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6.4. A mayfly naiad (nymph) of Hexagenia (above), which has tusks protruding from its mandibles that are used for burrowing. A more typical mayfly naiad is below. Like the adults, immature mayflies have three terminal filaments, a groundplan feature of insects. Length 23 mm (above), 8 mm (below).

number into the tens of millions of individuals and form clouds of dense swarms. Within a day of swarming, their corpses can pile up on roadways or in towns where they had been attracted by lights. In temperate regions adults emerge in warm seasons such as late spring or early summer, usually during the evening hours. In the tropics they swarm earlier in the day or even in the morning. Typically swarms are timed to occur during a single evening or over a couple of days, although they can persist for months, slowly dribbling out individuals that quickly seek a partner before passing away. Once a male and female locate each other, copulation takes place in flight. The male grabs the female with his long, outstretched forelegs and then clasps her abdomen with his genital forceps (which are modified gonostyli). The paired penes (a defining feature of the order) are inserted into the female gonopores located between the seventh and eighth abdominal sterna. Most females oviposit directly into the water either by inserting the apex of the abdomen into the water, or they may "air drop" packages of eggs. A few species carefully place their eggs on aquatic substrates such as stones.

Immature, wingless mayflies, or naiads, are immediately notable for their three caudal filaments and lateral abdominal gills (Figure 6.4). Species occur in a diversity of aquatic systems, from small, cold streams to slow, murky rivers, and lakes, occupying littoral to benthic zones. Naiads are capable of "swimming" but typically adhere close to the substrate and overall are relatively flattened. Swimming occurs by ventraldorsal undulations of the body and terminal filaments, in stark contrast to the unique lateral undulations of the abdomen of stonefly nymphs. Naiads are herbivorous or detritivorous, typically scraping the substrate for algae or diatoms, though a few are carnivorous, such as some hepta-geniids (e.g., McCafferty and Provonsha, 1986). Naiads of the Ephemeroidea and some of the Leptophlebioidea have developed elaborate "tusks," which are actually mandibular processes that point forward (Figure 6.4). The tusks and sometimes modified forelegs are used for digging burrows or for feeding (Keltner and McCafferty, 1986; McCafferty and Bae, 1992; Bae and McCafferty, 1995).

Ephemeroptera are the only pterygote insects with a winged subimago; loss of this molt occurs in all other ptery-gote orders. Emergence is actually done as the subimago, later molting into true, reproductively mature adults. In some species mating takes place in the subimago instar, which probably are males mating with females that will soon emerge as imagos.

Recent phylogenetic and classificatory treatments of the order are by McCafferty and Edmunds (1979), Landa and Soldán (1985), Kluge (1989, 1998, 2000, 2004), McCafferty (1990, 1991, 1997a), and Tomka and Elpers (1991). Different classifications are advocated by various authors, and there is presently no stability in the application of suborders and infraorders within the Ephemeroptera. Numerous cladistic studies have been undertaken on families and superfamilies of mayflies (e.g., Bae and McCafferty, 1995; Wang and McCafferty, 1995; Wang et al., 1997; McCafferty and Wang, 2000), and a complete classification of the order is perhaps imminent. The traditional division into Schistonota and Pannota (McCafferty and Edmunds, 1979) is no longer followed because the latter is derived from the former (e.g., McCafferty, 1991). Presently three suborders are recognized: Furcatergalia (including the former Pannota), Carapacea, and Pisciforma (including the Setisura and Branchitergalia) (Figure 6.5).

Mayflies are relatively well represented in the fossil record, and, not surprisingly given their phylogenetic position, there are some ancient fossils of them. The earliest definitive mayflies stem from the earliest Permian, although some enigmatic fossils from the Carboniferous are likely close relatives to these. Among the most curious creatures representing early

6.5. Phylogeny of the mayflies, order Ephemeroptera, with significant characters indicated (Table 6.1). Modified from Willmann (1999), McCafferty (1997), McCafferty and Wang (2000), and Kluge (1998, 2000).

