A heat exchanger is a device used to transfer heat between two or more fluids. Heat exchangers are used in both heating processes; the fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. They are used in space heating, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing, sewage treatment; the classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. Another example is the heat sink, a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium air or a liquid coolant. There are three primary classifications of heat exchangers according to their flow arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, travel in parallel to one another to the other side.
In counter-flow heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is the most efficient, in that it can transfer the most heat from the heat medium per unit mass due to the fact that the average temperature difference along any unit length is higher. See countercurrent exchange. In a cross-flow heat exchanger, the fluids travel perpendicular to one another through the exchanger. For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger; the exchanger's performance can be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence. The driving temperature across the heat transfer surface varies with position, but an appropriate mean temperature can be defined. In most simple systems this is the "log mean temperature difference". Sometimes direct knowledge of the LMTD is not available and the NTU method is used.
Double pipe heat exchangers are the simplest exchangers used in industries. On one hand, these heat exchangers are cheap for both design and maintenance, making them a good choice for small industries. On the other hand, their low efficiency coupled with the high space occupied in large scales, has led modern industries to use more efficient heat exchangers like shell and tube or plate. However, since double pipe heat exchangers are simple, they are used to teach heat exchanger design basics to students as the fundamental rules for all heat exchangers are the same. Shell and tube heat exchangers consist of a series of tubes which contain fluid that must be either heated or cooled. A second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Shell and tube heat exchangers are used for high-pressure applications.
This is because the tube heat exchangers are robust due to their shape. Several thermal design features must be considered when designing the tubes in the shell and tube heat exchangers: There can be many variations on the shell and tube design; the ends of each tube are connected to plenums through holes in tubesheets. The tubes may be straight or bent in the shape of a U, called U-tubes. Tube diameter: Using a small tube diameter makes the heat exchanger both economical and compact. However, it is more for the heat exchanger to foul up faster and the small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube diameters can be used, thus to determine the tube diameter, the available space and fouling nature of the fluids must be considered. Tube thickness: The thickness of the wall of the tubes is determined to ensure: There is enough room for corrosion That flow-induced vibration has resistance Axial strength Availability of spare parts Hoop strength Buckling strength Tube length: heat exchangers are cheaper when they have a smaller shell diameter and a long tube length.
Thus there is an aim to make the heat exchanger as long as physically possible whilst not exceeding production capabilities. However, there are many limitations for this, including space available at the installation site and the need to ensure tubes are available in lengths that are twice the required length. Long, thin tubes are difficult to take out and replace. Tube pitch: when designing the tubes, it is practical to ensure that the tube pitch is not less than 1.25 times the tubes' outside diameter. A larger tube pitch leads to a larger overall shell diameter, which leads to a more expensive heat exchanger. Tube corrugation: this type of tubes used for the inner tubes, increases the turbulence of the fluids and the effect is important in the heat transfer giving a better performance. Tube Layout: refers to. There are four main types of tube layout, which are, rotated triangular and rotated square; the triangular patterns are employed to give greater heat transfer as they force the fluid to flow in a more turbulent fashion around the piping.
Square patterns are employed where high fouling is experienced and cleaning is more regular. B
Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared and ultraviolet light. On Earth, sunlight is filtered through Earth's atmosphere, is obvious as daylight when the Sun is above the horizon; when the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When it is blocked by clouds or reflects off other objects, it is experienced as diffused light; the World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per square meter. Other sources indicate an "Average over the entire earth" of "164 Watts per square meter over a 24 hour day"; the ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a requisite for vitamin D3 synthesis and a mutagen. Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun.
A photon starting at the center of the Sun and changing direction every time it encounters a charged particle would take between 10,000 and 170,000 years to get to the surface. Sunlight is a key factor in photosynthesis, the process used by plants and other autotrophic organisms to convert light energy from the Sun, into chemical energy that can be used to synthesize carbohydrates and to fuel the organisms' activities. Researchers can measure the intensity of sunlight using a sunshine recorder, pyranometer, or pyrheliometer. To calculate the amount of sunlight reaching the ground, both the eccentricity of Earth's elliptic orbit and the attenuation by Earth's atmosphere have to be taken into account; the extraterrestrial solar illuminance, corrected for the elliptic orbit by using the day number of the year, is given to a good approximation by E e x t = E s c ⋅, where dn=1 on January 1st. In this formula dn–3 is used, because in modern times Earth's perihelion, the closest approach to the Sun and, the maximum Eext occurs around January 3 each year.
