A simple eye or ocellus (sometimes called a pigment pit ) is a form of eye or an optical arrangement which has a single lens without the sort of elaborate retina that occurs in most vertebrates . These eyes are called "simple" to distinguish them from " compound eyes ", which have multiple lenses. They are not necessarily simple in the sense of being uncomplicated or basic.
78-505: Springtails (class Collembola ) form the largest of the three lineages of modern hexapods that are no longer considered insects . Although the three orders are sometimes grouped together in a class called Entognatha because they have internal mouthparts , they do not appear to be any more closely related to one another than they are to all insects, which have external mouthparts. Springtails are omnivorous , free-living organisms that prefer moist conditions. They do not directly engage in
156-447: A behavioural dimension to this still poorly understood vertical segregation. Experiments with peat samples turned upside down showed two types of responses to disturbance of this vertical gradient, called "stayers" and "movers". As a group, springtails are highly sensitive to desiccation , because of their tegumentary respiration , although some species with thin, permeable cuticles have been shown to resist severe drought by regulating
234-540: A biological warfare effort. The alleged associated diseases included anthrax , cholera , dysentery , fowl septicemia , paratyphoid , plague , scrub typhus , small pox , and typhoid . China created an international scientific commission for investigating possible bacterial warfare, eventually ruling that the United States probably did engage in limited biological warfare in Korea. The US government denied all
312-614: A ringtest , on the biology and ecotoxicology of Folsomia candida and comparison with the sexual nearby species Folsomia fimetaria (sometimes preferred to Folsomia candida ) are given in a document written by Paul Henning Krogh. Care should be taken that different strains of the same species may be conducive to different results. Avoidance tests have been also performed. They have been standardized, too. Avoidance tests are complementary to toxicity tests, but they also offer several advantages: they are more rapid (thus cheaper), more sensitive and they are environmentally more reliable, because in
390-409: A background of snow. In addition, a few species routinely climb trees and form a dominant component of canopy fauna, where they may be collected by beating or insecticide fogging. These tend to be the larger (>2 mm) species, mainly in the genera Entomobrya and Orchesella , though the densities on a per square meter basis are typically 1–2 orders of magnitude lower than soil populations of
468-707: A clasping organ located on their antenna . Many springtails, mostly those living in deeper soil horizons, are parthenogenetic, which favors reproduction to the detriment of genetic diversity and thereby to population tolerance of environmental hazards . Parthenogenesis (also called thelytoky ) is under the control of symbiotic bacteria of the genus Wolbachia , which live, reproduce and are carried in female reproductive organs and eggs of Collembola. Feminizing Wolbachia species are widespread in arthropods and nematodes , where they co-evolved with most of their lineages . Hexapoda The subphylum Hexapoda (from Greek for 'six legs') or hexapods comprises
546-431: A class of simple eyes. Many kinds of holometabolous larvae bear no other form of eyes until they enter their final stage of growth. Adults of several orders of hexapods also have stemmata, and never develop compound eyes at all. Examples include fleas , springtails , and Thysanura . Some other Arthropoda , such as some Myriapoda , rarely have any eyes other than stemmata at any stage of their lives (exceptions include
624-413: A clue to their ontogenetic role is their lateral position on the head; ocelli, that in other ways resemble stemmata, tend to be borne in sites median to the compound eyes, or nearly so. Among some researchers, this distinction has led to the use of the term "lateral ocelli" for stemmata. A number of genetic pathways are responsible for the occurrence and positioning of the ocelli. The gene orthodenticle
702-425: A continuous change in species assemblages can be observed from tree canopies to ground vegetation then to plant litter down to deeper soil horizons . This is a complex factor embracing both nutritional and physiological requirements, together with behavioural trends, dispersal limitation and probable species interactions . Some species have been shown to exhibit negative or positive gravitropism , which adds