TABLE 6.1. Significant Characters in Ephemeroptera Phylogeny0

1. Costal brace at base of forewing

2. Anal region in hind wing reduced

3. Aquatic naiads

4. Crossvenation more irregular and sparse; hind wings (where known) somewhat shortened

5. Heteronomous wings (hind wing reduced); male forelegs elongate; tarsi reduced; male terminalia paired; adult mouthparts reduced (unknown in most fossil forms)

a Numbers correspond to those on phylogeny, Figure 6.5.

mayflies or stem-group mayflies is Lithoneura lameerei (Syntonopteridae) from the Late Carboniferous of Mazon Creek, Illinois (Figure 6.6). The fossil was originally described as a member of the extinct order Palaeodicty-optera (Carpenter, 1938a, 1987, 1992), but was later assigned to the Ephemeroptera by several other authors (e.g., Edmunds and Traver, 1954; Edmunds, 1972; Wootton, 1981) or placed in its own order/suborder, Syntonopterodea (e.g., Laurentiaux, 1953). Hubbard and Kukalova-Peck (1980) and Kukalova-Peck (1985) were the first to report the putative presence of a haustellate beak, swollen clypeus, englarged compound eyes, and elongate antennae in Lithoneura, which would corroborate the palaeodictyopteran hypothesis. Willmann (1999), however, countered the reconstruction of

6.6. The Late Carboniferous Lithoneura lameerei (Syntonopteridae) from Mazon Creek has been a controversial fossil concerning the phy-logeny of early winged insects. Lithoneura and other "Syntonopterodea" are now understood to be stem-group Ephemeroptera. MCZ 4537; greatest width 65 mm.

Lithoneura, indicating that certain traits are not preserved in the fossil and that it indeed shares a derived formation of a triad between Rs, M, and Cu near the wing base with the Ephemeroptera. However, the fore- and hind wings of Lithoneura were very similar to each other (they were homonomous), slight differences being that the hind wing possessed a large anal fan and that the forewing lacked a costal brace (Willmann, 1999). Thus, Lithoneura cannot be considered a true mayfly. Instead, Lithoneura is representative of a stem group that eventually gave rise to the true Ephemeroptera.

Another enigmatic fossil is Triplosoba pulchella (Triploso-bidae) from the Late Carboniferous of Commentry, France (Brongniart, 1893). Like Lithoneura, this fossil has weighed considerably on the mind of systematists and generated wide opinions concerning its identity, although most have agreed that it is somehow related to the mayflies (e.g., Handlirsch, 1906b; Lameere, 1917; Martynov, 1923; Tillyard, 1932; Carpenter, 1963a; Rasnitsyn and Quicke, 2002). The body preserves three caudal appendages and is therefore excluded from the Metapterygota, which includes the Odonatoptera, Palaeodictyopterida, and Neoptera. Willmann (1999), however, noted that Triplosoba, unlike Lithoneura and Ephemeroptera, has the stems of R and Rs fused near the wing base, a feature considered by him to be a derived trait of Metapterygota and Triplosoba. The Triplosobidae may represent a lineage sister to the Metapterygota, the group that consists of all other winged insects.

Various Permian-Jurassic families of Ephemeroptera are believed to be stem groups to modern Ephemeroptera, such as the extinct families Protereismatidae, Bojophlebiidae, Mis-thodotidae, Mesoplectopteridae, and Mesephemeridae (and perhaps also Jarmilidae and Oboriphlebiidae, but these are known only from naiads); indeed, these families possess the distinctive costal brace in the forewing and have a reduced

6.7. An early mayfly from the Early Permian of Elmo, Kansas, Pro-tereisma permianum (Protereismatidae). These early mayflies, while sharing some traits with modern Ephemeroptera, had homonomous wings. YPM; forewing length 18 mm.

anal region in the hind wing, similar to modern mayflies (Figures 6.7, 6.8, 6.9). However, unlike modern Ephemeroptera, these families possessed homonomous wings and apparently had well-developed mouthparts in adults. These families are often placed in a separate suborder, the "Permoplectoptera" (e.g., Tillyard, 1932; Hubbard, 1990; Kluge, 1998), but this group is likely paraphyletic and a stem group to the modern suborders of Ephemeroptera (the latter is sometimes referred to as Euplectoptera or Euephemeroptera: e.g., Kluge, 2000, 2004). These stem-group taxa persisted throughout the Triassic (e.g., Lin, 1986; Via and Calzada, 1987; Sinitshenkova, 2000c; Krzeminski and Lombardo, 2001) and, on the basis of a few fossils of Mesephemeridae, into the Late Jurassic, disappearing from the record at that time.

