Light-eyed horseflies

Small horseflies (8-16 mm), painted in light gray or ocher yellow. The eyes are usually bare (subgenus Atylotus ) or covered with hairs (subgenus Surcoufiella ). The color of the eyes in living insects is greenish, yellow or gray, in dried ones it changes to red-brown. The head is very convex. An ocellar tubercle is present only in males. It is located below eye level and is covered with a gray coating. Frontal calluses are small. The wings are transparent. Hummer brown. The fourth radial vein with an appendage (subgenus Atylotus ) or without it (subgenus Surcoufiella ) ^{[2] [4]} .

The body of the larvae is fusiform, painted in a pale green or yellowish color, sometimes with a purple hue. There are no lateral and dorsolateral spots of bristles on the terminal segment of the body. Length about 30 mm, weight up to 160 mg ^[5] . Pupa is light yellow or tan. The color of the cephalothorax and abdomen is similar, while in the closely related genus Hybomitra the cephalothorax is darker ^[6] . A description of the egg laying was done only for the species Atylotus pulchellus , Atylotus miser, and Atylotus horvathi . They consist of 3-4 layers of whitish-gray eggs and have a size of about 5-8 mm ^[7] .

The life cycle features of different species have been studied in only a few species. Larvae of European species ( Atylotus fulvus and Atylotus plebeius ) develop in shallow areas of water bodies, or mesotrophic sphagnum bogs . The development of the larva lasts from one to three years. The duration of the pupal phase is from 14 to 24 days ^{[4] [5] [6]} . In the Caucasus, larvae of Atylotus loewianus were found in moist soil under plant debris ^[8] . The preimaginal stages of Atylotus miser develop in floodplain meadows and rice fields ^[9] . In laboratory conditions, the larvae eat earthworms ^[10] . Females of most species are blood-sucking. The exceptions are Atylotus plebejus and Atylotus sublunaticornis . Females and males of these species feed on flower nectar ^[4] .

In the American species Atylotus bicolor and the Palearctic Atylotus fulvus , autogenous (without bloodsucking) development of the first portion of eggs was noted ^{[11] [12]} . Such females do not react to carbon dioxide and do not attack animals ^[11] . The number of adult horseflies in the European part of Russia and in Western Siberia is low ^{[13] [14] [15]} . Fertility of these species is estimated from 300 to 400 egg tubules. Embryonic development lasts from 6 to 11 days. Larvae of them in the south of the Far East hatch at the end of August and winter after the second molt. Before going to the pupal stage, 9–10 molts occur. The duration of larval development depends on external conditions and can be delayed up to 4-6 years. It takes about 10 days to form an adult in a chrysalis ^[7] .

In Europe, species of this genus are rare, the most common species, Atylotus fulvus and Atylotus rusticus, make up no more than 1-2% of the total number of horseflies ^{[6] [14]} . In the south of Central Asia , numerous species (up to 34%) include Atylotus chodukini and Atylotus pulchellus ^[16] . In the Far East, the species Atylotus miser and Atylotus horvathi are numerous, their share can reach 60% ^{[7] [17]} . In Africa, representatives of the genus are a typical part of the midge . The share of Atylotus agrestis in the complex of attacking species in Cameroon is more than 90% ^[18] . In North America, representatives of the genus are few ^{[19] [20]} .

Some species can tolerate trypanosomiasis and tularemia ^{[2] [21]} . To experimentally confirm the ability of horseflies, Atylotus agrestis built a cattle enclosure with a mosquito net. There were ten cows in the pen, two of them were infected. In different series of experiments, cows were infected with either Trypanosoma congolense or Trypanosoma vivax . For 20 days, mosquitoes Atylotus agrestis or Atylotus fuscipes were launched under the mosquito net in different versions of the experiment. After the completion of the experiments, Trypanosoma vivax infected 5-6 cows, and Trypanosoma congolense only two ^{[21] [22] [23]} . Phytopathogenic fungi Tabanomyces milkoi and Entomophthora tabanivora ^{[24] [25]} affect the number of horseflies of the genus Atylotus .

The genus was described by Robert Romanovich Osten-Saken in 1876 ^[3] . In the first half of the 20th century, the genus Atylotus was considered as a subgenus of Ochrops of the genus Tabanus ^{[26] [27]} . According to the presence of the appendage in the fourth radial vein and the presence of hairs in the eyes, the genus is divided into two subgenera: Atylotus and Surcoufiella. Only two species Atylotus plebejus and Atylotus sublunaticornis are included in the subgenus Surcoufiella ^[2] . Most taxonomists include the genus Atylotus in the Tabanini tribe ^[28] . The Polish entomologist considered the subgenus Surcoufiella as an independent genus called Ochrops and assigned this genus to the tribe Lepidoselagini described by him ^[29] . However, these taxonomic proposals were criticized and were not supported by other taxonomists ^[30] . About 70 species are known in the world fauna ^[28] .

The diploid set of chromosomes in most studied species ( Atylotus fulvus, Atylotus bicolor, Atylotus loewianus , Atylotus obioensis ) consists of 18 chromosomes. Only the species Atylotus horvathi has 12 chromosomes. Representatives of the genus Atylotus have the smallest sex chromosomes among all karyologically studied horsefly species. Sex chromosomes X and Y do not visually differ from each other ^[31] .

The complete sequence of the mitochondrial genome was determined in the species Atylotus miser . Mitochondrial DNA is a double-stranded molecule closed in a ring with a length of 15,858 nucleotides . The mitochondrial genome is typical for dipterans and contains 37 genes, including 13 genes that encode polycomb proteins , 22 genes are responsible for tRNA synthesis, and two rRNA genes . In the genome there is a control region with a length of 993 nucleotide pairs. The total A + T nucleotide content is 77.7%. The stop codons most commonly used by Atylotus miser are the TAA and TAG nucleotide sequences ^[32] .

The range of the genus covers the Holarctic and Afrotropical zoogeographic regions ^{[2] [28] [1]} . Three species ( Atylotus cryptotaxis , Atylotus sudharensis, and Atylotus virgo ) were recorded only within the Indomalai region ^[33] . 34 species ^{[1] [34] are} found in the Palaearctic, the greatest species diversity of the genus in this region is concentrated in the steppe and desert zones ^[2] . In the Afrotropic region, 22 species are known, many of them are rare, with the exception of Atylotus agrestis ^[28] . Most species of Atylotus do not go beyond the boundaries of one zoogeographic region. The maximum range is the species Atylotus agrestis , which is found in Africa and Madagascar, in the south of the Palaearctic and South and Central Asia ^[33] . In the Palaearctic and Afrotropical regions, Atylotus agricola , Atylotus farinosus , Atylotus loewianus were noted ^{[28] [1]} . The species Atylotus sublunaticornis and Atylotus plebejus have a torn (polydisjunctive) range ^[29] . The first of these lives in the Palaearctic and Nearctic, the second only in the Palearctic ^{[2] [35]} . The formation of the ranges of these species is associated with the formation of bog ecosystems after the end of the last ice age ^[29] . In mountainous regions, individual species ( Atylotus fulvus ) rise to a height of up to 1900 m ^[16] .

The relict species Atylotus sublunaticornis is listed in the Red Book of the Komi Republic . He is assigned status 3 - a rare species with naturally low abundance. This species lives in swamps. Males and females feed on the nectar of the flowers of the serpentine highlander , whitebird , wild rosemary and cassandra ^[36] .