780-458: A distinct retina, lens, and cornea. Many snails and slugs also have ocelli, either at the tips or bases of their tentacles. Some other gastropods, such as the Strombidae , have much more sophisticated eyes. Giant clams have ocelli that allow light to penetrate their mantles . Spiders do not have compound eyes, but instead have several pairs of simple eyes with each pair adapted for
858-409: A family of sawflies) are only "simple" in that they represent immature or embryonic forms of the compound eyes of the adult. They can possess a considerable degree of acuity and sensitivity, and can detect polarized light. They may be optimized for light sensitivity, as opposed to detailed image formation. In the pupal stage, such stemmata develop into fully fledged compound eyes. One feature offering
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#1732780375062936-400: A food source for spiders and harvestmen in the same environment, such as the endangered harvestman Texella reyesi . To protect themselves, some species have evolved chemical defenses. Springtails are cryptozoa frequently found in leaf litter and other decaying material, where they are primarily detritivores and microbivores , and one of the main biological agents responsible for
1014-421: A full set of ocelli (photoreceptors). Epedaphic species inhabit upper litter layers and fallen logs. They are slightly smaller and have less pronounced pigments, as well as less developed limbs and ocelli than the atmobiotic species. Hemiedaphic species inhabit the lower litter layers of decomposing organic material. They are 1-2 millimeters (about 1/16") in length, have dispersed pigmentation, shortened limbs, and
1092-742: A negative effect on mesofauna biomass and diversity in drier parts and a positive effect in moist sub-areas. Furthermore, a study with 20 years of experimental warming in three contrasting plant communities found that small scale heterogeneity may buffer springtails to potential climate warming. Sexual reproduction occurs through the clustered or scattered deposition of spermatophores by male adults. Stimulation of spermatophore deposition by female pheromones has been demonstrated in Sinella curviseta . Mating behavior can be observed in Symphypleona . Among Symphypleona, males of some Sminthuridae use
1170-649: A neurological problem, such as delusional parasitosis , a psychological rather than entomological problem. Researchers themselves may be subject to psychological phenomena. For example, a publication in 2004 claiming that springtails had been found in skin samples was later determined to be a case of pareidolia ; that is, no springtail specimens were actually recovered, but the researchers had digitally enhanced photos of sample debris to create images resembling small arthropod heads, which then were claimed to be springtail remnants. However, Steve Hopkin reports one instance of an entomologist aspirating an Isotoma species and in
1248-404: A number of neighbouring retinulae. Some jellyfish , sea stars , flatworms , and ribbonworms have the simplest "eyes" – pigment spot ocelli – which have randomly distributed pigment, and which have no other structure (such as a cornea , or lens ). The apparent "eye color" in these animals is red or black. Certain groups such as box jellyfish have more complex eyes, including some with
1326-429: A patchwork of closed ( woodland ) and open ( meadows , cereal crops ) environments, most soil -dwelling species are not specialized and can be found everywhere, but most epigeal and litter -dwelling species are attracted to a particular environment, either forested or not. As a consequence of dispersal limitation, landuse change, when too rapid, may cause the local disappearance of slow-moving, specialist species ,
1404-561: A phenomenon the measure of which has been called colonisation credit. Springtails are well known as pests of some agricultural crops. Sminthurus viridis , the lucerne flea, has been shown to cause severe damage to agricultural crops, and is considered as a pest in Australia. Onychiuridae are also known to feed on tubers and to damage them to some extent. However, by their capacity to carry spores of mycorrhizal fungi and mycorrhiza helper bacteria on their tegument, soil springtails play
1482-509: A positive role in the establishment of plant-fungal symbioses and thus are beneficial to agriculture. They also contribute to controlling plant fungal diseases through their active consumption of mycelia and spores of damping-off and pathogenic fungi. It has been suggested that they could be reared to be used for the control of pathogenic fungi in greenhouses and other indoor cultures. Various sources and publications have suggested that some springtails may parasitize humans, but this