The value of 0.033412 is determined knowing that the ratio between the perihelion squared and the aphelion squared should be 0.935338. The solar illuminance constant, is equal to 128×103 lux; the direct normal illuminance, corrected for the attenuating effects of the atmosphere is given by: E d n = E e x t e − c m, where c is the atmospheric extinction and m is the relative optical airmass. The atmospheric extinction brings the number of lux down to around 100 000 lux; the total amount of energy received at ground level from the Sun at the zenith depends on the distance to the Sun and thus on the time of year. It is 3.3 % lower in July. If the extraterrestrial solar radiation is 1367 watts per square meter the direct sunlight at Earth's surface when the Sun is at the zenith is about 1050 W/m2, but the total amount hitting the ground is around 1120 W/m2. In terms of energy, sunlight at Earth's surface is around 52 to 55 percent infrared, 42 to 43 percent visible, 3 to 5 percent ultraviolet. At the top of the atmosphere, sunlight is about 30% more intense, having about 8% ultraviolet, with most of the extra UV consisting of biologically damaging short-wave ultraviolet.
Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux. Multiplying the figure of 1050 watts per square metre by 93 lumens per watt indicates that bright sunlight provides an illuminance of 98 000 lux on a perpendicular surface at sea level; the illumination of a horizontal surface will be less than this if the Sun is not high in the sky. Averaged over a day, the highest amount of sunlight on a horizontal surface occurs in January at the South Pole. Dividing the irradiance of 1050 W/m2 by the size of the Sun's disk in steradians gives an average radiance of 15.4 MW per square metre per steradian. Multiplying this by π gives an upper limit to the irradiance which can be focused on a surface using mirrors: 48.5 MW/m2. The spectrum of the Sun's solar radiation is close to that of a black body with a temperature of about 5,800 K; the Sun emits EM radiation across most of the electromagnetic spectrum. Although the Sun produces gamma rays as a result of the nuclear-fusion process, internal absorption and thermalization convert these super-high-energy photons to lower-energy photons before they reach the Sun's surface and are emitted out into space.
As a result, the Sun does not emit gamma rays from this process, but it does emit gamma rays from solar flares. The Sun emits X-rays, vis
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries when the surrounding temperature is different. A thermoconforming organism, by contrast adopts the surrounding temperature as its own body temperature, thus avoiding the need for internal thermoregulation; the internal thermoregulation process is one aspect of homeostasis: a state of dynamic stability in an organism's internal conditions, maintained far from thermal equilibrium with its environment. If the body is unable to maintain a normal temperature and it increases above normal, a condition known as hyperthermia occurs. For humans, this occurs when the body is exposed to constant temperatures of 55 °C, with prolonged exposure at this temperature and up to around 75 °C death is inevitable. Humans may experience lethal hyperthermia when the wet bulb temperature is sustained above 35 °C for six hours; the opposite condition, when body temperature decreases below normal levels, is known as hypothermia.
It results when the homeostatic control mechanisms of heat within the body malfunction, causing the body to lose heat faster than producing it. Normal body temperature is around 37 °C, hypothermia sets in when the core body temperature gets lower than 35 °C. Caused by prolonged exposure to cold temperatures, hypothermia is treated by methods that attempt to raise the body temperature back to a normal range, it was not until the introduction of thermometers that any exact data on the temperature of animals could be obtained. It was found that local differences were present, since heat production and heat loss vary in different parts of the body, although the circulation of the blood tends to bring about a mean temperature of the internal parts. Hence it is important to identify the parts of the body that most reflect the temperature of the internal organs. For such results to be comparable, the measurements must be conducted under comparable conditions; the rectum has traditionally been considered to reflect most the temperature of internal parts, or in some cases of sex or species, the vagina, uterus or bladder.
The temperature of the urine as it leaves the urethra may be of use in measuring body temperature. More the temperature is taken in the mouth, ear or groin; some animals undergo one of various forms of dormancy where the thermoregulation process temporarily allows the body temperature to drop, thereby conserving energy. Examples include hibernating bears and torpor in bats. Thermoregulation in organisms runs along a spectrum from endothermy to ectothermy. Endotherms create most of their heat via metabolic processes, are colloquially referred to as warm-blooded; when the surrounding temperatures are cold, endotherms increase metabolic heat production to keep their body temperature constant, thus making the internal body temperature of an endotherm more or less independent of the temperature of the environment. One metabolic activity, in terms of generating heat, that endotherms are able to do is that they possess a larger number of mitochondria per cell than ectotherms, enabling them to generate more heat by increasing the rate at which they metabolize fats and sugars.
