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The long history of Diptera (From Paleozoic to Mesozoic - The evolutionary flow of Insects - The evolutionary flow of Dipterans) - The invasion of Earth - Bibliography

Flies, gnats, mosquitoes, or more generally, Diptera, are familiar insects, frequently found in houses or nearby. People not receiving their utility and, indeed, consider them a plague. Diptera, in fact, include species of medical interest, because they practice stings, suck blood, or transmit diseases or are direct agents of diseases, generally known as myiasis. Diptera include also economically harmful species, which may destroy entire crops or make food inedible. When actual damage is not obvious, flies are then perceived as intrusive, annoying, and repulsive animals. Think about the discomfort caused by the continuous drone, the insistent settling onto the skin, the dirty on car windshield, the horror that strikes the view of "worms" in decaying organic matter.

So, Diptera are insects more hated than a butterfly, of which we can admire the beauty, than a ladybug, which gives sympathy for her shape and colors, or than a bee, of which all people recognize the utility. Like all insects feared or hated, the flies become animals which we have to kill when possible, because they are not interesting and no reason to respect them is barely visible. Consequence of this approach is that the flies are animals virtually unknown, despite their close association with humans, because they do not arouse curiosity. Diptera are for us a planet almost entirely unexplored, despite we can see daily their fly, we can hear their incessant buzzing, we can feel the discomfort of their walks on our skin.

However, flies are fascinating when viewed from a different perspective. The history of these insects, which has ancient origins, their extraordinary capacity for adaptation, evolution and differentiation, the high degree of biodiversity, making them one of the groups of living organisms that has better interpreted the goal of biological colonization of the world.

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The long history of Diptera

The history of this order has remote origins, dating back 250 million years ago (Mya), although the knowledge about that is poor and controversial: first dipterans, now extinct and little known, would appear near the end of Permian, the last period of Paleozoic, in the same time of first beetles, but their presence is proven from the Triassic, 220 Mya. Anyway, Flies and Beetles are the most ancient of the large orders that are placed at the top of the evolutionary flow of Insects: Lepidoptera and Hymenoptera will appear later, during the Mesozoic, together with the appearance of Angiosperms. The occurrence of Dipterans on Earth an their differentiation come at a time when both the geoclimatic conditions and the Life had drastic changes. This events falls during the conquest of land by Amphibians and Reptiles and the flow of radiation of many dipteran clades occurs during the Mesozoic, best known as the age of dinosaurs. We can not avoid to include the flow of radiation of Dipterans in a global context which relates the large geoclimatic mutations of late Paleozoic and Mesozoic with the parallel evolutionary adaptation of main taxonomic groups of living organisms.

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From Paleozoic to Mesozoic

The temporal range spanning the late Paleozoic (Permian) and the early Mesozoic (Triassic) was marked by geoclimatic events which caused drastic changes on the planet's ecosystems, breaking down the composition of flora and fauna.

The period that extend from late Paleozoic (Permian) and early Mesozoic (Triassic) was characterized by geological and climatic events that caused strong changes in the world ecosystems, upsetting their flora and fauna composition. Among the more important geological events there are the following:

Among the climatic conditions of major importance, there was a shift from warm and wet climate of Early Carboniferous to basically dry climate of Permian and Triassic. The tropical regions were invaded by marshes, while most of internal regions of subcontinents housed dry or desertic environments. No less important is the changing composition of the air, that have a significant reduction in the level of oxygen during the Permian and the Triassic periods.

The evolution of biosphere is affected by these changes and possibly some other events whose justification is not entirely satisfied. It is found that at the end of Permian there was the most dramatic of the five mass extinctions that occurred during the Phanerozoic. The Permian-Triassic extinction caused the end of half of Paleozoic families. An extinction of lower intensity also occurred later, in the Triassic-Jurassic transition.

About the vegetation, the lush forests of the Carboniferous, composed predominantly of Pteridophytes, gave way to poor ecosystems of Permian. The Gymnosperms took over and came at the top of their evolutionary flow during the Mesozoic. The Angiosperms came later, during the Early Cretaceous.

