The Hemiscorpiidae are a family of scorpions with 72 described species in 12 genera. Their old name was Ischnuridae, which had to be changed due to a naming conflict with the damselfly family of the same name, they at one point held the name Liochelidae. Most species have a flat and broad body plan, due to their main habitat in tight rock crevices; some long-lived species can reach more than 20 cm in length. Hemiscorpiidae are distributed throughout the world's subtropics; the family's center of distribution appears to be Africa where several genera are found, although it is known from Australasia, the Middle East, South America Many species are held as pets. Species in this family are some of the less venomous scorpions. Only genus Hemiscorpius has a strong venom. Cheloctonus Pocock, 1892 Chiromachetes Pocock, 1899 Chiromachus Pocock, 1893 Habibiella Vachon, 1974 Hadogenes Kraepelin, 1894 Hemiscorpius Peters, 1861 Heteroscorpion Birula, 1903 Iomachus Pocock, 1893 Liocheles Sundevall, 1833 Monodopisthacanthus Lourenço, 2001 Opisthacanthus Peters, 1861 Paleocheloctonus Lourenço, 1996 Hemiscorpiidae bei The Scorpion Files Hadogenes bicolor Hadogenes troglodytes
Arachnids are a class of joint-legged invertebrate animals, in the subphylum Chelicerata. All adult arachnids have eight legs, although the front pair of legs in some species has converted to a sensory function, while in other species, different appendages can grow large enough to take on the appearance of extra pairs of legs; the term is derived from the Greek word ἀράχνη, from the myth of the hubristic human weaver Arachne, turned into a spider. Spiders are the largest order in the class, which includes scorpions, mites and solifuges. In 2019, a molecular phylogenetic study placed horseshoe crabs in Arachnida. All extant arachnids are terrestrial, living on land. However, some inhabit freshwater environments and, with the exception of the pelagic zone, marine environments as well, they comprise over 100,000 named species. All adult arachnids have eight legs, arachnids may be distinguished from insects by this fact, since insects have six legs. However, arachnids have two further pairs of appendages that have become adapted for feeding and sensory perception.
The first pair, the chelicerae, serve in defense. The next pair of appendages, the pedipalps, have been adapted for feeding, and/or reproductive functions. In Solifugae, the palps are quite leg-like; the larvae of mites and Ricinulei have only six legs. However, mites are variable: as well as eight, there are adult mites with six or four legs. Arachnids are further distinguished from insects by the fact, their body is organized into two tagmata, called the prosoma, or cephalothorax, the opisthosoma, or abdomen. The cephalothorax is derived from the fusion of the cephalon and the thorax, is covered by a single, unsegmented carapace; the abdomen is segmented in the more primitive forms, but varying degrees of fusion between the segments occur in many groups. It is divided into a preabdomen and postabdomen, although this is only visible in scorpions, in some orders, such as the Acari, the abdominal sections are fused. A telson is present in scorpions, where it has been modified to a stinger, in the Schizomida, whip scorpions and Palpigradi.
Like all arthropods, arachnids have an exoskeleton, they have an internal structure of cartilage-like tissue, called the endosternite, to which certain muscle groups are attached. The endosternite is calcified in some Opiliones. Most arachnids lack extensor muscles in the distal joints of their appendages. Spiders and whipscorpions extend their limbs hydraulically using the pressure of their hemolymph. Solifuges and some harvestmen extend their knees by the use of elastic thickenings in the joint cuticle. Scorpions and some harvestmen have evolved muscles that extend two leg joints at once; the equivalent joints of the pedipalps of scorpions though, are extended by elastic recoil. There are characteristics that are important for the terrestrial lifestyle of arachnids, such as internal respiratory surfaces in the form of tracheae, or modification of the book gill into a book lung, an internal series of vascular lamellae used for gas exchange with the air. While the tracheae are individual systems of tubes, similar to those in insects, ricinuleids and some spiders possess sieve tracheae, in which several tubes arise in a bundle from a small chamber connected to the spiracle.
