies is an essential information source for Australia’s aquatic biologists and for flyfishers, novice and experienced alike, who chase the insects and the fish that feed on them.

• Language: English
• Publisher: CSIRO PUBLISHING
• Release date: Sep 1, 2023
• ISBN: 9781486316137

Ron Thresher

Ron Thresher has worked for over 30 years on the behavioural ecology of aquatic animals. He has published more than 100 scientific papers, plus books and patents, and occasionally catches a trout.

Book preview

1

Why mayflies?

Sub-adult Olive Nousia (Sp 4), Fish River, 24 Feb. 2020.

Because:

1. I could not work out what most of the mayflies were when I arrived in Tasmania 30-odd years ago;

2. very little information seemed available on the natural history of Australian mayflies, other than a few mainland studies and anecdotes in flyfishing books;

3. their often extreme abundance means they play major roles in aquatic and peripheral ecosystems;

4. working out these roles and their natural history depends on being able to identify the insects; but

5. the published literature on Australian mayflies is scattered, and some really good (particularly taxonomic) work is in the unpublished ‘grey literature’ and needs to be highlighted; and

6. not least of all, mayflies are the mainstay of Tasmania’s vaunted freshwater flyfishing, as they are in most parts of the world.

This book mainly covers Tasmanian mayflies. In that sense, it is an extension and up-date of the mayfly section of David Scholes’s classic Fly-fisher in Tasmania (which in turn, at the time up-dated and extended Robin Tillyard’s original 1930s descriptions of Tasmanian mayflies). However, this book is also relevant to Victoria and southern New South Wales, for three reasons. First, all but one of the genera found in Tasmania are also found on the south-eastern mainland of Australia. In many cases, even the same species is found in both areas (Red Spinners, Atalophlebia australis, for example) or the species are so similar that you need a microscope to tell them apart (e.g. the Tasmanian Tasmanophlebia lotis and the mainland T. nigrescens). Hence, much of the species descriptions, natural history and relevance to fishers is as applicable on the mainland as it is in Tasmania. Second, where things are different on the mainland, the mainland families, genera or species are briefly described in the relevant sections throughout the book. With this book in hand, identification to at least the genus level is do-able on the mainland, even if the level of detail is less than for Tasmanian species. Third, much of the reported information on mayfly ecology and natural history directly applies to south-eastern mainland species because a lot of the work was actually done there.

Keeping the information-by-listing approach going, the book is divided into four components, sometimes interwoven.

First, most of the book details the mayfly species found in Tasmania. Where I have the information, each is described and pictured, the features that distinguish it from similar appearing species are given, its distribution, natural history and ecology are detailed to the extent that these are known, and its relevance to and role in flyfishing discussed. The taxonomic information is based in part of the tremendous work done by Robin Tillyard, Ian Campbell, John Dean and Phillip Suter, supplemented by my own observations, whereas the natural history, behaviour, distribution and the like are mostly new information based on my field and laboratory work. I take blame for most of the fishing information as well, based on years of trying to catch trout.

Second, the species accounts are sandwiched between chapters on general biology. In the beginning, there are details on how to identify mayflies and a summary of their general biology. Afterwards, there is a synthesis chapter on the status of mayflies in Tasmania. These chapters are more technical and scientific than the rest of the book. If you are mainly interested in figuring out what mayfly something is and how to use this information to catch more trout, you can skip these chapters.

Third, with very few exceptions I do not cite in the text sources of information (standard scientific publishing protocol). This is to keep the text comfortably readable for an interested, but non-scientific audience. The sources of information, however, are listed in the ‘Further reading’ chapter, organised by subject area.

Fourth, along with the flyfishing comments in each species/genus account, two chapters in the book focus on mayflies for fishers: one on the hatches (where, when and involving what species), and a second on flies I have used more-or-less successfully to catch trout.

Serving two audiences – biologists involved with mayflies and the ecosystems in which they reside, and flyfishers – made determining the level of detail to include in the book a challenge, as well as how best to present it. Apologies if neither group feels that I got their perspective done perfectly; it means I probably got the balance about right.

Finally, I wish to thank those who helped make this book possible (including CSIRO Publishing). Individuals too numerous to list gave me specimens I could sometimes identify and told me their observations of mayflies and flyfishing around the state. I am grateful for every bit of information, which I hope I have conveyed accurately. I also sincerely thank Denis Abbott, Ed Brothers, Will Fletcher, Rod Griffin, Don Scofield, Rob Sloane and three not altogether anonymous referees for comments on the book’s content and format as they evolved. Their advice very much helped in producing a book that, I hope, serves both the scientific and angler audiences. I particularly thank Denis and Rod for their encouragement throughout to get the book done and their input to it, in Denis’s case even allowing me to include his flies in the book. I also thank Chris Bobbi, Scott Hardie and Tom Krasnicki for alerting me to the tremendous Tasmanian River Health dataset they assembled; it helped place my observations in a statewide context.

