

CHAPTER 3 



Ephemeroptera 



By W. C. Day 

 Oakland, California 



Mayflies are the familiar insects with fragile bodies 

 and slender tails which swarm in the vicinity of ponds 

 and streams. They occupy an important place in 

 writing and folklore because of their ephemeral exist- 

 ence in the adult stage. The angler's fly box contains 

 many feather and fur imitations of the mayflies, and 

 his literature has been filled for five hundred years 

 with descriptions of "duns" and "spinners" as the 

 winged forms are called. 



The nymphs of mayflies occupy an important place 

 in the economy of aquatic communities. They are the 

 "cattle" or "rabbits" of the aquatic environment, 

 transforming plant tissue into animal. They are practi- 

 cally defenseless, making up in numbers for their 

 vulnerability. Thus they fulfill all the requirements 

 of a basic herbivore. Since they are so readily avail- 

 able, mayfly nymphs are preyed upon by practically 

 every aquatic predator. 



Adult mayflies also play their role in the economy 

 of nature; they are eaten by birds, dragonflies, and 

 fish, to mention only a few of their more important 

 enemies. 



Mayfly nymphs respire primarily by means of artic- 

 ulated gills which are situated at the sides of the 

 abdominal segments. Of the ten abdominal segments, 

 from four to seven bear gills at their posterolateral 

 corners. The various nymphs are well adapted, each 

 to particular types of aquatic habitats, and Needham, 

 Traver, and Hsu (1935) proposed the following classi- 

 fication based upon body form and other adaptations 

 which show a close correlation with particular habitats. 



A. Still water forms 



Climbers amid vegetation (Siphlonurus, Callibaetis) 

 Sprawlers on the bottom (Choroterpes, Tricorythod.es) 

 Burrowers in the bottom (Hexagenia) 



B. Rapid water forms 



Agile, free-ranging, streamlined forms (Isonychia, 



Ameletus, Baetis) 



Close-clinging, limpet forms found under stones (Hep- 



tagenia, Ironodes) 



Stiff-legged trash-, silt-, and moss-inhabiting forms 



(some Ephemerella) 



Although the nymphs of some species mature within 

 a few months, and other species require a period of 



two years, the great majority of mayfly nymphs live 

 in the water for one year. Each species has its own 

 time for emergence — from February through November 

 in California. The nymphs of a small number of species 

 leave the water by climbing onto rocks or stems of 

 plants. Most of them swallow air, rise to the surface 

 film, break the nymphal skin along the back and, 

 clambering from it, dry their wings for a moment and 

 fly into the surrounding vegetation. 



When they first emerge from the water, mayflies 

 are dull in appearance and more or less pubescent 

 and at this stage are known as "subimagoes." In 

 a period of from a few minutes to forty-eight hours 

 (about twenty-four hours for most species) the subi- 

 mago molts to become the final adult or imago. No 

 other order of insects undergoes such a second stage 

 of development between nymph or naiad and final adult. 



The true adult of the mayfly is ordinarily smooth 

 and shining, usually with glassy but perhaps lightly 

 tinted wings, and with longer tails and legs than the 

 subimago. Adults live only for a few hours or, at the 

 most, for a few days, and the mouth parts are vestigial 

 and nonfunctional. The sole function of the adults is 

 reproduction, and to this end they expend most of 

 their energy. 



Mating swarms consist of males with an occasional 

 female which flies in for mating. Swarms of many 

 millions of these insects occur in the Sacramento 

 Valley, but large flights are rarely encountered else- 

 where in California; most swarms consist of just a 

 few hundred. Gravid females deposit eggs on the 

 water surface, and the eggs sink to the bottom. An 

 adult mayfly at rest, Ameletus amador Mayo, is shown 

 in figure 3:1. 



COLLECTING MAYFLIES 



Nymphs. — The one piece of essential equipment for 

 collecting mayfly nymphs, effective in taking un- 

 damaged specimens in most lotic or moving California 

 waters, is the simple hand screen. A good screen is 

 made from a three-foot length of sixteen-mesh copper 



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