Mayfly

Mayflies are the group of aquatic insects that belong to the genus Ephemeroptera which is a part of an ancient group of insects that fall under the Paleoptera genus that also contains fireflies and damselflies. The common names with which mayflies are popularly known are drake, dayflies, fishflies and sandflies. There are over 3000 types of mayflies known worldwide which belong to 42 families and are divided into 400 genera. They have a very short life span and populate in huge numbers during summers. Mayflies are known to exhibit a lot of ancestral traits of the first known flying insect such as long tails and wings that do not fold in a flat position over the abdomen area. The interesting fact about mayflies is that as soon as they enter into the winged stage, they become unable to feed. 

Thus the appearance of a winged mayfly can be summarized as large eyes, short and bristle-like antennae, a nonfunctional mouth and a digestive system. Usually, the wings of the mayfly are in an upright position as that of a butterfly and it consists of a large memberous triangular-shaped forewing part and a small and rounded hindwing part. In many species, the rounded hind wing is reduced to the smallest part or is completely absent. The adult mayflies have a thin long tail that is either as long as their body or even longer than that. Mayfly hatches, that is, emerges as an adult from spring to autumn in large numbers but not in the month of May. These hatches usually attract many tourists and fly fishermen use these mayflies hatches by creating artificial flies that help the fisherman to catch some definite fish species that are targeted. 

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Description of Mayfly

Mayflies in their immature stage are in the form of aquatic freshwater known as naiads or nymphs. Their presence indicates the water is fresh and unpolluted. 

1. Nymph: Nymphs are the immature form of mayfly that is an aquatic freshwater form and unlike their adult counterpart with a short lifespan, they can live for years in freshwater. They have a cylindrical, elongated and flattened body that undergoes several stages, moulting that results in the formation of puma which is also known as mayfly cocoon and every time they increase in size. The various species of mayflies vary in their sizes before they are ready to get out of water surpassing the nymph stage. Their size usually varies from 3to 30mm. 

Their head is usually covered with a hard covering of sclerotin and has various hard projections and rigids that are usually pointed forward or downwards with a mouth attached in front. They have two large compounded eyes and three normal eyes known as ocelli attached to their head. They also have two antennas either between or in front of the eyes and vary in their length. The mouthpart of the mayfly is designed primarily for chewing and has a flap-like structure that is attached just in front of the mouth and is present in all the Euarthropoda and a pair of strong appendages near its mouth. It also possesses a pair of maxillae and hypopharynx and labium that is membranous in structure. 

The thorax consists of three parts, namely, the two hindmost mesothorax and metathorax that are fused with one another. Each segment of the thorax bears a pair of legs that terminates in a single claw. Their legs are quite robust in nature and are covered in spines, hairs or bristles. Their abdomen is divided into ten parts out of which some of them are covered with a large pair of gills, a thoracic shield and a developing pair of wing pads. The abdomen finally terminates into two or three projections that look like slender threads. 

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2. Subimago: Subimago is the final moult of a nymph that is in the winged stage but is not fully adult. Though they develop wings, they are not sexually mature and dull in colour. Subimago has partially developed wings that are covered with microtrichia which are basically minute hair. Its eyes, legs as well as genitalia are not fully grown and mature. But often the female mayflies do not evolve from subimago and become fully adult and sexually mature while they still appear like a subimago with subimago on the wing membrane. 

Submigo as not being a full adult lake in colour and pattern to attract the males, are poor fliers and have short projections on them. The male in the white mayfly species that are in the subimago stage has shorter and compressed folds resembling accordion which usually expands to double its actual length after the moult. The subimagos in some species moult in a certain period of time that may take a few days and in some they take a few minutes to molt into a fully winged adult. Thus mayflies are the only insects that undergo moulting even after being a complete winged formed adult. 

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3. Imago: Imago is the adult and fully mature form of mayfly whose structure resembles that of an ancient form of the insect with two wings that do not fall flat over the abdomen. They are very delicate looking flies that possess long tails and a pair of giant rectangular-shaped membranous wings that are very thin and are covered with numerous veins. The forewings are always in the upright position while they are resting. The hind wings are small and round in shape and are often absent. The second segment of the thorax that is responsible for holding the forewings is enlarged as they are responsible for holding the forewing muscles. They usually have a fully functioning compound eye with three ocelli, short but flexible antennae and a non-functional abdomen and mouthpart. 

Male imagos usually have large eyes and the foreleg to be able to locate and catch the female imago during air mating. In a few families of mayflies, the males possess turban eyes that are in the upward direction in addition to the lateral eyes. These turban eyes are ultraviolet radiation-sensitive and are often used to detect any female mayflies flying overhead during courtship. In some species apart from a pair of legs in male, all the legs are nonfunctional for the rest of the mayflies in the family. Among insects only the imago, that is, fully adult mayflies have two genitalia, that is two aedeagi in males and two gonopores in females.   

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Types of Mayfly

1. Giant Mayfly: Giant mayflies are often seen in the Eastern United States in huge numbers in the summer season. They are the only type of mayflies that primarily reside in the Eastern United States and are mostly found in and around the water sources. The giant mayfly can be identified by its yellow and brown pattern on their bodies with two triangular-shaped giant darkened wings and two filamented tails. They have a very short lifespan after their adulting that may last for some days before they die. Thus they channelize all their energy for reproduction before they die and thus generally fly in large groups. 

The females lay their eggs in clean and unpolluted water with slight sedimentation so that the larvae are allowed to get buried into the silt and can hide before they are hatched into the nymphs that have identical three-pronged tails and resemble more like their adult counterparts. The larvae of the giant mayflies are more yellow, brown to black in colour. They mostly feed on algae and underwater plants and are then eaten by the fishes when they turn into the subimago stage. Just because of their colour and appearance they are also known as golden mayfly as they belong to the Hexagenia genus.