Two groups deserve special thanks. I am extraordinarily grateful to Australia’s two leading mayfly experts – John Dean and Phil Suter – for the information they generously shared with me and, also generously, for correcting my too frequent mistakes. This book is much more comprehensive, accurate and, I hope, useful as a result of their help and deep knowledge of Australian mayflies. And the book would have been much less fun to do without the encouragement and help of my spouse (Ann M.) and two kids (Ann C. and Harold Everett), who chased mayflies all over Tasmania with me, and sometimes without me, took photographs, fished with me, made drawings, listened politely while I described in no doubt painful detail my latest exciting ‘discovery’, and accepted with straight faces frequent claims that I really needed to get to a particularly fishable hatch ‘to get information for the book’. Their enthusiasm, patience and participation made the book possible and a joy to work on.

2

The biology of mayflies

Emerging Brown Spinner, Atalophlebia superba, at Little Pine Lagoon.

Mayflies are ‘primitive’, in the sense that they have retained a very basic insect body plan, and ancient. By comparison with mayflies, dinosaurs are modern. A fossil of a ‘proto-mayfly’ dates from 312 million years ago and is the oldest known winged insect. In reality, the fossil is not that of an actual mayfly, as defined in modern terms, but something that was evolving towards one. These ancient insects, the Syntonopteroidea, had two pairs of equal-sized wings, like dragonflies (Order Odonata), which are also considered primitive and to which mayflies are arguably closely related. In modern mayflies, the fore- and hindwings (when present at all) differ greatly in size. The hindwings are what has changed over time. The length of the forewings relative to body size was about the same for the proto-mayflies as it is for the modern mayflies, reflecting their importance in flight. The syntonopteroids were abundant and diverse through to about the age of dinosaurs (the Mesozoic Era), and sometimes large. Bojophlebia prokopi, from the Carboniferous Period (roughly 300 million years ago), had a wingspan of almost half a metre!

Almost 3500 species of mayflies have been described globally, in 42 families and ~450 genera. Taxonomists recently estimated that there are probably another thousand or so species to be described. The largest families are the Baetidae and Leptophlebiidae, both well represented in Tasmania and Australia in general. Globally, there are over 100 genera of baetids and almost a 1000 described species, and almost 150 genera and more than 600 species of leptophlebiids.

The mayfly order, Ephemeroptera, is ‘cosmopolitan’, which means that mayflies are found worldwide except for Antarctica and a few isolated island groups (e.g. Tristan da Cunha and the Falkland/Malvinas Islands). They peak in abundance and species richness in the tropics and at low elevations, including in Australia. Here, however, there is a second peak in the south-east. The reason for this is the biogeographic history of Australia. Taxonomists recognise three sources of the Australian mayfly fauna. The first is the true cosmopolitan families – in Australia, mainly baetids and caenids, which are widely distributed and thought to be ‘old’ migrants into Australia from the tropics. The second group consists of two small, wholly tropical families, prosopistomatids and austromerellids. These are thought to be recent arrivals from the north. The third group, and the reason for the south-eastern peak in species numbers, is of Gondwanan origin. ‘Amphinotic’ families are wholly or predominantly found in the Southern Hemisphere, with different but related genera in Australia, New Zealand and South America. The inference is that these families were widely distributed in the ancient southern super-continent, Gondwanaland, and were split as the continent fragmented due to continental drift, the newly separated land masses carrying off parts of the original fauna, which then further evolved. As a result, leptophlebiids are very well represented in temperate regions of all three Southern Hemisphere land masses, including south-east Australia, much more so than in the Northern Hemisphere. There are also several primitive mayfly families that are found only in this part of the world, in Australia again mainly in the south-east. These amphinotic families include the Oniscigastridae, the Nesameletidae, the Coloburiscidae and the Ameletopsidae. All have large hindwings.

The mayfly life cycle

The life cycle of mayflies consists of four stages: egg, nymph, sub-adult and adult.