Ectotherms use external sources of temperature to regulate their body temperatures. They are colloquially referred to as cold-blooded despite the fact that body temperatures stay within the same temperature ranges as warm-blooded animals. Ectotherms are the opposite of endotherms. In ectotherms, the internal physiological sources of heat are of negligible importance. Living in areas that maintain a constant temperature throughout the year, like the tropics or the ocean, has enabled ectotherms to develop a wide range of behavioral mechanisms that enable them to respond to external temperatures, such as sun-bathing to increase body temperature, or seeking the cover of shade to lower body temperature. Vaporization: Evaporation of sweat and other bodily fluids. Convection: Increasing blood flow to body surfaces to maximize heat transfer across the advective gradient. Conduction: Losing heat by being in contact with a colder surface. For instance: Lying on cool ground. Staying wet in a river, lake or sea.
Covering in cool mud. Radiation: releasing heat by radiating it away from the body. Convection: Climbing to higher ground up trees, rocks. Entering a warm water or air current. Building an insulated nest or burrow. Conduction: Lying on a hot surface. Radiation: Lying in the sun. Folding skin to reduce exposure. Concealing wing surfaces. Exposing wing surfaces. Insulation: Changing shape to alter surface/volume ratio. Inflating the body. To cope with low temperatures, some fish have developed the ability to remain functional when the water temperature is below freezing. Amphibians and reptiles cope with heat loss by behavioral adaptations. An example of behavioral adaptation is that of a lizard lying in the sun on a hot rock in order to heat through radiation and conduction. An endotherm is an animal that regulates its own body temperature by keeping it at a constant level. To regulate body temperature, an organism may need to prevent heat gains in arid environments. Evaporation of water, either across respiratory surfaces or across the skin
The Ancient Greek language includes the forms of Greek used in Ancient Greece and the ancient world from around the 9th century BCE to the 6th century CE. It is roughly divided into the Archaic period, Classical period, Hellenistic period, it is succeeded by medieval Greek. Koine is regarded as a separate historical stage of its own, although in its earliest form it resembled Attic Greek and in its latest form it approaches Medieval Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects. Ancient Greek was the language of Homer and of fifth-century Athenian historians and philosophers, it has contributed many words to English vocabulary and has been a standard subject of study in educational institutions of the Western world since the Renaissance. This article contains information about the Epic and Classical periods of the language. Ancient Greek was a pluricentric language, divided into many dialects; the main dialect groups are Attic and Ionic, Aeolic and Doric, many of them with several subdivisions.
Some dialects are found in standardized literary forms used in literature, while others are attested only in inscriptions. There are several historical forms. Homeric Greek is a literary form of Archaic Greek used in the epic poems, the "Iliad" and "Odyssey", in poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic and other Classical-era dialects; the origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period, they differ in some of the detail. The only attested dialect from this period is Mycenaean Greek, but its relationship to the historical dialects and the historical circumstances of the times imply that the overall groups existed in some form. Scholars assume that major Ancient Greek period dialect groups developed not than 1120 BCE, at the time of the Dorian invasion—and that their first appearances as precise alphabetic writing began in the 8th century BCE.
The invasion would not be "Dorian" unless the invaders had some cultural relationship to the historical Dorians. The invasion is known to have displaced population to the Attic-Ionic regions, who regarded themselves as descendants of the population displaced by or contending with the Dorians; the Greeks of this period believed there were three major divisions of all Greek people—Dorians and Ionians, each with their own defining and distinctive dialects. Allowing for their oversight of Arcadian, an obscure mountain dialect, Cypriot, far from the center of Greek scholarship, this division of people and language is quite similar to the results of modern archaeological-linguistic investigation. One standard formulation for the dialects is: West vs. non-west Greek is the strongest marked and earliest division, with non-west in subsets of Ionic-Attic and Aeolic vs. Arcadocypriot, or Aeolic and Arcado-Cypriot vs. Ionic-Attic. Non-west is called East Greek. Arcadocypriot descended more from the Mycenaean Greek of the Bronze Age.
