Subimago of the March tan ( Rhithrogena germanica )
|Hyatt & Arms , 1890|
The mayflies (Ephemeroptera;. Of gr ephemeros - one day, pteron - wing) are the most primitive among the winged insects (Pterygota). They form an order within the class of insects (Insecta). With just over 3000 species in 42 families and more than 400 genera, they belong to the species-poorer orders. About 300 of these species live in Europe and more than 100 in Central Europe .
The animals reach a body size of 3 to 38 mm and a wingspan of a maximum of 80 mm in the species of the genus Euthyplocia . The largest European species is the Tisza mayfly ( Palingenia longicauda ), which can be up to 120 mm long including abdominal appendages.
The adult mayflies are characterized by stunted mouthparts and an intestine that is functionless for food processing . The midgut serves the animals like a body-stabilizing skeleton thanks to a firm air filling - the connection with the foregut and the rectum is closed. These structures could be modified after the original function of the intestinal tract no longer played a role here, since the animals no longer consume any food in the adult stage. The large compound eyes , which are designed as double eyes or turban eyes in the males of many species, are striking . The original compound eye is divided into two parts, one facing the side and the other facing up. There are also two short, bristle-shaped antennae on the head.
The mayflies have large wings , the front wings being significantly larger than the hind wings. The hind wings are reduced in many species or completely absent (Fam. Caenidae and Baetidae partly, e.g. in the Cloeon dipterum shown below ). Both wings have strong and clearly visible wing veins . The wings are stiffened by a corrugated cardboard-like structure made of normal veins and secondary intercalary veins lying deeper between them. In contrast to the similarly built dragonfly wings, however, they have a higher mobility in the rear part against rotation. The flight mechanism is thus similar to that of grasshoppers or cockroaches. In contrast to all new winged birds, the wings can not be laid flat on the abdomen, instead they are folded up over the back while at rest. The wings do not have a coupling mechanism, so they are not connected to each other in flight, but the wing beat is synchronous (in contrast to the dragonflies , but similar to all other pterygota).
The abdomen is generally more or less elongated and cylindrical. Mayflies have two or three long abdomen filaments , the outer two representing the cerci and the middle one representing the terminal filum . The appendages, which are sometimes several times the length of the body, serve to stabilize in flight and as parachute-like structures that slow down the descent when flying without flapping wings; they can be folded or spread apart as required. Below the tail threads sit two three- or four-membered gonopods with which the female holds the female during mating. The front legs of the males are also modified, usually greatly elongated, and form grasping organs for copulation.
The larvae of the mayflies resemble the imagines in their body shape , although besides the wings they lack the mating organs and all the special formations associated with the mating (turban eyes, elongated front legs of the males). The larvae develop directly into the imago, which is why one speaks of nymphs . The wing systems are designed as wing sheaths on the back of the thorax segments and become somewhat larger with each moult. In the last larval stage, the folded wings of the (sub) imago are visible through the larval skin. From the ovary vein to the subimago a great number (generally 15 to 25) moults.
All mayflies spend the larval stage in the water . The larvae are usually round, in some forms also flattened, they can be elongated or compact depending on the species. The larvae have large compound eyes and well-developed mouthparts . A special feature of the mouthparts in mayfly larvae is a particularly prominent tongue-shaped lobe, the hypopharynx. The paired tracheal gills on the first five to seven abdominal segments (eight in a single species), which are usually formed as external gill leaves, are noticeable . The shape, arrangement and number of the gill leaflets are very characteristic of the different families. The gills can be tufted or slit in the shape of a tree, thread-like, plate-like, they are mobile in some genera (through their movement, the larvae generate a water current on the gill plates, which constantly brings fresh water to these respiratory organs). Some groups (Heptageniidae and Oligoneuriidae) have characteristic combinations of tufts and leafy lamellae. In Cloeon (Baetidae) the gill leaflets are doubled. The gills can protrude sideways from the body or be worn sitting on the back. Another noticeable feature of the mayfly larvae are the abdominal filaments, which with a few exceptions (in Central Europe: genus Epeorus) are formed in threes; in contrast to the stonefly larvae with two threads.