Mayfly eggs are typically elongated ovoids less than 0.2 mm in length. As adaptations to avoid being washed downstream or buried in silt, the eggs can be sticky or, very often, covered with minute hairs and ridges. The sculpturing and texture of the surface differ among species and is often used in taxonomic studies. Some baetid genera (in Australia, Cloeon) are ovoviviparous, with developing eggs retained in the female for days to weeks before being released into the water, where they hatch immediately into minute larvae. Otherwise, the duration of the aquatic egg stage can range even within a species from as little as 10 days to several months, depending mainly on water temperature. In many temperate species, development pauses during the winter, resuming when waters warm up in the spring.

The eggs hatch into nymphs (sometimes referred to as larvae or naiads). Mayflies are essentially aquatic insects, the overwhelming majority of their lives spent underwater. The duration of the nymph stage ranges from as little as a few weeks to 2 years, longer for large species and for those living in cool climates. In many, if not most, temperate species, there tends to be at least two generations a year: a generation that overwinters as nymphs and emerges in the spring as relatively large adults, and one or more quickly developing cohorts that emerge over the warm summer as smaller adults. These, in turn, lay the eggs that form the next batch of overwintering nymphs. Small species, such as baetids, are often ‘multivoltine’, with five or more cohorts a year (species with only one generation are ‘univoltine’). During their development, the nymphs go through a series of stages, referred to as instars. For most species, there are usually 15 to 25 stages, between which they moult, shedding their external skeletons and growing as they progress from one instar to the next.

Tillyardophlebia tristis eggs. Line indicates 1 mm.

The ecology of the nymphs varies enormously: some live in burrows dug into the mud, some graze algae and detritus off rocks in fast current environments or off plants in still-water ponds; some filter the water, using fine hairs on their forelegs, for organic matter including floating detritus; and some few, very uncommonly, are predators. Consequently, mayfly nymphs are much more diverse morphologically than the adults, ranging from conspicuously flattened top-to-bottom to elongate and streamlined. Some have tusks that help them burrow while others are almost fish-shaped and strong (if usually short burst) swimmers. All nymphs, however, share several features. Their heads have a single pair of antennae, three small eyes (ocelli) and a pair of large, compound eyes, as well as a complex set of mouthparts. Behind the head is a thorax, consisting of three parts (pronotum, mesonotum and metanotum), each of which bears a pair of laterally spreading jointed legs. The wings also develop on the thorax, initially as small, somewhat triangular flat structures held tight over the back. These wing pads enlarge and darken just before emergence, at which stage the nymphs are ‘mature’. Behind the thorax is the long, 10-segmented abdomen. In most groups, there is a set of gills on each side of the first seven abdominal segments. The gills themselves can be long and slender, short and round, fringed with one or many filaments, plate-like, disc-like, single or split into two filaments. In some genera, the gills are located on top of the abdomen, are restricted to the first few segments and often are covered. Not coincidentally, nymphs with these gills typically burrow, where the gills need protection from abrasion. At the end of the abdomen trails a pair of caudal filaments, one on each side and emerging from the 10th segment. A third ‘terminal filament’ also usually arises from the 10th segment and usually, but not always, reaches as far back as the tips of the two widely spread caudal filaments. The filaments themselves are finely segmented, often long and simple, but can also be short and covered with fine setae, giving them a leaf-like appearance. In the ‘minnow’ mayflies (baetids, oniscigastrids and their relatives), these leaf-like caudal filaments are flipped up and down while swimming, rather like the horizontal tail fluke of a whale or dolphin.

Two other aspects of the nymphs are worth noting. First, nymphs of many riverine species ‘drift’. Essentially, they release from the bottom and drift downstream for up to several kilometres before reattaching to the bottom. In one study in the North American arctic region, about a third of the Baetis population drifted an average of 2 km downstream during the short summer. It is widely accepted that behavioural (as opposed to accidental or catastrophic) drift is both an anti-predator device and a means by which nymphs seek better food resources. Whether it is an ‘active’ process in which the nymphs deliberately launch into the water column, or ‘passive’ in the sense that they are dislodged while feeding by, for example, a predator, is widely debated, and the answer probably varies between species and situations. In at least some experimental studies, however, drift rates by Baetis were higher in the presence of predatory stoneflies, when food supplies were low or where competitor densities were high. One experimental study noted that, in food-rich patches, large Baetis nymphs waited until predators got much closer before drifting than in food-poor ones. Interestingly, drift rates were lower in the presence of trout and shifted from being frequent during both day and night to primarily nocturnal. Both responses were interpreted as a means of avoiding predation by visually foraging trout. The nymphs detect the trout by ‘smelling or tasting’ chemicals released into the water from the mucous covering the fish.

Blue Winged Olive (Offadens frater) nymphs feeding in an algal mat, Macquarie River.