This type of tracheal system has certainly evolved from the book lungs, indicates that the tracheae of arachnids are not homologous with those of insects. Further adaptations to terrestrial life are appendages modified for more efficient locomotion on land, internal fertilisation, special sensory organs, water conservation enhanced by efficient excretory structures as well as a waxy layer covering the cuticle; the excretory glands of arachnids include up to four pairs of coxal glands along the side of the prosoma, one or two pairs of Malpighian tubules, emptying into the gut. Many arachnids have the other type of excretory gland, although several do have both; the primary nitrogenous waste product in arachnids is guanine. Arachnid blood is variable in composition, depending on the mode of respiration. Arachnids with an efficient tracheal system do not need to transport oxygen in the blood, may have a reduced circulatory system. In scorpions and some spiders, the blood contains haemocyanin, a copper-based pigment with a similar function to haemoglobin in vertebrates.
The heart is located in the forward part of the abdomen, may or may not be segmented. Some mites have no heart at all. Arachnids are carnivorous, feeding on the pre-digested bodies of insects and other small animals. Only in the harvestmen and among mites, such as the house dust mite, is there ingestion of solid food particles, thus exposure to internal parasites, although it is not unusual for spiders to eat their own silk. Several groups secrete venom from specialized glands to kill prey or enemies. Several mites and ticks are parasites. Arachnids produce digestive juices in their stomachs, use their pedipalps and chelicerae to pour them over their dead prey; the digestive juices turn the prey into a broth of nutrients, which the arachnid sucks into a pre-buccal cavity located in front of the mouth. Behind the mouth is a muscular, sclerotised pharynx, which acts as a pump, sucking the food through the mouth and on into the oesophagus and stomach. In some arachnids, the oesophagus a
East Africa or Eastern Africa is the eastern region of the African continent, variably defined by geography. In the United Nations Statistics Division scheme of geographic regions, 20 territories make up Eastern Africa: Tanzania, Uganda, Rwanda and South Sudan are members of the East African Community; the first five are included in the African Great Lakes region. Burundi and Rwanda are at times considered to be part of Central Africa. Djibouti, Eritrea and Somalia – collectively known as the Horn of Africa; the area is the easternmost projection of the African continent, is sometimes considered a separate region from East Africa. Comoros and Seychelles – small island nations in the Indian Ocean. Réunion and Mayotte – French overseas territories in the Indian Ocean. Mozambique and Madagascar – considered part of Southern Africa, on the eastern side of the sub-continent. Madagascar has close cultural ties to the islands of the Indian Ocean. Malawi and Zimbabwe – also included in Southern Africa, constituted the Central African Federation.
Sudan and South Sudan – collectively part of the Nile Valley. Situated in the northeastern portion of the continent, the Sudans are included in Northern Africa. Members of the Common Market for Eastern and Southern Africa free trade area. Due to colonial territories of the British East Africa Protectorate and German East Africa, the term East Africa is used to refer to the area now comprising the three countries of Kenya and Uganda. However, this has never been the convention in many other languages, where the term had a wider geographic context and therefore included Djibouti, Eritrea and Somalia; some parts of East Africa have been renowned for their concentrations of wild animals, such as the "big five": the elephant, lion, black rhinoceros, leopard, though populations have been declining under increased stress in recent times those of the rhino and elephant. The geography of East Africa is stunning and scenic. Shaped by global plate tectonic forces that have created the East African Rift, East Africa is the site of Mount Kilimanjaro and Mount Kenya, the two tallest peaks in Africa.
It includes the world's second largest freshwater lake, Lake Victoria, the world's second deepest lake, Lake Tanganyika. The climate of East Africa is rather atypical of equatorial regions; because of a combination of the region's high altitude and the rain shadow of the westerly monsoon winds created by the Rwenzori Mountains and Ethiopian Highlands, East Africa is cool and dry for its latitude. In fact, on the coast of Somalia, many years can go by without any rain whatsoever. Elsewhere the annual rainfall increases towards the south and with altitude, being around 400 mm at Mogadishu and 1,200 mm at Mombasa on the coast, whilst inland it increases from around 130 mm at Garoowe to over 1,100 mm at Moshi near Kilimanjaro. Unusually, most of the rain falls in two distinct wet seasons, one centred on April and the other in October or November; this is attributed to the passage of the Intertropical Convergence Zone across the region in those months, but it may be analogous to the autumn monsoon rains of parts of Sri Lanka and the Brazilian Nordeste.
