A Preliminary Introduction of Mayfly Nymphal Structures and Their Adaptive Variation
general characters of mayfly nymphs and their variation
The life of mayflies (Order Ephemeroptera) has four stages: egg, nymph, subimago and imago. Among them, the nymph stage is the longest development period
adult: a few hours to 3-8 weeks; nymph:14 days to 2-3 years (Edmunds et Allen 1966)
adult:1-2 hours to a few days even up to 14 days; nymph: 3-4 weeks to about 2.5 years (Brittain 1982)
egg: a week to one year, diapause : no to a winter
Because of selection and adaptation, the characters of nymph vary more greatly than that of subimago and imago. That is very useful to classification.
Body form: flat body (Hetagenia) and streamline body (Isonychina)
Head: compound eyes, occelli (Bracycercus), fron (Ephemera), angles (Vietnamella), mouthparts
Thorax: prothorax (wing pad); mesothorax, and metathorax (maybe elongened in Baetisca and Prosopistoma); leg
Abdomen: gills
the kinds of mayfly nymphal habitats
lentic water
1. free, active swimmer and clamber in lentic water (Siphlonuridae, Baetidae)
2. the sprawlers amid the silt in still waters (Caenidae, Neoephemeridae, Ephemerellidae, Tricorythodidae)
3. the burrowers in the lake or pond bed (Ephemeroidea)
lotic water
4. the free-ranging stream-lined forms of rapid waters (Baetidae, Isonychiidae, Siphlonuridae, Leptophlebiidae)
5. the close-clinging, stone-loving forms (Heptageniidae, Leptophlebiidae)
6. the trash-, silt-, and moss-inhabiting nymph (Ephemerellidae, Potamanthidae)
typical mayflies of each habitat
- free, active swimmer and clamber in lenitic water (Siphlonuridae, Baetidae)
Siphlonurus: They have better eyes than have other nymphs, better locomotor muscles, and a better developed tail fin. One or two gill plates, with leading margin, accompany apparatus in swimming and location. Its tails bear dense lateral fringes of swimming hairs. The middle tail is fringed on both sides; the lateral tails are fringed on the inner side only. The fringes of the three together compose the webbing of an excellent tail fin; the half-bare laterals are its cutting edges, and the ready adjustment of its position gives wonderful control of the direction of movement. Claws moderately long and slender. When the nymphs are at rest, their large gills are in constant motion. They swim by rapidly vibrating the caudal filaments. The legs are held posterolaterally when the nymphs swim but are extended when they rest on the bottom. All or most species are omnivorous, opportunistically feeding on plant and insect remains and capturing soft-bodied bottom-feeding insects, especially tube-dwelling chironomids and mosquitoes. - the sprawlers amid the silt in still waters (Caenidae, Neoephemeridae, Ephemerellidae, Tricorythodidae)
Caenis: stiff-legged, hairy, dirt-covered forms with stout bodies and thinly fringed tails. These nymphs settle downn amid the silt, either on the bottom or amid plant sems, moving aout but little, and that little, slowly. Their safety lies in escaping observation. The legs are spread out laterally. The body is more or less depressed; the tails are thinly hairy on all sides and of little use in swimming. Body and legs are more or less hairy all over, and silt adheres copiously to the hairs, giving excellent concealment. In such places the gills need protection from the dirt which would clog them and interfere with air taking. One pair of gills in front has become thickened and broadened and has developed into large poercula of gill covers, overlapping and protecting the gills behind them from falling silt.This cover is alternately lifted and lowered again in breathing. Interlacing fringes upon the margins of the thin underlying gills strain the silt out of the water as it enters the chamber beneath the gill cover, keeping the inner area clean. Some species are tolerant of considerable pollution. Omnivorous. The nymph feed chiefly on plant material but sometimes on dead nymphs or other organic material. - the burrowers in the lake or pond bed (Ephemeroidea)