1560-443: A reduced number of ocelli. Euedaphic species inhabit upper mineral layers known as the humus horizon. They are smaller than hemiedaphic species; have soft, elongated bodies; lack pigmentation and ocelli; and have reduced or absent furca. Poduromorphs are characterized by their elongated bodies and conspicuous segmentation – three thoracic segments, six abdominal segments, including a well-developed prothorax with tergal chaetae, while
1638-419: A separate evolutionary line from the other Hexapoda , but others disagree; this seems to be caused by widely divergent patterns of molecular evolution among the arthropods . The adjustments of traditional taxonomic rank for springtails reflect the occasional incompatibility of traditional groupings with modern cladistics : when they were included with the insects, they were ranked as an order ; as part of
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#17327803750621716-409: A specific task or tasks. The principal and secondary eyes in spiders are arranged in four, or occasionally fewer, pairs. Only the principal eyes have moveable retinas. The secondary eyes have a reflector at the back of the eyes. The light-sensitive part of the receptor cells is next to this, so they get direct and reflected light. In hunting or jumping spiders, for example, a forward-facing pair possesses
1794-512: A various number of annuli. This type of antenna is therefore called annulated antenna. Johnston's organ , which is found on the pedicel, is absent in the Entognatha. The thorax is composed of three segments, each of which bears a single pair of legs. As is typical of arthropods adapted to life on land, each leg has only a single walking branch composed of five segments, without the gill branches found in some other arthropods and with gill on
1872-705: Is a muscular sphincter. Malpighian tubules are absent. Traditionally, the springtails were divided into the orders Arthropleona , Symphypleona , and occasionally also Neelipleona . The Arthropleona were divided into two superfamilies , the Entomobryoidea and the Poduroidea . However, recent phylogenetic studies show Arthropleona is paraphyletic . Thus, the Arthropleona are abolished in modern classifications, and their superfamilies are raised in rank accordingly, being now orders Entomobryomorpha and
1950-445: Is absent. The sideways-facing ocelli can be called "lateral ocelli", referring to their direction and position in the triplet, however this is not to be confused with the stemmata of some insect larvae, which are also known as lateral ocelli. A dorsal ocellus consists of a lens element ( cornea ) and a layer of photoreceptors ( rod cells ). The ocellar lens may be strongly curved or flat. The photoreceptor layer may also be separated from
2028-515: Is allelic to ocelliless, a mutation that stops ocelli from being produced. In Drosophila , the rhodopsin Rh2 is only expressed in simple eyes. While (in Drosophila at least) the genes eyeless and dachshund are both expressed in the compound eye but not the simple eye, no reported 'developmental' genes are uniquely expressed in the simple eye. Epidermal growth factor receptor ( Egfr ) promotes
2106-498: Is also becoming a genomic model organism for soil toxicology. With microarray technology the expression of thousands of genes can be measured in parallel. The gene expression profiles of Folsomia candida exposed to environmental toxicants allow fast and sensitive detection of pollution, and additionally clarifies molecular mechanisms causing toxicology. Collembola have been found to be useful as bio-indicators of soil quality. Laboratory studies have been conducted that validated that
2184-401: Is believed to be associated with fluid uptake and balance, excretion, and orientation of the organism itself. Most species have an abdominal, tail-like appendage known as a furcula (or furca). It is located on the fourth abdominal segment of springtails and is folded beneath the body, held under tension by a small structure called the retinaculum (or tenaculum). When released, it snaps against
2262-541: Is composed of a presegmental acron that usually bears eyes (absent in Protura and Diplura), followed by six segments, all closely fused together, with the following appendages: The mouth lies between the fourth and fifth segments and is covered by a projection from the sixth, called the labrum (upper lip). In true insects (class Insecta) the mouthparts are exposed or ectognathous , while in other groups they are enveloped or endognathous . Similar appendages are found on
2340-412: Is determined by the environment in which it lives, and the behavioural tasks it must fulfill to survive. Arthropods differ widely in the habitats in which they live, as well as their visual requirements for finding food or conspecifics , and avoiding predators. Consequently, an enormous variety of eye types are found in arthropods to overcome visual problems or limitations. Use of the term simple eye