West of the Rwenzoris and Ethiopian highlands, the rainfall pattern is more tropical, with rain throughout the year near the equator and a single wet season in most of the Ethiopian Highlands from June to September – contracting to July and August around Asmara. Annual rainfall here ranges from over 1,600 mm on the western slopes to around 1,250 mm at Addis Ababa and 550 mm at Asmara. In the high mountains rainfall can be over 2,500 mm. Rainfall in East Africa is influenced by El Niño events, which tend to increase rainfall except in the northern and western parts of the Ethiopian and Eritrean highlands, where they produce drought and poor Nile floods. Temperatures in East Africa, except on the hot and humid coastal belt, are moderate, with maxima of around 25 °C and minima of 15 °C at an altitude of 1,500 metres. At altitudes of above 2,500 metres, frosts are common during the dry season and maxima about 21 °C or less; the unique geography and apparent suitability for farming made East Africa a target for European exploration and colonialization in the nineteenth century.
Today, tourism is an important part of the economies of Kenya, Tanzania and Uganda. The easternmost point of the continent, Ras Hafun in Somalia, is of archaeological and economical importance. According to the theory of the recent African origin of modern humans, the predominantly held belief among most archaeologists, East Africa is the area where anatomically modern humans first appeared. There are differing theories on whether there was several. A growing number of researchers suspect that North Africa was instead the original home of the modern humans who first trekked out of the continent; the major competing hypothesis is the multiregional origin of modern humans, which envisions a wave of Homo sapiens migrating earlier from Africa and interbreeding with local Homo erectus populations in multiple regions of the globe. Most multiregionalists still view Africa as a major wellspring of human genetic diversity, but allow a much greater role for hybridization. Some
Scorpions are predatory arachnids of the order Scorpiones. They have eight legs and are recognized by the pair of grasping pedipalps and the narrow, segmented tail carried in a characteristic forward curve over the back, ending with a venomous stinger. Scorpions range in size from 9 mm / 0.3 in. to 23 cm / 9 in.. The evolutionary history of scorpions goes back to the Silurian period 430 million years ago, they have adapted to a wide range of environmental conditions, they can now be found on all continents except Antarctica. Scorpions number about 1,750 described species, with 13 extant families recognised to date; the taxonomy has undergone changes and is to change further, as genetic studies are bringing forth new information. All scorpions have a venomous sting, but the vast majority of the species do not represent a serious threat to humans, in most cases, healthy adults do not need any medical treatment after being stung. Only about 25 species are known to have venom capable of killing a human.
In some parts of the world with venomous species, human fatalities occur in areas with limited access to medical treatment. The word scorpion is thought to have originated in Middle English between 1175 and 1225 AD from Old French scorpion, or from Italian scorpione, both derived from the Latin scorpius, the romanization of the Greek word σκορπίος – skorpíos. Scorpions are found on all major land masses except Antarctica. Scorpions did not occur in Great Britain, Japan, South Korea, New Zealand and some of the islands in Oceania, but now have been accidentally introduced in some of these places by human trade and commerce; the greatest diversity of scorpions in the Northern Hemisphere is to be found in regions between the latitudes 23° N and 38° N. Above these latitudes, the diversity decreases with the northernmost natural occurrence of scorpions being the northern scorpion Paruroctonus boreus at Medicine Hat, Canada 50° N. Five colonies of scorpions have established themselves in Sheerness on the Isle of Sheppey in the United Kingdom.
This small population has been resident since the 1860s, having arrived with imported fruit from Africa. This scorpion species is small and harmless to humans. At just over 51 ° N, this marks the northernmost limit. Today, scorpions are found in every terrestrial habitat including: high-elevation mountains and intertidal zones, with the exception of boreal ecosystems such as: the tundra, high-altitude taiga, the permanently snow-clad tops of some mountains; as regards microhabitats, scorpions may be tree-living, rock-loving or sand-loving. Some species, such as Vaejovis janssi, are versatile and are found in every type of habitat in Baja California, while others occupy specialized niches such as Euscorpius carpathicus, endemic to the littoral zone of rivers in Romania. Thirteen families and about 1,750 described species and subspecies of scorpions are known. In addition, 111 described; this classification is based on that of Soleglad and Fet, which replaced the older, unpublished classification of Stockwell.