The diet of the mayfly larvae generally consists of living or dead plant matter. Some species, e.g. B. from the family Ephemerellidae, can additionally (optional) also feed predatory, z. B. from mosquito larvae. In the case of non-European species, predatory diets also occur. Most species collect fine food particles in a relatively unspecialized manner, e.g. B. algae and dead plant debris, from the substrate surface, some (e.g. the Caenidae family) prefer to "sieve" the upper layers of fine, muddy substrate in which they live buried. Other groups, e.g. B. the family Heptageniidae, specialize in grazing the organic coating of algae etc. (" biofilm ") from stone surfaces. A few species (e.g. Ephoron virgo , Ephemera spp.) Are filter feeders .
A distinction can be made between the larvae:
- Digging forms (e.g. ephemera ). The body is cylindrical, the gill threads lie against the body. Front legs and (in some forms) mouthparts are redesigned for digging.
- Floating forms (e.g. Baetis , Siphlonurus ). The body shape is similar to the imago, the gills are usually designed as laterally protruding leaflets. The cerci and terminal filum are very hairy on the sides and serve as a drive when swimming.
- Stone clamps (e.g. Heptagenia ) are usually flattened in order to offer little resistance to the flow. You can barely swim.
The last molting of the larvae takes place on the surface of the water or on land, from the larval skin a not yet sexually mature, but airworthy subimago hatches , which within a few minutes (Tisza mayfly) up to three days (pond spiny Siphlonurus lacustris ) to a full-grown Insect ( Imago ) sheds its skin. Oligoneuriidae and the (Asian to Eastern European and North American) Behningiidae shed their skin in the air during their wedding flight, whereby the wings are not shed with the skin. In some families, the females are mated as subimago. Subimago and Imago are easy to distinguish: the wings of the Subimago are cloudy and covered with fine hair, the rear edge is bristled or fringed. Adults have hairless (exception: genus Caenis ), clear wings, which in some species have brownish spots. Mayflies are the only recent insects in which animals with fully developed wings shed their skin again.
Life cycle, reproduction and development
Most species develop one generation per year (“ univoltin ”), some species can have two (“bivoltin”), exceptionally three or more generations per year (“polyvoltin”). Some larger species need two years to develop, exceptionally up to four years for non-European species.
The adult animals usually only live one to four days, sometimes only a few minutes or hours, such as the Oligoneuriella rhenana , which only lives about 40 minutes. They rarely live longer than a week. This period of time is used exclusively for mating and oviposition. The last molting of the larvae to the adult animals usually takes place synchronously, which is why large swarms of male mayflies often arise. Depending on the species, these swarms fly over the water or, sometimes for miles, away from it. Females fly into these "wedding flocks" and are caught by the males, mating takes place in flight. The females lay the eggs in the water of rivers and streams (some species prefer to be in still waters ), although they have sometimes flown a few kilometers against the direction of flow of the water ( compensation flight ). Whether such directed compensation flights are actually carried out on a larger scale is, however, controversial in science. In some species there are predominantly or exclusively females; they develop parthenogenetically .
Mayfly larvae are among the most species- and individual-rich colonists of Central European rivers. In some streams they can represent up to 60% of the species of macrozoobenthos . Typically, the number of species in streams corresponds to that of the stonefly and caddis flies , only the two-winged species are sometimes significantly more species-rich. Mayflies clearly prefer brooks (the rhithral ), in the source region (the krenal ) they are only represented with a few individuals and species, whereby not a single Central European species is definitely specialized in this stretch of water (possibly some rare Heptageniidae such as Ecdyonurus subalpinus or Rhithrogena picteti as To classify source stream species). In rivers (the Potamal ) there are significantly fewer species than in streams, but there are a number of highly specialized river species that can sometimes develop in mass occurrences. Due to their sensitivity to water pollution, many of the Potamal species are threatened with extinction throughout Central Europe. Some types, e.g. B. from the genera Cloeon and Caenis , preferentially or exclusively colonize standing waters. In general, however, mayfly larvae play a much smaller role in stagnant waters and occur with only a few species and a small proportion of individuals. Mayfly larvae are important bioindicators for water quality. Most species prefer clean or slightly polluted water, and they are rich in individuals and species up to water quality class II (beta-mesosaprobe level). In quality class II-III, i.e. H. Critically polluted waters, a few species can still survive, but sometimes develop here in large numbers (often the species Baetis rhodani ). From quality class III (alpha-mesosaprobe level) onwards, mayfly larvae usually no longer occur.