2418-436: Is entirely inconsistent with their biology, and no such phenomenon has ever been scientifically confirmed, though it has been documented that the scales or hairs from springtails can cause irritation when rubbed onto the skin. They may sometimes be abundant indoors in damp places such as bathrooms and basements, and incidentally found on one's person. More often, claims of persistent human skin infection by springtails may indicate
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2496-609: Is flexible, and must be interpreted in proper context; for example, the eyes of most large animals are camera eyes and are sometimes considered "simple" because a single lens collects and focuses an entire image onto the retina (analogous to a camera ). By other criteria, the presence of a complex retina distinguishes the vertebrate camera eye from the simple stemma or ommatidia which make up compound eyes. Additionally, not all invertebrate ocelli and ommatidium have simple photoreceptors . Many have various forms of retinula (a retina-like cluster of photoreceptor cells), including
2574-402: Is more inclusive. Though taxonomic rank cannot be used for absolute comparisons, it is notable that nematodes are a phylum and crustaceans a subphylum . Most springtails are small and difficult to see by casual observation, but one springtail, the so-called snow flea ( Hypogastrura nivicola ), is readily observed on warm winter days when it is active and its dark color contrasts sharply with
2652-564: The Devonian period. This fossil may help to fill the arthropod gap from 385 million to 325 million years ago, although some researchers oppose this view and suggest that the fossil may instead represent a decomposed crustacean or other non-insect. In 2023, a hexapod-like arthropod fossil from the Ordovician marine fossil site Castle Bank was reported, although further study is needed. Ocelli The structure of an animal's eye
2730-855: The Early Devonian . The fossil from 400 million years ago , Rhyniella praecursor , is the oldest terrestrial arthropod, and was found in the famous Rhynie chert of Scotland . Given its morphology resembles extant species quite closely, the radiation of the Hexapoda can be situated in the Silurian , 420 million years ago or more. Additional research concerning the coprolites (fossilized feces) of ancient springtails allowed researchers to track their lineages back some 412 million years. Fossil Collembola are rare. Instead, most are found in amber. Even these are rare and many amber deposits carry few or no collembola. The best deposits are from
2808-563: The Krubera Cave . The horizontal distribution of springtail species is affected by environmental factors which act at the landscape scale, such as soil acidity , moisture and light . Requirements for pH can be reconstructed experimentally. Altitudinal changes in species distribution can be at least partly explained by increased acidity at higher elevation. Moisture requirements, among other ecological and behavioural factors, explain why some species cannot live aboveground, or retreat in
2886-670: The Poduromorpha . Technically, the Arthropleona are thus a partial junior synonym of the Collembola. The term "Neopleona" is essentially synonymous with Symphypleona + Neelipleona. The Neelipleona was originally seen as a particularly advanced lineage of Symphypleona, based on the shared global body shape, but the global body of the Neelipleona is realized in a completely different way than in Symphypleona. Subsequently,
2964-935: The Canadian Cretaceous specimens. There are about 3,600 different species. Specific feeding strategies and mechanisms are employed to match specific niches. Herbivorous and detritivorous species fragment biological material present in soil and leaf litter, supporting decomposition and increasing the availability of nutrients for various species of microbes and fungi. Carnivorous species maintain populations of small invertebrates such as nematodes, rotifers, and other collembolan species. Springtails commonly consume fungal hyphae and spores, but also have been found to consume plant material and pollen, animal remains, colloidal materials, minerals and bacteria. Springtails are consumed by mesostigmatan mites in various families, including Ascidae , Laelapidae , Parasitidae , Rhodacaridae and Veigaiidae . Cave-dwelling springtails are
3042-429: The Entognatha, they are ranked as a subclass . If they are considered a basal lineage of Hexapoda, they are elevated to full class status. Members of the Collembola are normally less than 6 mm (0.24 in) long, have six or fewer abdominal segments , and possess a tubular appendage (the collophore or ventral tube) with reversible, sticky vesicles, projecting ventrally from the first abdominal segment. It