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The evolutionary flow of Insects

The appearance of Insects on Earth dates back to the Devonian period. The insect fauna of Paleozoic was largely composed of primitive organisms, provided of rudimentary anatomical and morphological structures and unspecialized physiological adaptations. The first radiation of Insects occurs during the early steps of conquest of land. Insects are protagonists of this colonization with other terrestial arthropods (like Myriapoda and Arachnida) and with the first Tetrapods, represented by Amphybians and the parents of Reptiles. Until the Middle Carboniferous, environmental conditions were probably so favorable that did not require special skills. Paleozoic insects are represented by organisms with primitive chewing mouthparts, wings not improved, polyphagous diets, life cycles unsophisticated. Food habits involved the phytophagy and the predation unspecialized, with the broad occurrence of the polyphagy. The omnivory came later, probably because it was more advantageous than the phytophagy with the changed environmental conditions of the Permian. An interesting aspect is the development of large bodies, related to an atmosphere rich in oxygen: some dragonflies of the Paleozoic reach 70 cm of wingspan; they were real giant insects, able to preying also small vertebrates. During the Paleozoic other flying animals did not exists and the Odonata were the only rulers of the skies. It is also considerable the broad diffusion of the hemimetabolism, or incomplete metamorphosis, the primitive condition of Insects.

The hard environmental conditions of the Permian are probably a factor of selection pressure that will cause more differentiation lineages within the Insects. This differentiation flows started during the Permian-Triassic transition, with the appearance of primitive dipterans and coleopters and the evolution of Hemiptera. These trends are reflected in the Mesozoic, with the next coming of butterflies and Hymenoptera and the differentiation of more recent clades within Diptera and Coleoptera. The broad diffusion of these orders is contextual to three important conditions:

Overall, these events have contributed to improve adataptions based on specialization, with the evolution of morphological and anatomical structures more improved, and life histories and habits more sophisticated. These adaptations are a complex set of responses to drastic mutations occurred during some hundred million years, and they focus on four orders of holometabolous insects: Diptera, Hymenoptera, Lepidoptera, and Coleoptera. Their biological success is indicated by numbers: species currently described are approssimately 120,000 among the Diptera, 130,000 among the Hymenoptera, 180,000 among the Lepidoptera, and 350,000 among the Coleoptera. Taken together, these four orders would include approssimately 780,000 known species, nearly 80% of the entire class of Insects, which, according to latest estimates, amount to a million species (Chapman, 2009). These numbers refer, in fact, to species classified, but the estimates assume that the true number of species is much higher. To offer an idea of proportion, considering that insects alone include more than half of all species of classified living organisms (Chapman, 2009; Gatto & Casagrandi, 2003), and that the class of Mammals, according to a recent estimate, include just 5,500 species (Chapman, 2009), despite being considered the current rulers of the planet.

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The evolutionary flow of Dipterans

The flies, despite the apparent morphological and ethological homogeneity, reflect in their context the considerable evolutionary differentiation that is present in the class of Insects. This diversification also places the flies at the top of the evolutionary tree of the Panorpoidea clade, putting them, in some ways, to level of sophistication comparable to those of Hymenoptera. Although phylogenetically distant, Diptera and Hymenoptera often have common features, index of convergent evolution: the nectarivory and pollinivory, the development of parasitoidism, the differentiation of specialized mouthparts, the specialization of trophic relations, both in insectivory and phytophagy, the improvement of wing structure, of the connect apparatus, of the structure of thorax, and therefore of the flight dynamics, etc.

The differentiation of flies occurs during the Mesozoic and the first period of Cenozoic (Paleogene) and fits into three distinct environmental contexts, sequantial chronologically. First step extended from Triassic to early Jurassic, second step from Middle Jurassic to entire Cretaceous, third step from late Cretaceous to Paleogene. These three steps correspond to the differentiation of the four main taxonomic groups of Diptera: first there was the differentiation of Nematocerous, the dipterans most primitives, later the differentiation of lower Brachycerous, during the Jurassic, and of lower Cyclorrhaphous during the early Cretaceous. They are traditionally known respectively as Orthorrhapha and Aschiza. The third step, the most recent, gives the differentiation of the large clade of Schizophora, representative of the most evolutionary level in this order.