Two fragmentary Triassic fossils, at times associated with the Ephemeroptera, deserve particular mention. The family Litophlebiidae was originally described as a mayfly from the Triassic of South Africa (Riek, 1976b; Hubbard and Riek, 1978). The family was moved into the Megasecoptera by Hubbard and Kukalova-Peck (1980), where it would

6.8. Forewing detail of Protereisma permianum (Protereismatidae) from the Early Permian of Kansas. Note the costal brace near the base of the wing, which is a defining feature of Ephemeroptera. MCZ 34056; forewing length 20 mm.
6.9. A naiad (nymph) of Protereisma americanum (Protereismatidae) from the Early Permian of Kansas. The wingpads of many Paleozoic naiads protruded from the body, unlike modern insects. MCZ 80356; wingpad length 6.5 mm.

potentially represent a post-Permian persistence of this latter, palaeodictyopterid order. As considered by Rasnit-syn and Quicke (2002), the fossil does not permit assignment to Megasecoptera and instead is an ephemeropteran proper, like other Triassic mayflies. Similarly, the Triassic fossil Thuringopteryx from Germany was described as a dragonfly (Kuhn, 1937), moved into the Orthoptera (Zeuner, 1939), and then into the Palaeodictyoptera (Bechly, 1997). Like, Litophlebia, the assignment of Thuringopteryx to a palaeodictyopterid order would suggest a post-Permian survival of this otherwise Paleozoic superorder and would be an extremely significant discovery. However, the fossil is based solely on fragments of a hind wing that possesses no derived features of any of these lineages and instead more closely resembles a mayfly

6.10. Like early relatives of dragonflies, some extinct mayflies reached impressive proportions, such as this naiad of Ephemeropsis melanurus from the Early Cretaceous of Baissa. PIN 3064/3313; length 37 mm (1.5 in.).

hind wing (e.g., Rasnitsyn and Quicke, 2002). Indeed, a definitive placement of Thuringopteryx is not possible until more complete material is discovered, but an ephemeropteran assignment presently seems more justified than any other ordinal placement.

Ephemeroptera with fore- and hind wings very different in structure (heteronomous) are first known from

6.11. A mayfly naiad, Protoligoneura limai, from the Early Cretaceous of Brazil's Santana Formation. Mayfly naiads are abundant in this deposit. AMNH; length 9 mm (excluding terminal filaments).

6.12. An adult mayfly from the Santana Formation of Brazil. AMNH; body length 16 mm (excluding terminal filaments).

6.13. Although superficially resembling a myriapod, this is actually an extremely unusual, laterally compressed mayfly naiad with stubby gills that appear like legs. It is preserved in 120 myo limestone from Brazil. AMNH; length 18.5 mm.

6.12. An adult mayfly from the Santana Formation of Brazil. AMNH; body length 16 mm (excluding terminal filaments).

the Early Jurassic, and several modern families were already present by the end of this period. It would appear that a lineage consisting of heteronomous Ephemeroptera came into existence sometime in the Triassic and were diverse by the end of the Jurassic. Certainly in Jurassic and Early Cretaceous deposits mayfly naiads are not uncommon, and some, like Ephemeropsis melanurus (Figure 6.10), could reach remarkable proportions, with a forewing length of nearly 40 mm (1.7 inches) and wingspans near 90 mm (3.5 inches). By the Cretaceous mayflies are more abundant and can be extremely numerous regionally, although not very diverse (e.g., Tsher-nova and Sinitshenkova, 1974; Sinitshenkova, 1975, 1976, 1986; Jell and Duncan, 1986; McCafferty, 1990; Lin and Huang, 2001) (Figure 6.12). Naiads can be so abundant in some Early Cretaceous lacustrine deposits that they are used for stratigraphic dating (Figures 6.11, 6.13). Mayflies are rare in amber but have been discovered in several Cretaceous (e.g., Tshernova, 1971; Kluge, 1993, 1997; McCafferty, 1997b; Peters and Peters, 2000; Sinitshenkova, 2000a,b; Grimaldi etal., 2002) (Figure 6.14) and Tertiary (e.g., Demoulin, 1954a,

6.13. Although superficially resembling a myriapod, this is actually an extremely unusual, laterally compressed mayfly naiad with stubby gills that appear like legs. It is preserved in 120 myo limestone from Brazil. AMNH; length 18.5 mm.

1955, 1956, 1965, 1968, 1970a,b; Hong, 1979; McCafferty, 1987; Staniczek and Bechly, 2002) resins (Figure 6.15). Numerous fossils of a diversity of families are known as compression fossils from throughout the Tertiary (e.g. Lewis, 1978; McCafferty and Sinitshenkova, 1983; Fujiyama, 1985; Zhang, 1989; Richter and Krebs, 1999; Sinitshenkova, 1999; Masselot and Nel, 1999), and, although not providing insight into the higher-level relationships within the order, these do provide a perspective on Cenozoic taphonomy, biogeogra-phy, and evolution of mayflies.

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