3120-548: The Korean War were Isotoma (Desoria) negishina (a local species) and the "white rat springtail" Folsomia candida . Captive springtails are often kept in a terrarium as part of a clean-up crew . Springtails are currently used in laboratory tests for the early detection of soil pollution . Acute and chronic toxicity tests have been performed by researchers, mostly using the parthenogenetic isotomid Folsomia candida . These tests have been standardized. Details on
3198-677: The Neelipleona were considered as being derived from the Entomobryomorpha. Analysis of 18S and 28S rRNA sequence data, though, suggests that they form the most ancient lineage of springtails, which would explain their peculiar apomorphies . This phylogenetic relationship was also confirmed using a phylogeny based on mtDNA and whole-genome data . The latest whole-genome phylogeny supporting four orders of Collembola: Neelipleona [REDACTED] Poduromorpha [REDACTED] Symphypleona [REDACTED] Entomobryomorpha [REDACTED] Springtails are attested to since
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3276-406: The abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment. The myriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity. These were then considered subclasses of a subphylum called Uniramia or Atelocerata. In the first decade of the 21st century, however, this
3354-420: The abdominal segments of some immature aquatic insects. In most insects the second and third thoracic segments also support wings. It has been suggested that these may be homologous to the gill branches of crustaceans, or they may have developed from extensions of the segments themselves. The abdomen follow epimorphic development, where all segments are already present at the end of embryonic development in all
3432-465: The allegations, and instead proposed that the United Nations send a formal inquiry committee to China and Korea, but China and Korea refused to cooperate. U.S. and Canadian entomologists further claimed that the accusations were ridiculous and argued that anomalous appearances of insects could be explained through natural phenomena. Springtail species cited in allegations of biological warfare in
3510-638: The best resolution (and even some telescopic ability) to help spot prey from a distance. Nocturnal spiders' eyes are very sensitive in low light levels and are large to capture more light, equivalent to f/0.58 in the rufous net-casting spider . The term "ocellus" (plural ocelli) is derived from the Latin oculus (eye), and literally means "little eye". In insects, two distinct ocellus types exist: dorsal (top-most) ocelli, and lateral ocelli (often referred to as ocelli and stemmata , respectively), most insects have dorsal ocelli while stemmata are found in
3588-585: The compound eyes. Additionally, given the relatively simple neural arrangement of the eye (small number of synapses between detector and effector ), as well as the extremely large diameter of some ocellar interneurons (often the largest diameter neurons in the animal's nervous system), the ocelli are typically considered to be "faster" than the compound eyes. One common theory of ocellar function in flying insects holds that they are used to assist in maintaining flight stability. Given their underfocused nature, wide fields of view , and high light-collecting ability,
3666-405: The conclusion that, with some exceptions in predatory insects, the ocelli are incapable of perceiving proper images and are thus solely suitable for light-metering functions. Given the large aperture and low f -number of the lens, as well as high convergence ratios and synaptic gains (amplification of photoreceptor signals), the ocelli are generally considered to be far more sensitive to light than
3744-688: The control and the dissemination of soil microorganisms . In a mature deciduous woodland in temperate climate, leaf litter and vegetation typically support 30 to 40 species of springtails, and in the tropics the number may be over 100. In sheer numbers, they are reputed to be one of the most abundant of all macroscopic animals, with estimates of 100,000 individuals per square meter of ground, essentially everywhere on Earth where soil and related habitats ( moss cushions, fallen wood , grass tufts, ant and termite nests) occur. Only nematodes , crustaceans , and mites are likely to have global populations of similar magnitude, and each of those groups except mites
3822-475: The decomposition of organic matter, but contribute to it indirectly through the fragmentation of organic matter and the control of soil microbial communities. The word Collembola is from the ancient Greek κόλλα kólla "glue" and ἔμβολος émbolos "peg"; this name was given due to the existence of the collophore , which was previously thought to stick to surfaces to stabilize the creature. Early DNA sequence studies suggested that Collembola represent