Ephemera: pale, more or less thinly hairy forms, with narrow heads, tusked mandibles, digging front tibie and plumose gills, close-laid over the back. Mandibular tusks with long setae principally in the basal half; with scattered spines. Head with frontal process bifid; usually widest near apex. Flagella of antennae with whorls of setae at least in basal two-thirds. Gills on segment 1 bifid near base. Anterior lamella of gill 2 with outer margin of plate-like portion expanded only slightly at base. These live in the mud and sand of the bottom, more or less out of harm's way. The front feet are flattened like scrapers, and the head is narrowed forward. The short legs are drawn close up under the body. Most remarkable of all, the mandibles are produced forward into long, strong, tapering, horn-like tusks that may be longer than the head. These are used for lifting the roof of the burrow. When the nymphs are taken from their positions in the sand, they immediately begin burrowing. They exhibit very pronounced negative phototropism. The gills are moved in an undulating fashion, the movement beginning at the anterior end of the body and progressing toward the posterior end. The nymphs are fair swimmers, using undulating movements of the entire abdomen. The food of the nymphs apparently varies by species. - the free-ranging stream-lined forms of rapid waters (Baetidae, Isonychiidae, Siphlonuridae, Leptophlebiidae)
Isonychia: streamline body, little, slender, agile, and protectively colored. Mouthparts hairy. Antennal length up to 2.5 times width of head. Gill tufts present at bases of maxillae. Vigorous swinmmers. Negatively phototropic. Their enemies here are numerous. Isonychia dwells on the steam bed and is a fine, big fellow. This nymph anchors itself to the rock bed with its claws and its erect with the current sweeping the sides of its shapely body, its head up, its stout gill plates outspread, its caudal plumes trailing, its long fore feet extended forward.
Most curious and interesting is the food gathering equipment of these big fore legs. They bear two rows of long, stiff, brownish bristles along the anterior side. These rows of bristles are set at an angle with each other, opening forward, upstream. Together they constitute a collecting basket that is held ever ready to receive such contributions of food as the current may sweep into it. The bottom of the basket lies beneath the head, convenient to the jaws. - the close-clinging, stone-loving forms (Heptageniidae, Leptophlebiidae)
Heptagenia: very flat, widened toward the front, with eyes and antennae dorsal in position. They have broad, depressed femora, slender tibiae, and wide-spreading tails. - The trash-, silt-, and moss-inhabiting nymph (Ephemerellidae, Potamanthidae)
Drunella: Sprawling, stiff-legged, with tusks and spines. Abdomen with or without dorsal paired submedian tubercles; gills present on segments 3-7 with rudimentary gills present on segment 1. Omnivorous. Most nymphs cling very tightly to the objects to which they are attached. When removed from the water, they remain quiescent; as they dry out, some slow, deliberate movements can be seen. Those living in moss will leave the moss as it begins to dry. In some forms the nymphs flick the tail forward over the abdomen, then straighten it. Repeating the act continually. Peckarsky (1987, cited by Gupta et al 1999) that an abdomen-bending "scorpion"-like posture by Ephemerella nymphs resulted in the cerci and the flexed abdominal spines acting as a defence mechanism against predatory stoneflies.
Potamanthus: the flatter body, mandibular tusks projecting forward and visible from above; with only inconspicuous setae and short spines ondorsal surface and lateral margins. Femora flattened; tibiae and tarsi slender, cylindrical; gills lateral in position; gills on segment 1 rudimentary and unbranched; gills on segments 2-7 paired, each lamella slender and pointed, margins with ling fringes. Three caudal filaments; bare at extreme base and at tip, terminal filament with setae on both sides, cerci with setae on mesal side only. Smoother nymphs live on the bottom amid rather clean dirt, silt, and sand, and they possess a rather pretty, highly protective color-pattern. Filtering !
MAIN IDEAS
- In contrast with the uniformity of the adults, the nymphs of mayflies are remarkably differentiated in adaptation to diverse environments, that is useful to taxonomy;
- According to the nymph characters, we can get some information of their habitats and habits; vice versa.
- In order to increase the diversity of sample or collection, we should collect in different eco-environments as much as possible
- When we analyze the phylogeny of mayflies, the characters of nymph should be selected carefully because of their variation, convergence and parallelism may be common.
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