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2. White Mayfly: White mayflies are biologically named Ephoron Leukon whose hatches fly off in such huge numbers in a group that it appears like a blaze of snow. The two categories of white mayflies,  Ephoron Leukon and Ephoron album, are commonly found in giant numbers in many territories around and by many streams and rivers. They are mostly seen in the month of august in overwhelming numbers. Ephoron Leukon mostly in their larvae stage remains in the burrows of the slit where they are hatched into nymphs that come to the surface of the water. 

They quickly turn into the winged subimago that is not fully adult and fly off. The male counterpart quickly molts into spinners and comes back to the water bodies for mating. Their hatching starts in the month of august at dawn and continues till the twilight hours of the night. Their hatches last for 2 to 3 weeks. Ephoron Leukon adults are highly sensitive to light. 

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3. Black Mayfly: The black mayflies are the submigoes of the genus H. Bilineata and that is the reason they look darker in their colour. But as they grow to be fully adult with fully grown wings, their wings become more transparent and shimmery in nature with a typical body structure where a double lateral line runs along its abdomen. They mostly last for 24 hours as the lifespan of the adult black mayfly is very short. Thus when the black mayfly is in their subimago stage that often last for 24 hrs or less they are either resting in the silt or are in shoreline vegetation. As they quickly molt into an adult mayfly they initiate air mating.

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Reproduction and Life Cycle

Soon after the final molting stage happens and the mayflies become fully adult and sexually active, the mating begins as their lifespan is very short and they mostly live for a few hours or a day. They die as soon as mating and egg deposition takes place. Though the males of the Hexagenia can live up to two consecutive days to engage in flight matings and the female imagos who are carrying the eggs and haven’t engaged in mating can also live up to two successive days. As dusk approaches, the male imagos takes a flight in groups over the water body performing a flight mating, flying into any air current or the breeze. The individual male imago within the group may fly up and forward and then swing downwards in a dancing motion. 

Soon the female imagos join them in the upward and forward motions and then fall downwards as the dancing continues. The male counterpart comes closer to the female from below and behind and grasps her thorax with their long elongated legs and the mating is completed on the wings. Soon after the female imago hatches the egg and dies and in few species the female imago is ovoviviparous, which means, while the female mayfly is dead and floating over the river, the hatching of the egg takes place within the body. Though the oviposition varies with the varied species. Some of the female mayflies drop their egg into the water from a height of a few feet while some others take a flight to the surface of the water at certain intervals, touching their abdomen on the water surface. 

And while doing that they hatch a few eggs each time. Few females exert out the eggs from the two oviducts in form of two elongated packet-like structures that adhere to one another while they are dropped. But most of the time the female mayflies fall on the surface of the water with their wings spread and squeeze out the egg before they die. The rarest form of oviposition is when the female imago aligns herself to the object that is emerging out of the water surface and then crawls into the sedimentation bed underneath the water level to hatch their eggs into the slit so that they can be borrowed into the silt for security. The female imagos that flies close to the waterbody is subjected to the fish attack and fish feeding.

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The life cycle after hatching of eggs occurs in four stages, namely, nymph, subimago and imago. The size of the eggs varies widely in size and shape, from round to oval to elongated pocket shaped. A female can produce eggs that may vary from 50 to 1000 eggs at a point in time depending on the species. Usually, the eggs laid are submerged into the silt or adhere to the surface immersed in water. The hatching of larvae takes place within 2 weeks of the egg hatching, but depending on circumstances, there is no growth seen for a certain period of varying duration. This stage of no development post-hatching is known as diapause which is a very crucial stage adopted by the larvae to avoid any environmental conditions that may affect the growth of nymphs. 

The life of a nymph can be from two weeks to about two years underwater for certain species but the annual cycle is common for all. Almost 50 nymphs undergo molts (shedding of skin) periodically depending on the environmental conditions. Then the skin of the nymphs split into two parts giving rise to the wings as they fly and step in the subimago stage. Then the subimagos fly off the surface of the water and take shelter nearby. Within the duration of a few hours to a day time, they finally molt one last time before they reach the imago stage where they become fully adult to mate and reproduce with their counterparts.

Ecology

Mayflies are subjected to prey by many invertebrates that are carnivorous in nature and by fishes who capture the mayflies that come closer to the water surface while hatching. During their winged stages, they are quickly eaten up by birds, bats and other predatory insects like hornets, dragonflies and hornets. At the subimago stage while the mayflies are resting they are hunted by spiders, beetles, birds and often by flying squirrels mostly in North America. At the imago stage, while they are about to hatch eggs, that is, during oviposition of female mayflies near the water surface are preyed upon by the fishes.

Distribution

There are almost 2500 species of mayflies that are known today out of which 700 species belong to North America and Mexico. Their orders are found equally among all the countries of the world except Antarctica. The areas that have temperature zoned streams, hard water as well as gravel are considered to be their high productivity areas. In such areas, as much as 1400 lymph are found in just one square foot of area and almost 33 species can be supported by one gravelled riffle.

Palaeontology and Classifications

About 298 to 323 million years ago the fossils of the mayflies were recovered from the basin of the Pennsylvanian subperiod and they were predicted to be in avoidance during the time period known as Permian, that is, 252 to 298 million years ago. The only fossil records that are viable for the study is their wing impression and that is so incomplete that their characteristics and classification are based on the study of the recent forms and species available in different parts of the world, chiefly the morphology of the mayflies.