Second, although mayfly adults do the reproducing, the eggs and sperm develop fully in the nymphs. This makes sense in light of the sometimes very short sub-adult and adult stages, during which there would not be enough time for eggs and sperm to develop. On the other hand, the apparent full development of eggs and sperm in mature nymphs suggests the possibility of ‘neoteny’. Neoteny is the evolutionary step whereby organisms that are morphologically juvenile become fully reproductive. It is common in insects, apparently more so in habitats where resources are limited and the physiological cost of transforming to an adult would be high. In mayflies, mortality rates suffered by sub-adults, in particular, during emergence can be very high, and it seems odd that no species, so far as is known, has by-passed the risky adult stages and become fully aquatic. Three hypotheses to explain this situation seem likely. First, the insects are evolutionarily conservative. Second, for neoteny to develop, both males and females need to evolve it at the same time and place; a solitary mature nymph would be unable to mate and therefore the trait would not persist in the population. This argument fails, however, for parthenogenic mayflies, in which a single female can produce more females (but usually not males) without the need to breed with a male. Parthenogenesis is common in mayflies. Third, although reproduction is the primary role of adults, perhaps an important secondary role is dispersal. Reproduction as nymphs could result in isolated populations subject to high rates of extinction, and hence be selected against if water bodies dry or are otherwise rendered unfit for survival.

At the end of the nymph stage, mayflies ‘hatch’. Hatching, or emergence, takes one of three forms: on the water’s surface, where the nymph floats to the surface courtesy of a gas bubble it forms between the nymphal and sub-adult cuticle; on the shoreline, whereby the nymph crawls wholly or partially out of the water to transform; or wholly underwater, with the emerger either crawling or floating to the water’s surface. The last has been reported for several mayfly families, including a North American leptophlebiid (Choropteres mexicanus), but not yet for any Australian species. Emergence starts with a split along the centre line of the thorax, followed by waves of contraction in the abdomen that forces tissue up and into the thorax, splitting it widely. The thorax of the sub-adult pushes out of the nymphal skin, followed by the eyes and head, resulting in the emerging insect sitting slanting forward over the remnant nymphal body. The insect then jerks and contracts the abdomen repeatedly, freeing its legs, abdomen and caudal filaments. In surface emergers, the wings unfold before the body is fully out, whereas in crawlers I have watched, it happens afterwards. The wings seem to need time to dry before taking flight which, in moist conditions, can be prolonged, to the benefit of foraging fish, birds and flyfishers.

In most tropical species, emergence occurs at night, apparently to avoid predation by visual hunters such as dragonflies and birds. In temperate regions, emergence occurs most often from mid-afternoon through to dusk, possibly the result of low temperatures at night that slow emergence and development. Numerous factors have been suggested as important cues for the daily and seasonal timing of emergences. In North American species, varying water temperature seems to be particularly important: daily hatches tend to be largest when there has been a large diurnal cycle of water temperature, and small when the flux has been small. Light intensity has also been strongly implicated in diverse taxa, including caenids which, in Tasmania and elsewhere, emerge during low light periods: mornings in most places, but also at dusk for some overseas species.

Seasonally, mayflies are renowned among flyfishers for having short, intense and dramatic species-specific emergence periods. Flyfishing books are full of emergence charts for different species, so that fishers can select the correct fly patterns to imitate the insects likely to be on the water at a particular time and place. This tight seasonality is especially well developed in some Northern Hemisphere species, in which truly massive hatches occur over a few days, so large that the ground and rivers are covered with mayflies and the swarms can be seen on radar. A recent study of Hexagenia limbata in North America estimated a single highly synchronised emergence event consisted of 87.9 billion sub-adults, collectively weighing over 3000 tonnes. In practice, these short-duration emergence periods are primarily a temperate latitude feature. Tropical species tend to emerge year-round. Among temperate species, there are two contrasting general patterns: short, burst (synchronous) emergences and long (dispersed) emergence periods. Both appear to be devices to reduce predation on the clumsy flying sub-adults. It has been suggested that short-period emergers have evolved a predator swamping system: there are so many insects in the air at the same time that predators are confused and/or quickly satiated, so that relatively few sub-adults get taken. In contrast, long-period emergences keep predators from targeting periods of high prey abundance, resulting in overall low levels of predation. Among short-period emergers, related species often have non-overlapping emergence periods, which is seen to be a means of reducing competition amongst nymphs of the different species, while also minimising risks of hybridisation and wastage of reproductive effort (because hybrids would most likely be infertile).

Atalophlebia emergence sequence. A. mature nymph drifting