Boeotian had come under a strong Northwest Greek influence, can in some respects be considered a transitional dialect. Thessalian had come under Northwest Greek influence, though to a lesser degree. Pamphylian Greek, spoken in a small area on the southwestern coast of Anatolia and little preserved in inscriptions, may be either a fifth major dialect group, or it is Mycenaean Greek overlaid by Doric, with a non-Greek native influence. Most of the dialect sub-groups listed above had further subdivisions equivalent to a city-state and its surrounding territory, or to an island. Doric notably had several intermediate divisions as well, into Island Doric, Southern Peloponnesus Doric, Northern Peloponnesus Doric; the Lesbian dialect was Aeolic Greek. All the groups were represented by colonies beyond Greece proper as well, these colonies developed local characteristics under the influence of settlers or neighbors speaking different Greek dialects; the dialects outside the Ionic group are known from inscriptions, notable exceptions being: fragments of the works of the poet Sappho from the island of Lesbos, in Aeolian, the poems of the Boeotian poet Pindar and other lyric poets in Doric.
After the conquests of Alexander the Great in the late 4th century BCE, a new international dialect known as Koine or Common Greek developed based on Attic Greek, but with influence from other dialects. This dialect replaced most of the older dialects, although Doric dialect has survived in the Tsakonian language, spoken in the region of modern Sparta. Doric has passed down its aorist terminations into most verbs of Demotic Greek. By about the 6th century CE, the Koine had metamorphosized into Medieval Greek. Ancient Macedonian was an Indo-European language at least related to Greek, but its exact relationship is unclear because of insufficient data: a dialect of Greek; the Macedonian dialect (or l
Nocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite. Nocturnal creatures have developed senses of hearing and specially adapted eyesight; such traits can help animals such as the Helicoverpa zea moths avoid predators. Some animals, such as cats and ferrets, have eyes that can adapt to both low-level and bright day levels of illumination. Others, such as bushbabies and bats, can function only at night. Many nocturnal creatures including tarsiers and some owls have large eyes in comparison with their body size to compensate for the lower light levels at night. More they have been found to have a larger cornea relative to their eye size than diurnal creatures to increase their visual sensitivity: in the low-light conditions. Nocturnality helps wasps, such as avoid hunting in intense sunlight. Diurnal animals, including squirrels and songbirds, are active during the daytime.
Crepuscular species, such as rabbits, skunks and hyenas, are erroneously referred to as nocturnal. Cathemeral species, such as fossas and lions, are active both at night. While it is difficult to say which came first, nocturnality or diurnality, there is a leading hypothesis out in the evolutionary biology community. Known as the "bottleneck theory", it postulates that millions of years ago in the Mesozoic era, many ancestors of modern-day mammals evolved nocturnal characteristics in order to avoid contact with the numerous diurnal predators. A recent study attempts to answer the question as to why so many modern day mammals retain these nocturnal characteristics though they are not active at night; the leading answer is that the high visual acuity that comes with diurnal characteristics isn't needed anymore due to the evolution of compensatory sensory systems, such as a heightened sense of smell and more astute auditory systems. In a recent study extinct elephant birds and modern day nocturnal kiwi bird skulls were examined to recreate their brain and skull formation.
They indicated that olfactory bulbs were much larger in comparison to their optic lobes, indicating they both have a common ancestor who evolved to function as a nocturnal species, decreasing their eyesight in favor of a better sense of smell. The anomaly to this theory were anthropoids, who appeared to have the most divergence from nocturnality than all organisms examined. While most mammals didn't exhibit the morphological characteristics expected of a nocturnal creature and birds fit in perfectly. A larger cornea and pupil correlated well with whether these two classes of organisms were nocturnal or not. Being active at night is a form of niche differentiation, where a species' niche is partitioned not by the amount of resources but by the amount of time. Hawks and owls can hunt the same field or meadow for the same rodents without conflict because hawks are diurnal and owls are nocturnal; this means. Nocturnality is a form of an adaptation to avoid or enhance predation. One of the reasons that lions prefer to hunt at night is that many of their prey species have poor night vision.
Many species of small rodents, such as the Large Japanese Field Mouse, are active at night because most of the dozen or so birds of prey that hunt them are diurnal. There are many diurnal species. For example, many seabirds and sea turtles only gather at breeding sites or colonies at night to reduce the risk of predation to themselves and/or their offspring. Nocturnal species take advantage of the night time to prey on species that are used to avoiding diurnal predators; some nocturnal fish species will use the moonlight to prey on zooplankton species that come to the surface at night. Some species have developed unique adaptations. Bats are famous for using echolocation to hunt down their prey, using sonar sounds to capture them in the dark. Another reason for nocturnality is avoiding the heat of the day; this is true in arid biomes like deserts, where nocturnal behavior prevents creatures from losing precious water during the hot, dry daytime. This is an adaptation. One of the reasons that lions prefer to hunt at night is to conserve water.