Additional taxonomic changes are from papers by Soleglad et al.. This classification covers extant taxa to the rank of family: Order ScorpionesInfraorder Orthosterni Pocock, 1911 Parvorder Pseudochactida Soleglad et Fet, 2003 Superfamily Pseudochactoidea Gromov, 1998 Family Pseudochactidae Gromov, 1998 Parvorder Buthida Soleglad et Fet, 2003 Superfamily Buthoidea C. L. Koch, 1837 Family Buthidae C. L. Koch, 1837 Family Microcharmidae Lourenço, 1996 Parvorder Chaerilida Soleglad et Fet, 2003 Superfamily Chaeriloidea Pocock, 1893 Family Chaerilidae Pocock, 1893 Parvorder Iurida Soleglad et Fet, 2003 Superfamily Chactoidea Pocock, 1893 Family Chactidae Pocock, 1893 Family Euscorpiidae Laurie, 1896 Family Superstitioniidae Stahnke, 1940 Family Vaejovidae Thorell, 1876 Superfamily Iuroidea Thorell, 1876 Family Caraboctonidae Kraepelin, 1905 Family Iuridae Thorell, 1876 Superfamily Scorpionoidea Latreille, 1802 Family Bothriuridae Simon, 1880 Family Hemiscorpiidae Pocock, 1893 Family Scorpionidae Latreille, 1802 Scorpions have been found in many fossil records, including marine Silurian and estuarine Devonian deposits, coal deposits from the Carboniferous Period and in amber.
The oldest known scorpions lived around 430 million years ago in the Silurian period. Though once believed to have lived on the bottom of shallow tropical seas, early scorpions are now believed to have been terrestrial and to have washed into marine settings together with plant matter; these first scorpions were believed to have had gills instead of the present forms' book lungs, though this has subsequently been refuted. The oldest Gondwanan scorpions comprise the earliest known terrestrial animals from Gondwana. 111 fossil species of scorpion are known. Unusually for arachnids, there are more species of Palaeozoic scorpion than Mesozoic or Cenozoic ones. Fossil of ancestral scorpions had compound eyes, but as they adapted to a nocturnal lifestyle, their eyes became simplified; the eurypterids called "sea scorpions", were aquatic creatures that lived during the Palaeozoic era that share several physical traits with scorpions and may be related to them. Various species of Eurypterida could grow to be anywhere from 10 centimetres to 2.5 metres in length.
However, they exhibit anatomical differences marking them off as a group distinct from t
Malawi the Republic of Malawi, is a landlocked country in southeast Africa, known as Nyasaland. It is bordered by Zambia to the northwest, Tanzania to the northeast, Mozambique on the east and west. Malawi is over 118,000 km2 with an estimated population of 18,091,575. Lake Malawi takes up about a third of Malawi's area, its capital is Lilongwe, Malawi's largest city. The name Malawi comes from an old name of the Nyanja people that inhabit the area; the country is nicknamed "The Warm Heart of Africa" because of the friendliness of the people. The part of Africa now known as Malawi was settled by migrating Bantu groups around the 10th century. Centuries in 1891 the area was colonised by the British. In 1953 Malawi known as Nyasaland, a protectorate of the United Kingdom, became a protectorate within the semi-independent Federation of Rhodesia and Nyasaland; the Federation was dissolved in 1963. In 1964 the protectorate over Nyasaland was ended and Nyasaland became an independent country under Queen Elizabeth II with the new name Malawi.
Two years it became a republic. Upon gaining independence it became a totalitarian one-party state under the presidency of Hastings Banda, who remained president until 1994. Malawi has a democratic, multi-party government headed by an elected president Arthur Peter Mutharika; the country has a Malawian Defence Force that includes a navy and an air wing. Malawi's foreign policy is pro-Western and includes positive diplomatic relations with most countries and participation in several international organisations, including the United Nations, the Commonwealth of Nations, the Southern African Development Community, the Common Market for Eastern and Southern Africa, the African Union. Malawi is among the world's least-developed countries; the economy is based in agriculture, with a rural population. The Malawian government depends on outside aid to meet development needs, although this need has decreased since 2000; the Malawian government faces challenges in building and expanding the economy, improving education, environmental protection, becoming financially independent amidst widespread unemployment.
Since 2005, Malawi has developed several programs that focus on these issues, the country's outlook appears to be improving, with a rise in the economy and healthcare seen in 2007 and 2008. Malawi has a low life expectancy and high infant mortality. There is a high prevalence of HIV/AIDS, a drain on the labour force and government expenditures. There is a diverse population of native peoples and Europeans, with several languages spoken and an array of religious beliefs. Although there was periodic regional conflict fuelled in part by ethnic divisions in the past, by 2008 it had diminished and the concept of a Malawian nationality had reemerged; the area of Africa now known as Malawi had a small population of hunter-gatherers before waves of Bantu peoples began emigrating from the north around the 10th century. Although most of the Bantu peoples continued south, some remained permanently and founded ethnic groups based on common ancestry. By 1500 AD, the tribes had established the Kingdom of Maravi that reached from north of what is now Nkhotakota to the Zambezi River and from Lake Malawi to the Luangwa River in what is now Zambia.