The structure of the basal groups of the winged insects (pterygota) is an old problem that has not yet been finally solved. Traditionally, the mayflies were usually compared with the dragonflies as "Paleoptera" (also: Palaeoptera) to the other pterygota. An alternative interpretation compares the mayflies, then also called Archipterygota, with the other flying insects (then called Metapterygota as a taxon ). A basal position of the odonata has also been considered by some taxonomists, but has few supporters. To date, neither morphological nor molecular studies have finally been able to decide the problem, both points of view still have supporters. The monophyly of the recent Ephemeroptera has been confirmed in all modern molecular studies.
According to McCafferty and Edmunds 1979, the Central European mayflies are assigned to two sub-orders (as shown in the list). Other taxonomists have suggested other divisions.
The species of mayflies occurring in Central Europe are assigned to the following families (information with selected species):
- Subordination Schistonota
- Siphlonuridae with five species of the genus Siphlonurus
- Ameletidae (syn .: Rallidentidae) with two species
- Ametropodidae with a species ( Ametropus fragilis )
- Baetidae with 30 species
- Oligoneuriidae with three species of the genus Oligoneuriella
- Isonychiidae with a species ( Isonychia ignota )
- Arthropleidae with a species ( Arthroplea congener )
- Heptageniidae with approx. 60 species (partly newly described and controversial)
- Leptophlebiidae with eleven species
- Potamanthidae with a species ( Potamanthus luteus )
- Polymitarcyidae with a species ( Ephoron virgo )
- Ephemeridae with four species of the genus Ephemera
- Palingeniidae with a species ( Palingenia longicauda )
- Subordination Pannota
According to Fauna Europaea, there are 339 species in Europe, and 344 according to Freshwaterecology.info.
Mayflies are a very old group of insects. Fossil evidence that is assigned to the parent group of this order is already known from the Upper Carboniferous . However, the assignment of these fossils (which have been handed down as imprints on layer joints of sedimentary rocks) is questionable. At that time there probably existed numerous similar groups of insects (a "group of roots"), all of which, with the exception of today's Ephemeroptera, have become extinct. Most of the characteristic features of the order are ancient peculiarities ( plesiomorphies ) which all winged urine insects have. A special structure of the wing veins, the so-called costal brace ("costal brace"), is one of the few apomorphic features that have survived in fossil form . From the Permian there are known many reliable members of the mayflies, which are usually grouped together as a separate group of Permoplectoptera (e.g. Protereismatidae and Misthodotidae ). The larvae of the Permoplectoptera still had nine pairs of abdominal gills, and the hind wings of the adults were not yet reduced in size. The Permoplectoptera possibly also belonged to the Cretereismatidae family from the Lower Cretaceous Period of Brazil, from which otherwise fossil representatives of modern mayflies and the fossil (but modern) family Hexagenitidae were found. From the same site, however, the larvae and imago of the Mickoleitiidae ( Coxoplectoptera ) were described, which are the fossil sister group of the modern mayflies, but had very strange adaptations (e.g. fangs). The oldest inclusion in amber is the species Cretoneta zherichini (Leptophlebiidae), which is described from Siberian amber ( Albium to Santonium ). Representatives of the families Ephemeridae, Potamanthidae, Leptophlebiidae, Ametropodidae, Siphlonuridae, Isonychiidae, Heptageniidae and Ephemerellidae are known from Baltic amber ( Eocene ).
Colloquial naming: Weißwurm and Uferaas
White worm is the term for dried riverside macaws , a colloquial term for the species of the genera Palingenia (especially Palingenia longicauda ) and Mayfly Polymitarcis .
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