3900-412: The dorsal ocelli vary markedly throughout insect orders. They tend to be larger and more strongly expressed in flying insects (particularly bees, wasps, dragonflies and locusts) where they are typically found as a triplet. Two ocelli are directed to either side of the head, while a central (median) ocellus is directed forwards. In some terrestrial insects (e.g. some ants and cockroaches), the median ocellus
3978-575: The early Eocene of Canada and Europe, Miocene of Central America, and the mid-Cretaceous of Burma and Canada. They display some unexplained characteristics: first, all but one of the fossils from the Cretaceous belong to extinct genera, whereas none of the specimens from the Eocene or the Miocene are of extinct genera; second, the species from Burma are more similar to the modern fauna of Canada than are
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#17327803750624056-402: The exceptional acrobatic abilities of these animals. Research on the ocelli is of high interest to designers of small unmanned aerial vehicles . Designers of these craft face many of the same challenges that insects face in maintaining stability in a three-dimensional world. Engineers are increasingly taking inspiration from insects to overcome these challenges. Stemmata (singular stemma) are
4134-411: The first springtails to have been described by Carl Linnaeus ), spends its entire life at the surface of water, its wettable eggs dropping in water until the non-wettable first instar hatches then surfaces. A few genera are capable of being submerged, and after molting young springtails lose their water repellent properties and are able to survive submerged under water. In a variegated landscape, made of
4212-466: The first thoracic segment in Entomobryomorpha is clearly reduced and bears no chaetae. The digestive tract of springtails consists of three main components: the foregut, midgut, and hindgut. The midgut is surrounded by a network of muscles and lined with a monolayer of columnar or cuboidal cells. Its function is to mix and transport food from the lumen into the hindgut through contraction. Many species of syntrophic bacteria, archaea, and fungi are present in
4290-694: The head and the thorax , leading to a rudimentary, although fully functional, tracheal system. The anatomical variance present between different species partially depends on soil morphology and composition. Surface-dwellers are generally larger, have darker pigments, have longer antennae and functioning furcula. Sub-surface-dwellers are usually unpigmented, have elongated bodies, and reduced furcula. They can be categorized into four main forms according to soil composition and depth: atmobiotic, epedaphic, hemiedaphic, and euedaphic. Atmobiotic species inhabit macrophytes and litter surfaces. They are generally 8-10 millimeters (about ⅓") in length, pigmented, have long limbs, and
4368-408: The heads of Myriapoda and Crustacea , although the crustaceans have secondary antennae . Collembolans and diplurans have segmented antenna; each segment has its own set of muscles. The antennea of insects consist of just three segments; the scape, the pedicel and the flagellum. Muscles occur only in the first two segments. The third segment, the flagellum, don't have any muscles and is composed of
4446-529: The hexapod groups except for Protura, which has an anamorphic development where the hatched juveniles has an incomplete complement of segments, and goes through a post-embryonic segment addition with each molting before the final adult number of segments is reached. All true insects have eleven segments (often reduced in number in many insect species), but in Protura there are twelve, and in Collembola only six (sometimes reduced to only four). The appendages on
4524-468: The hexapods diverged from their sister group, the Anostraca (fairy shrimps), at around the start of the Silurian period 440 million years ago , coinciding with the appearance of vascular plants on land. Since then remipedians have been revealed as closest living relative of hexapods. Several hypotheses about their internal relationships have been suggested over the years, with proturans as
4602-578: The jumping ability of springtails can be used to evaluate the soil quality of Cu- and Ni-polluted sites. In polar regions that are expected to experience among the most rapid impact from climate warming, springtails have shown contrasting responses to warming in experimental warming studies. There are negative, positive and neutral responses reported. Neutral responses to experimental warming have also been reported in studies of non-polar regions. The importance of soil moisture has been demonstrated in experiments using infrared heating in an alpine meadow, which had
