Fly
Flies are insects of the order Diptera, the name being derived. Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced mechanosensory organs known as halteres, which act as high-speed sensors of rotational movement and allow dipterans to perform advanced aerobatics. Diptera is a large order containing more than 150,000 species including horse-flies, crane flies, hoverflies, mosquitoes and others.
Flies have a mobile head, with a pair of large compound eyes, and mouthparts designed for piercing and sucking, or for lapping and sucking in the other groups. Their wing arrangement gives them great manoeuvrability in flight, and claws and pads on their feet enable them to cling to smooth surfaces. Flies undergo complete metamorphosis; the eggs are often laid on the larval food-source and the larvae, which lack true limbs, develop in a protected environment, often inside their food source. Other species are ovoviviparous, opportunistically depositing hatched or hatching larvae instead of eggs on carrion, dung, decaying material, or open wounds of mammals. The pupa is a tough capsule from which the adult emerges when ready to do so; flies mostly have short lives as adults.
Diptera is one of the major insect orders and of considerable ecological and human importance. Flies are major pollinators, second only to the bees and their Hymenopteran relatives. Flies may have been among the evolutionarily earliest pollinators responsible for early plant pollination. Fruit flies are used as model organisms in research, but less benignly, mosquitoes are vectors for malaria, dengue, West Nile fever, yellow fever, encephalitis, and other infectious diseases; and houseflies, commensal with humans all over the world, spread foodborne illnesses. Flies can be annoyances especially in some parts of the world where they can occur in large numbers, buzzing and settling on the skin or eyes to bite or seek fluids. Larger flies such as tsetse flies and screwworms cause significant economic harm to cattle. Blowfly larvae, known as gentles, and other dipteran larvae, known more generally as maggots, are used as fishing bait, as food for carnivorous animals, and in medicine in debridement, to clean wounds.
Taxonomy and phylogeny
Relationships to other insects
Dipterans are holometabolous, meaning that they undergo radical metamorphosis. They belong to the Mecopterida, alongside the Mecoptera, Siphonaptera, Lepidoptera and Trichoptera. The possession of a single pair of wings distinguishes most true flies from other insects with "fly" in their names. However, some true flies such as Hippoboscidae have become secondarily wingless.The cladogram represents the current consensus view.
Relationships between subgroups and families
The first true dipterans known are from the Middle Triassic, and they became widespread during the Middle and Late Triassic. Modern flowering plants did not appear until the Cretaceous, so the original dipterans must have had a different source of nutrition other than nectar. Based on the attraction of many modern fly groups to shiny droplets, it has been suggested that they may have fed on honeydew produced by sap-sucking bugs which were abundant at the time, and dipteran mouthparts are well-adapted to softening and lapping up the crusted residues. The basal clades in the Diptera include the Deuterophlebiidae and the enigmatic Nymphomyiidae. Three episodes of evolutionary radiation are thought to have occurred based on the fossil record. Many new species of lower Diptera developed in the Triassic, about 220 million years ago. Many lower Brachycera appeared in the Jurassic, some 180 million years ago. A third radiation took place among the Schizophora at the start of the Paleogene, 66 million years ago.The phylogenetic position of Diptera has been controversial. The monophyly of holometabolous insects has long been accepted, with the main orders being established as Lepidoptera, Coleoptera, Hymenoptera and Diptera, and it is the relationships between these groups which has caused difficulties. Diptera is widely thought to be a member of Mecopterida, along with Lepidoptera, Trichoptera, Siphonaptera, Mecoptera and possibly Strepsiptera. Diptera has been grouped with Siphonaptera and Mecoptera in the Antliophora, but this has not been confirmed by molecular studies.
File:Lutzomyia adiketis.jpg|thumb|right|upright=0.7 |Fossil nematoceran in Dominican amber. Sandfly, Lutzomyia adiketis, Early Miocene, c. 20 million years ago
Diptera were traditionally broken down into two suborders, Nematocera and Brachycera, distinguished by the differences in antennae. The Nematocera are identified by their elongated bodies and many-segmented, often feathery antennae as represented by mosquitoes and crane flies. The Brachycera have rounder bodies and much shorter antennae. Subsequent studies have identified the Nematocera as being paraphyletic with modern phylogenies placing the Brachycera within grades of groups formerly placed in the Nematocera. The construction of a phylogenetic tree has been the subject of ongoing research. The following cladogram is based on the FLYTREE project.
A 2023 study revised the phylogeny of the Nematocera. The grouping remains paraphyletic with respect to the Brachycera, but is rearranged, with Deuterophlebiidae basal, Nymphomyiidae placed inside Culicomorpha, and Blephariceridae within Psychodomorpha. Finally, Anisopodidae becomes sister to the Brachycera.
Diversity
Flies are often abundant and are found in almost all terrestrial habitats. They include many familiar insects such as house flies, blow flies, mosquitoes, gnats, black flies, midges and fruit flies. More than 150,000 have been formally described and the actual species diversity is much greater, with the flies from many parts of the world yet to be studied intensively. The suborder Nematocera include generally small, slender insects with long antennae such as mosquitoes, gnats, midges and crane-flies, while the Brachycera includes broader, more robust flies with short antennae. Many nematoceran larvae are aquatic. There are estimated to be a total of about 19,000 species of Diptera in Europe, 22,000 in the Nearctic region, 20,000 in the Afrotropical region, 23,000 in the Oriental region and 19,000 in the Australasian region. While most species have restricted distributions, a few like the housefly are cosmopolitan. Gauromydas heros, with a length of up to, is generally considered to be the largest fly in the world, while the smallest is Euryplatea nanaknihali, which at is smaller than a grain of salt.Brachycera are ecologically very diverse, with many being predatory at the larval stage and some being parasitic. Animals parasitised include molluscs, woodlice, millipedes, insects, mammals, and amphibians. Flies are the second largest group of pollinators after the Hymenoptera. In wet and colder environments flies are significantly more important as pollinators. Compared to bees, they need less food as they do not need to provision their young. Many flowers that bear low nectar and those that have evolved trap pollination depend on flies. It is thought that some of the earliest pollinators of plants may have been flies.
