Mayflies

Ephemeroptera (= Ephemerida).

Portuguese: Siriruias (Brazil).

This is considered the most primitive of all the living orders of winged insects. Only mayflies undergo a molt after acquiring functional wings. The wings are incapable of being folded rearward and often possess a very complex, netlike venation, both ancestral characters indicative of early origin. The hind wing is always much smaller than the triangular fore wing and in many cases is lost altogether.

Adult mayflies are further recognized by their strongly developed eyes, particu larly in the males. These are often divided into upper and lower portions of larger and smaller ommatidia. The antennae are mere stylets, but the two or three terminal sensory filaments (cerci plus median caudal filament) are very long, extending from the tip of the abdomen. Mouthparts are vestigial and the legs weak or reduced, or even vestigial in Campsurus (fig. 6.1a). Adults of most species are dull colored. Those in the widespread genus Thraulodes (fig. 6.1c) may have striking patterns; male Tricorythodes are black with milky wings. The wings of the males in many species have partially maculate wings.

The biology of mayflies has been extensively studied by entomologists and limnolo-gists (Flannagan and Marshall 1980). The body form of the aquatic nymphs differs much among families but usually little from that of its own adult. Eyes, antennae, and mouthparts are well formed, as are the cerci and median caudal filament. Conspicuous also are four to seven pairs of articulated, lateral, platelike gills (often double plates) on most of the abdominal segments. Nymphs also display varied and sometimes bizarre shapes as specializations to their different submerged aquatic habitats. Slender, cylindrical types are strong, "minnowlike" swimmers, occupying still water in ponds and stream pools and sometimes mountain torrents. There are flattened forms that lodge between and under rocks, and some have splayed legs that cling to exposed rock surfaces in fast currents (e.g., Thraulodes, fig. 6.id). Others are robust, with heavy, shovel-shaped, spiny legs and head used for burrowing in bottom muds or sand (e.g., Campsurus, fig. 6.1b).

The nymphs of most species are herbivores or scavengers, taking detritus and aquatic microorganisms, especially diatoms. A minority, such as Chiloporter and Chaquihua in Chile and Argentina, are predatory on other small aquatic invertebrates (Edmunds pers. comm.).

On maturing, the nymphs transform

Figure 6.1 MAYFLIES, (a) Legless mayfly (Campsurus albicans, Polymitarcyidae). (b) Legless mayfly (Campsurus sp.) nymph, (c) Tropical mayfly (Thraulodes sp., Leptophlebiidae). (d) Tropical mayfly (Thraulodes sp.) nymph.

into the alate, flying, but sexually immature subimago (the "dun"). The transformation usually takes place at the water's surface but may also occur below the water or after the nymph has crawled out of the water onto some object. This stage soon metamorphoses into a reproductive adult (imago). Subimagos have infúscate wings and the integument covered with mi-crospines to distinguish them from the glassy-winged, glossy-surfaced imagos (Edmunds 1988).

Imagos are ephemeral, their active lives lasting only a few hours or, at most, days. They do not feed and spend most of their short existence on the wing, mating and egg laying. They generally remain near their breeding grounds where they are seen flying or resting. Many species swarm, some (as in the genera Tortopus and Campsurus) in such great numbers as to constitute a plague, especially when drawn to street lights or house lights in urbanized areas. Thousands of individuals, mostly females, pile up in the streets or press indoors to make themselves a nuisance. Within these swarms, the sexes copulate in flight, and females fall into the water as they release their eggs.

Mayflies in all stages form a large part of the diet of freshwater fish and small riparian birds. They are also eaten by many types of carnivorous aquatic insects. The unportance of some prolific species in food chains has caused them to be referred to as "insect cattle" (e.g., Callibaetis).

Mayfly classification and evolutionary history are complex (Edmunds 1972). There are 13 families, perhaps containing, when all are discovered, about 500 or more species in all of Latin America, extrapolating from Hubbard's (1982) list of 300 currently named species. Their poor abilities to disperse make them useful as biogeographic indicators. Although some common genera in South America may extend to North America, members of several groups in southern South America show closer relationships with species from other austral areas (Australia, New Zealand, and southern Africa) than with species to the north. Examples include the genera Metamonius, Chiloporter, Chaquihua, and Siphlonella of the Siphlonuridae and Nousia of the Leptophlebiidae (Pescador and Peters 1982, 1985). For general reviews of the order in Latin America, see Hubbard and Peters (1977, 1981) and Edmunds (1982). An important taxonomic paper is the review of the Neotropical Leptophlebiidae by Savage (1987).

References

Edmunds, Jr., G. F. 1972. Biogeography and evolution of Ephemeroptera. Ann. Rev. Entomol. 17: 21-42. Edmunds, Jr., G. F. 1982. Ephemeroptera. In S. H. Hurlbert and A. Villalobos Figueroa, eds., Aquatic biota of Mexico, Central America and the West Indies. San Diego State Univ., San Diego. Pp. 242-248. Edmunds, Jr., G. F. 1988. The mayfly sub-imago. Ann. Rev. Entomol. 33: 509-529.

Flannagan, J. F., and K. E. Marshall, eds. 1980. Advances in Ephemeroptera biology. Plenum, New York. Hubbard, M. D. 1982. Catálogo abreviado de Ephemeroptera da América do sul. Pap. Avul. Zool. (Sáo Paulo) 34(24): 257-282. Hubbard, M. D., and W. L. Peters. 1977. Ephemeroptera. In S. H. Hurlbert, ed., Biota acuática de sudamérica austral. San Diego State Univ., San Diego. Pp. 165-169. Hubbard, M. D., and W. L. Peters. 1981. Ephemeroptera. In S. H. Hurlbert, G. Rodriguez, and N. Dias dos Santos, eds., Aquatic biota of tropical South America. Pt 1. Arthro-poda. San Diego State Univ., San Diego. Pp. 55-63.

Pescador, M. L., and W. L. Peters. 1982. Four new genera of Leptophlebiidae (Ephemeroptera: Atalophlebiinae) from southern South America. Aquatic Ins. 4: 1 — 19. Pescador, M. L., and W. L. Peters. 1985. Biosystematics of the genus Nousia from southern South America (Ephemeroptera: Leptophlebiidae: Aetalophlebiinae). Kans. Entomol. Soc. J. 58: 91-123. Savage, H. M. 1987. Biogeographic classification of the Neotropical Leptophlebiidae (Ephemeroptera) based upon geological centers of ancestral origin and ecology. Stud. Neotrop. Fauna Environ. 22: 199-222.

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