4680-420: The large and well-developed compound eyes of the house centipedes, Scutigera ). Behind each lens of a typical functional stemma lies a single cluster of photoreceptor cells, termed a retinula. The lens is biconvex , and the body of the stemma has a vitreous or crystalline core. Although stemmata are simple eyes, some kinds (such as those of the larvae of Lepidoptera and especially those of Tenthredinidae ,
4758-441: The largest clade of arthropods and includes most of the extant arthropod species. It includes the crown group class Insecta (true insects), as well as the much smaller clade Entognatha , which includes three classes of wingless arthropods that were once considered insects: Collembola (springtails), Protura (coneheads) and Diplura (two-pronged bristletails). The insects and springtails are very abundant and are some of
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#17327803750624836-768: The larvae of some insect orders. Despite the shared name, they are structurally and functionally very different. Simple eyes of other animals may also be referred to as ocelli, but again the structure and anatomy of these eyes is quite distinct from those of insect dorsal ocelli. Dorsal ocelli are light-sensitive organs found on the dorsal surface or frontal surface of the head of many insects, including Hymenoptera ( bees , ants , wasps , sawflies ), Diptera (flies), Odonata ( dragonflies , damselflies ), Orthoptera ( grasshoppers , locusts ) and Mantodea (mantises). These ocelli coexist with compound eyes; thus, most insects possess two anatomically separate and functionally different visual pathways. The number, forms, and functions of
4914-470: The lens by a clear vitreous humour . The number of photoreceptors also varies widely, but may number in the hundreds or thousands for well-developed ocelli. In bees, locusts, and dragonflies, the lens is strongly curved; while in cockroaches it is flat. Locusts possess vitreous humour while blowflies and dragonflies do not. Two somewhat unusual features of ocelli are particularly notable and generally common between insect orders. These two factors have led to
4992-404: The lumen. These different digestive regions have varying pH to support specific enzymatic activities and microbial populations. The anterior portion of the midgut and hindgut is slightly acidic (with a pH of approximately 6.0) while the posterior midgut portion is slightly alkaline (with a pH of approximately 8.0). Between the midgut and hindgut is an alimentary canal called the pyloric region, which
5070-568: The most important pollinators , basal consumers , scavengers / detritivores and micropredators in terrestrial environments. Hexapods are named for their most distinctive feature: a three-part body plan with a consolidated thorax and three pairs of legs . Most other arthropods have more than three pairs of legs. Most recent studies have recovered Hexapoda as a subgroup of Crustacea . Hexapods have bodies ranging in length from 0.5 mm to over 300 mm which are divided into an anterior head, thorax , and posterior abdomen. The head
5148-488: The ocelli are superbly adapted for measuring changes in the perceived brightness of the external world as an insect rolls or pitches around its body axis during flight. Locusts and dragonflies in tethered flight have been observed to try and "correct" their flight posture based on changes in light. Other theories of ocellar function have ranged from roles as light adaptors or global excitatory organs to polarization sensors and circadian entrainers . Recent studies have shown
5226-442: The ocelli of some insects (most notably the dragonfly, but also some wasps) are capable of "form vision" similar to camera eyes, as the ocellar lens forms an image within, or close to, the photoreceptor layer. In dragonflies it has been demonstrated that the receptive fields of both the photoreceptors and the second-order neurons can be quite restricted. Further research has demonstrated these eyes not only resolve spatial details of
5304-403: The ommatidia of most insects and the central eyes of camel spiders . Jumping spiders and some other predatory spiders with seemingly simple eyes also emulate retinal vision in various ways. Many insects have unambiguously compound eyes consisting of multiple lenses (up to tens of thousands), but achieve an effect similar to that of a camera eye, in that each ommatidium lens focuses light onto