The greatest diversity of gall-forming insects is found among the flies, principally among the gall midges. Many flies lay their eggs in the mesophyll tissue of leaves with larvae feeding between the surfaces forming blisters and mines. Some families feed on fungi. These include the cave dwelling fungus gnats whose larvae are bioluminescent, and the Sciaridae. Some plants are pollinated by fungus feeding flies that visit fungus-infected male flowers.
The larvae of Megaselia scalaris are omnivorous and may consume such substances as paint and shoe polish. Exorista mella is a generalist and parasitoid of multiple hosts. The larvae of shore flies and some chironomid midges survive in extreme environments including glaciers, hot springs, geysers, saline pools, sulphur pools, septic tanks and even crude oil. Adult hoverflies are well known for their mimicry and the larvae adopt diverse lifestyles including being inquiline scavengers inside the nests of social insects. Some brachycerans are agricultural pests, some bite animals and humans and suck their blood, and some transmit diseases.
Anatomy and morphology
Flies are adapted for aerial movement and typically have short and streamlined bodies. The first tagma of the fly, the head, bears the eyes, the antennae, and the mouthparts. The second tagma, the thorax, bears the wings and contains the flight muscles on the second segment, which is greatly enlarged; the first and third segments have been reduced to collar-like structures, and the third segment bears the halteres, which help to balance the insect during flight. The third tagma is the abdomen consisting of 11 segments, some of which may be fused, and with the three hindmost segments modified for reproduction. Some Dipterans are mimics and can only be distinguished from their models by very careful inspection. An example of this is Spilomyia longicornis, which is a fly but mimics a vespid wasp.File:Tabanus atratus, U, Face, MD 2013-08-21-16.06.31 ZS PMax.jpg|thumb|left|Head of a horse-fly showing large compound eyes and stout piercing mouthparts
Flies have a mobile head with a pair of large compound eyes on the sides of the head, and in most species, three small ocelli on the top. The compound eyes may be close together or widely separated, and in some instances are divided into a dorsal region and a ventral region, perhaps to assist in swarming behaviour. The antennae are well-developed but variable, being thread-like, feathery or comb-like in the different families. The mouthparts are adapted for piercing and sucking, as in the black flies, mosquitoes and robber flies, and for lapping and sucking as in many other groups. Female horse-flies use knife-like mandibles and maxillae to make a cross-shaped incision in the host's skin and then lap up the blood that flows. The gut includes large diverticulae, allowing the insect to store small quantities of liquid after a meal.
For visual course control, flies' optic flow field is analyzed by a set of motion-sensitive neurons. A subset of these neurons is thought to be involved in using the optic flow to estimate the parameters of self-motion, such as yaw, roll, and sideward translation. Other neurons are thought to be involved in analyzing the content of the visual scene itself, such as separating figures from the ground using motion parallax. The H1 neuron is responsible for detecting horizontal motion across the entire visual field of the fly, allowing the fly to generate and guide stabilizing motor corrections midflight with respect to yaw. The ocelli are concerned in the detection of changes in light intensity, enabling the fly to react swiftly to the approach of an object.
Like other insects, flies have chemoreceptors that detect smell and taste, and mechanoreceptors that respond to touch. The third segments of the antennae and the maxillary palps bear the main olfactory receptors, while the gustatory receptors are in the labium, pharynx, feet, wing margins and female genitalia, enabling flies to taste their food by walking on it. The taste receptors in females at the tip of the abdomen receive information on the suitability of a site for ovipositing. Flies that feed on blood have special sensory structures that can detect infrared emissions, and use them to home in on their hosts. Many blood-sucking flies can detect the raised concentration of carbon dioxide that occurs near large animals. Some tachinid flies which are parasitoids of bush crickets, have sound receptors to help them locate their singing hosts.
File:Halteres-Tipule.jpg|thumb|A crane fly, showing the hind wings reduced to drumstick-shaped halteres
Diptera have one pair of fore wings on the mesothorax and a pair of halteres, or reduced hind wings, on the metathorax. A further adaptation for flight is the reduction in number of the neural ganglia, and concentration of nerve tissue in the thorax, a feature that is most extreme in the highly derived Muscomorpha infraorder. Some flies such as the ectoparasitic Nycteribiidae and Streblidae are exceptional in having lost their wings and become flightless. The only other order of insects bearing a single pair of true, functional wings, in addition to any form of halteres, are the Strepsiptera. In contrast to the flies, the Strepsiptera bear their halteres on the mesothorax and their flight wings on the metathorax. Each of the fly's six legs has a typical insect structure of coxa, trochanter, femur, tibia and tarsus, with the tarsus in most instances being subdivided into five tarsomeres. At the tip of the limb is a pair of claws, and between these are cushion-like structures known as pulvilli which provide adhesion.
The abdomen shows considerable variability among members of the order. It consists of eleven segments in primitive groups and ten segments in more derived groups, the tenth and eleventh segments having fused. The last two or three segments are adapted for reproduction. Each segment is made up of a dorsal and a ventral sclerite, connected by an elastic membrane. In some females, the sclerites are rolled into a flexible, telescopic ovipositor.