Many plant species native to arid biomes have adapted so that their flowers only open at night when the sun's intense heat cannot wither and destroy their moist, delicate blossoms. These flowers are pollinated by another creature of the night. Climate-change and the change in global temperatures has led to an increasing amount of diurnal species to push their activity patterns closer towards crepuscular or nocturnal behavior; this adaptive measure allows species to avoid the heat of the day, without having to leave that particular habitat. The exponential increase in human expansion and technological advances in the last few centuries has had a major effect on nocturnal animals, as well as diurnal species; the causes of these can be traced to distinct, sometimes overlapping areas: light pollution and spatial disturbance. Light pollution is a major issue for nocturnal species, the impact continues to increase as electricity reaches parts of the world that had no access. Species in the tropics are more affected by this due to the change in their constant light patterns, but temperate species relying on day-night triggers for behavioral patterns are affected as well.
Many diurnal species see the benefit of a "longer day", allowin
A poikilotherm is an animal whose internal temperature varies considerably. It is the opposite of an animal which maintains thermal homeostasis. While the term in principle can apply to all organisms, it is only applied to animals, to vertebrates; the fluctuations are consequence of variation in the ambient environmental temperature. Many terrestrial ectotherms are poikilothermic; however some ectotherms remain in temperature-constant environments to the point that they are able to maintain a constant internal temperature. It is this distinction that makes the term "poikilotherm" more useful than the vernacular "cold-blooded", sometimes used to refer to ectotherms more generally. Poikilothermic animals include types of vertebrate animals some fish and reptiles, as well as a large number of invertebrate animals; the naked mole-rat is the only mammal, thought to be poikilothermic. The term derives from Greek poikilos, meaning "varied," from a root meaning "dappled" or “painted,” and thermos, meaning "heat".
Poikilotherm animals must be able to function over a wider range of temperatures than homeotherms. The speed of most chemical reactions vary with temperature, in order to function poikilotherms may have four to ten enzyme systems that operate at different temperatures for an important chemical reaction; as a result, poikilotherms have larger, more complex genomes than homeotherms in the same ecological niche. Frogs are a notable example of this effect, though their complex development is an important factor in their large genome; because their metabolism is variable and below that of homeothermic animals, sustained high-energy activities like powered flight in large animals or maintaining a large brain is beyond poikilotherm animals. The metabolism of poikilotherms favors strategies such as sit-and-wait hunting over chasing prey for larger animals with high movement cost; as they do not use their metabolisms to heat or cool themselves, total energy requirement over time is low. For the same body weight, poikilotherms need only 5 to 10% of the energy of homeotherms.
Some adaptations are behavioral. Lizards and snakes bask in the sun in the early morning and late evening, seek shelter around noon; the eggs of the yellow-faced bumblebee are unable to regulate heat. A behavioral adaptation to combat this is incubation, where to maintain the internal temperatures of eggs, the queen and her workers will incubate the brood constantly, by warming their abdomens and touching them to the eggs; the bumblebee generates heat by shivering flight muscles though they are not flying. Termite mounds are oriented in a north-south direction so that they absorb as much heat as possible around dawn and dusk and minimise heat absorption around noon. Tuna are able to warm their entire bodies through a heat exchange mechanism called the rete mirabile, which helps keep heat inside the body, minimises the loss of heat through the gills, they have their swimming muscles near the center of their bodies instead of near the surface, which minimises heat loss. Gigantothermy means using a low ratio of surface area to volume to minimise heat loss, such as in sea turtles.
Camels, although they are homeotherms, thermoregulate using a method termed "temperature cycling" to conserve energy. In hot deserts, they allow their body temperature to rise during the day and fall during the night, adjusting their body temperature to cycle over 6°C, it is comparatively easy for a poikilotherm to accumulate enough energy to reproduce. Poikilotherms at the same trophic level have much shorter generations than homeotherms: weeks rather than years; such applies to animals with similar ecological roles such as cats and snakes. This difference in energy requirement means that a given food source can support a greater density of poikilothermic animals than homeothermic animals; this is reflected in the predator-prey ratio, higher in poikilothermic fauna compared to homeothermic ones. However, when homeotherms and poikilotherms have similar niches, compete, the homeotherm can drive poikilothermic competitors to extinction, because homeotherms can gather food for a greater fraction of each day.