5382-955: The osmotic pressure of their body fluid. The gregarious behaviour of Collembola, mostly driven by the attractive power of pheromones excreted by adults, gives more chance to every juvenile or adult individual to find suitable, better protected places, where desiccation could be avoided and reproduction and survival rates (thereby fitness ) could be kept at an optimum. Sensitivity to drought varies from species to species and increases during ecdysis . Given that springtails moult repeatedly during their entire life (an ancestral character in Hexapoda ) they spend much time in concealed micro-sites where they can find protection against desiccation and predation during ecdysis, an advantage reinforced by synchronized moulting. The high humidity environment of many caves also favours springtails and there are numerous cave adapted species, including one, Plutomurus ortobalaganensis living 1,980 metres (6,500 ft) down
5460-611: The process accidentally inhaling some of their eggs, which hatched in his nasal cavity and made him quite ill until they were flushed out. In 1952, China accused the United States military of spreading bacteria-laden insects and other objects during the Korean War by dropping them from P-51 fighters above rebel villages over North Korea . In all, the U.S. was accused of dropping ants, beetles, crickets, fleas, flies, grasshoppers, lice, springtails, and stoneflies as part of
5538-486: The real world Collembola move actively far from pollution spots. It may be hypothesized that the soil could become locally depauperated in animals (and thus improper to normal use) while below thresholds of toxicity. Contrary to earthworms , and like many insects and molluscs, Collembola are very sensitive to herbicides and thus are threatened in no-tillage agriculture, which makes a more intense use of herbicides than conventional agriculture. The springtail Folsomia candida
5616-413: The reduction in body size is advantageous to their survival. The Poduromorpha and Entomobryomorpha have an elongated body, while the Symphypleona and Neelipleona have a globular body. Collembola lack a tracheal respiration system, which forces them to respire through a porous cuticle , except for the two families Sminthuridae and Actaletidae, which exhibit a single pair of spiracles between
5694-423: The same species. In temperate regions, a few species (e.g. Anurophorus spp., Entomobrya albocincta , Xenylla xavieri , Hypogastrura arborea ) are almost exclusively arboreal. In tropical regions a single square meter of canopy habitat can support many species of Collembola. The main ecological factor driving the local distribution of species is the vertical stratification of the environment: in woodland
5772-543: The sister group to the other hexapods and collembolans and diplurans belonging together in Antennomusculata as the latest suggestion: The following cladogram is given by Kjer et al. (2016): Collembola (springtails) Protura (coneheads) Diplura (two-pronged bristletails) Archaeognatha (jumping bristletails) Zygentoma (silverfish) Pterygota (winged insects) An incomplete possible insect fossil, Strudiella devonica , has been recovered from
5850-417: The soil during dry seasons, but also why some epigeal springtails are always found in the vicinity of ponds and lakes, such as the hygrophilous Isotomurus palustris . Adaptive features, such as the presence of a fan-like wettable mucro, allow some species to move at the surface of water in freshwater and marine environments. Podura aquatica , a unique representative of the family Poduridae (and one of
5928-441: The substrate, flinging the springtail into the air and allowing for rapid evasion and travel. All of this takes place in as little as 18 milliseconds. Springtails also possess the ability to reduce their body size by as much as 30% through subsequent ecdyses (moulting) if temperatures rise high enough. The shrinkage is genetically controlled. Since warmer conditions increase metabolic rates and energy requirements in organisms,
6006-443: The world, but also perceive motion. Second-order neurons in the dragonfly median ocellus respond more strongly to upwards-moving bars and gratings than to downwards-moving bars and gratings, but this effect is only present when ultraviolet light is used in the stimulus; when ultraviolet light is absent, no directional response is observed. Dragonfly ocelli are especially highly developed and specialised visual organs, which may support
6084-628: Was called into question, and it appears the hexapods' closest relatives may be the crustaceans . The non-insect hexapods have variously been considered a single evolutionary line, typically treated as Class Entognatha , or as several lines with different relationships with the Class Insecta. In particular, the Diplura may be more closely related to the Insecta than to the Collembola (springtails). A 2002 molecular analysis suggests that
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