The first irradiation flow is marked by hard environmental conditions of the Triassic. The juvenil stages are adapted to aquatic life and colonize freshwater ecosystems (like streams, ponds, marshes). Food habits envolve, according to the groups, through different mechanisms of nutrition, like filtering organic particles and microorganisms present in the water, scraping algae and organic matter from submerged substrates. Adults colonize a harsh environment and develop two different food habits: one is the sucking of fluids rich on sugars, combined in some groups, with the integration of proteic substances, required to complete the maturation of eggs, the other is the aphagy. The glicophagy is probably the primitive condition of lower Diptera and it envolved in environments dominated by gymnosperms. It is assumed that the diet was based on the assumption of vegetal exudates rich in sugar and lipids and honeydew emitted by Homoptera. These have undoubtedly accounted, for at least 100-150 Mya, an important food resource for the development of glycophagy before the appearance of Angiosperms. The modern nematocerous glycophagous, descended from ancient primitive dipterans of Triassic, have oriented their glycophagy to the nectarivory. A particular aspect of glycophagy is the requirement of proteic integration in the diet to complete the maturation of eggs. Females of certain groups of lower Diptera have a secondary diet by sucking blood from vertebrates. The ancient parents of modern mosquitoes of modern, at that time adapted to suck blood from amphibians and reptiles. The other food habit, the aphagy, is the expression of adaptation to extreme habitats, characterized by the presence of aquatic ecosystems, rich in trophic resources, and terrestrial ecosystems dries and very poor. In this case, the life focus on larval development: the larva must accumulate reserves of nutrients needed by the adult for reproduction. Adults have a very short life, reduced to a few days; they spend the reserves accumulated in the larval stage for flight and reproduction, then will die. There are still some dipterans, real living fossils, that show this behavior. These are mostly rare species that colonize the streams of mountain areas poor of vegetation or any food resource.

The second flow is characterized by a strong biological diversification within Diptera. This steps occurred in a time where climatic conditions favored the development of complex food webs. This is the era of dinosaurs and lush forests, but also the era in which the Angiosperms began the colonization of plants associations. In many new systematic groups, the larval life move to terrestrial ecosystems, although these are often associated with high levels of humidity. Diets of larvae differ in several types: in addition to microphagy, saprophagy, phytophagy in acquatic environments, among larval diets many food behaviors appears in terrestrial or semiaquatic habitats: saprophagy on organic matter by plants or animals, predation or parasitoidism on invertebrates, phytophagy on seeds and roots, and, more important, the saprophagy associated with scavengers microorganisms, like fungi. Association with fungi will have a wide occurrence among the diptera and includes many degrees, from the micosaprophagy to mycophagy sensu stricto. In adults some proteic diets will spread such as hematophagy, insectivory, and pollinivory: these diets are designed to complement the protein requirements for reproduction and will occur with several behaviors. At this time appears on Earth the Tabanids, basically glycophagous, but better famous as bloodsucking insects because of the behavior common among females of many species; the Asilids, ruthless predators that hunt spiders and insects in flight, so as to deserve the common name of "robber flies"; the Dagger flies, other predators, whose males lure females offering a prey as nuptial gift. Pollinivory envolves contextually to Angiosperms radiation and marks the appearance of dipteran pollinators, represented in this phase by some important groups, such as Bee flies and Hover flies. Nectarivory, more or less associated with the assumption of vegetal or animal proteins, is the basic food in the ground plan of most clades of Diptera, evolved from the primitive glycophagy.

The third radiation flow, the most recent, intensified the degree of differentiation and produces, as a result, the high degree of biodiversity, both in the number of taxa and in variety of behaviors, subsequential to evolution of Schizophora. This is the biggest group of existing Diptera, including more than 50% of families. Essential aspects of this flow of radiation is the development of specialist trophic relations, but, more importantly, the occurrence of a complex net of convergent evolutions so, systematic groups that are phylogenetically distant may occup the same ecological niche. An example of this phenomenon is the wide spread of saprophagy and endophytic phytophagy, secondary trophic behaviors that evolved independently in different lineages.

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The invasion of Earth

The American biologist and paleontologist Stephen Jay Gould wrote:

Don't accept the chauvinistic tradition that labels our era the age of mammals. This is the age of arthropods. They outnumber us by any criterion – by species, by individuals, by prospects for evolutionary continuation. (S.J. Gould, 1989)

Diptera, althoug represent a small but significant subgroup of Arthropods, reflect the biological success of Arthropods. They appeared hundreds of million fo years befor humans, they attended the continental drift, the emergence and extinction of dinosaurs, they have accompanied the evolution of Angiosperms. Currently, the Diptera colonize a large range of environments, of several types: from intertidal zone of coral reefs to deserts, from rocks under the falls to swampy puddles rich of petroleum, from freshwaters of mountain streams to salt marshes, from mountain pastures to canopy of tropical rainforest. In Diptera we include formidable flyers, capable of long migrations or incredible aerial acrobatics but also agile runners, able to walk on any surface, including glass, regardless of the position.

Unlike many other organisms, the Flies were able to withstand the impact of anthropisation of Earth, adapting to new environments created by men, drawing also benefit. So they settled cultivation, houses, waste dumps, sewages, becoming also unsafe and intrusive commensals of humans.

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