In medicine, loss of normal thermoregulation in humans is referred to as "poikilothermia". This is seen with sedative and hypnotic drugs or in'compartment syndrome'. For example, barbiturates and chloral hydrate may precipitate this effect. REM sleep is considered a poikilothermic state in humans; the dictionary definition of poikilotherm at Wiktionary
Ambush predators or sit-and-wait predators are carnivorous animals that capture or trap prey by stealth or by strategy, rather than by speed or by strength. Ambush predators sit and wait for prey from a concealed position, launch a rapid surprise attack; the ambush may be set by hiding in a burrow, by camouflage, by aggressive mimicry, or by the use of a trap. The predator uses a combination of senses to assess the prey and to time the strike. Nocturnal ambush predators such as cats and snakes have vertical slit pupils, helping them to judge the distance to prey in dim light. Different ambush predators use a variety of means to capture their prey, from the long sticky tongues of chameleons to the expanding mouths of frogfishes. Ambush predation is distributed in the animal kingdom, spanning some members of numerous groups such as the starfish, crustaceans, insects such as mantises, vertebrates such as many snakes and fishes. Ambush predators remain motionless and wait for prey to come within ambush distance before pouncing.
Ambush predators are camouflaged, may be solitary. Pursuit predation becomes a better strategy than ambush predation when the predator is faster than the prey. Ambush predators use many intermediate strategies. For example, when a pursuit predator is faster than its prey over a short distance, but not in a long chase either stalking or ambush becomes necessary as part of the strategy. Ambush relies on concealment, whether by staying out of sight or by means of camouflage. Ambush predators such as trapdoor spiders on land and mantis shrimps in the sea rely on concealment and hiding in burrows; these provide effective concealment at the price of a restricted field of vision. Trapdoor spiders excavate a burrow and seal the entrance with a web trapdoor hinged on one side with silk; the most well-known door is the cork-type, thick and beveled to fit the opening. The other is a simpler sheet of silk and dirt; the top of the door is camouflaged with bits of debris such as twigs and rock, making it difficult to detect.
The spider spins trip wires, that radiate out of the burrow entrance. When the spider is using the trap to capture prey, its chelicerae hold the door shut on the end furthest from the hinge; the vibrations of passing prey are conducted by the silk and alert the spider whereupon it throws open the door, ambushes the prey and returns with it down the tube. Many ambush predators make use of camouflage so that their prey can come within striking range without detecting their presence. Among fishes, the warteye stargazer buries itself nearly in the sand and waits for prey; the devil scorpionfish lies buried on the sea floor or on a coral head during the day, covering itself with sand and other debris to further camouflage itself. The tasselled wobbegong is a shark whose adaptations as an ambush predator include a flattened and camouflaged body with a fringe that breaks up its outline. Many ambush predators attract their prey towards them before ambushing them; these animals are classified as aggressive mimics.
The promise of nourishment as a way of attracting prey. The alligator snapping turtle is a well-camouflaged ambush predator, its tongue bears a conspicuous pink extension that can be wriggled around. Some snakes employ caudal luring to entice small vertebrates into striking range; the zone-tailed hawk, which resembles the turkey vulture, flies among flocks of turkey vultures suddenly breaks from the formation and ambushes one of them as its prey. There is however some controversy about whether this is a true case of wolf in sheep's clothing mimicry. Flower mantises are aggressive mimics, resembling flowers convincingly enough to attract prey that come to collect pollen and nectar; the orchid mantis Hymenopus coronatus attracts its prey, pollinator insects, more than flowers do. Crab spiders are coloured like the flowers they habitually rest on, but again, they can lure their prey away from flowers; some ambush predators build traps to help capture their prey. Lacewings are a flying insect in the order Neuroptera.
In some species, their larval form, known as the antlion, is an ambush predator. Eggs are laid in the earth in caves or under a rocky ledge; the juvenile creates a small, crater shaped trap. The antlion hides under a light cover of earth; when an ant, beetle or other prey slides into the trap, the antlion grabs the prey with its powerful jaws. Some but not all web-spinning spiders are sit-and-wait ambush predators; the sheetweb spiders tend to stay with their webs for long periods and so resemble sit-and-wait predators, whereas the orb-weaving spiders tend to move from one patch to another. Ambush predators must time their strike carefully, they need to detect the prey, assess it as worth attacking, strike when it is in the right place. They have evolved a variety of adaptations. For example, pit vipers prey on small birds, choosing targets of the right size for their mouth gape: larger snakes choose larger prey, they prefer to strike prey, both warm and moving. The deep-sea tripodfish Bathypterois gra