Soon after 1600, with the area united under one native ruler, native tribesmen began encountering, trading with and making alliances with Portuguese traders and members of the military. By 1700, the empire had broken up into areas controlled by many individual ethnic groups; the Arab slave trade reached its height in the mid- 1800s, when 20,000 people were enslaved and considered to be carried yearly from Nkhotakota to Kilwa where they were sold. Missionary and explorer David Livingstone reached Lake Malawi in 1859 and identified the Shire Highlands south of the lake as an area suitable for European settlement; as the result of Livingstone's visit, several Anglican and Presbyterian missions were established in the area in the 1860s and 1870s, the African Lakes Company Limited was established in 1878 to set up a trade and transport concern working with the missions, a small mission and trading settlement was established at Blantyre in 1876 and a British Consul took up residence there in 1883.
The Portuguese government was interested in the area so, to prevent Portuguese occupation, the British government sent Harry Johnston as British consul with instructions to make treaties with local rulers beyond Portuguese jurisdiction. In 1889, a British protectorate was proclaimed over the Shire Highlands, extended in 1891 to include the whole of present-day Malawi as the British Central Africa Protectorate. In 1907, the protectorate was renamed Nyasaland, a name it retained for the remainder of its time under British rule. In a prime example of what is sometimes called the "Thin White Line" of colonial authority in Africa, the colonial government of Nyasaland was formed in 1891; the administrators were given a budget of £10,000 per year, enough to employ ten European civilians, two military officers, seventy Punjab Sikhs and eighty-five Zanzibar porters. These few employees were expected to administer and police a territory of around 94,000 square kilometres with between one and two million people.
In 1944, the Nyasaland African Congress was formed by the Africans of Nyasaland to promote local interests to the British g
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
An arthropod is an invertebrate animal having an exoskeleton, a segmented body, paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids and crustaceans; the term Arthropoda as proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora. Arthropods are characterized by their jointed limbs and cuticle made of chitin mineralised with calcium carbonate; the arthropod body plan consists of each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthopods are bilaterally symmetrical and their body possesses an external skeleton; some species have wings. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments, they have over a million described species, making up more than 80 per cent of all described living animal species, some of which, unlike most other animals, are successful in dry environments. Arthropods range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab.
Arthropods' primary internal cavity is a haemocoel, which accommodates their internal organs, through which their haemolymph – analogue of blood – circulates. Like their exteriors, the internal organs of arthropods are built of repeated segments, their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, their brains are formed by fusion of the ganglia of these segments and encircle the esophagus; the respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong. Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey.
Arthropods have a wide range of chemical and mechanical sensors based on modifications of the many setae that project through their cuticles. Arthropods' methods of reproduction and development are diverse; the evolutionary ancestry of arthropods dates back to the Cambrian period. The group is regarded as monophyletic, many analyses support the placement of arthropods with cycloneuralians in a superphylum Ecdysozoa. Overall, the basal relationships of Metazoa are not yet well resolved; the relationships between various arthropod groups are still debated. Aquatic species use either external fertilization. All arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and undergo a total metamorphosis to produce the adult form; the level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions. Arthropods contribute to the human food supply both directly as food, more indirectly as pollinators of crops.
Some species are known to spread severe disease to humans and crops. The word arthropod comes from the Greek ἄρθρον árthron, "joint", πούς pous, i.e. "foot" or "leg", which together mean "jointed leg". Arthropods are invertebrates with jointed limbs; the exoskeleton or cuticles consists of a polymer of glucosamine. The cuticle of many crustaceans, beetle mites, millipedes is biomineralized with calcium carbonate. Calcification of the endosternite, an internal structure used for muscle attachments occur in some opiliones. Estimates of the number of arthropod species vary between 1,170,000 and 5 to 10 million and account for over 80 per cent of all known living animal species; the number of species remains difficult to determine. This is due to the census modeling assumptions projected onto other regions in order to scale up from counts at specific locations applied to the whole world. A study in 1992 estimated that there were 500,000 species of animals and plants in Costa Rica alone, of which 365,000 were arthropods.
They are important members of marine, freshwater and air ecosystems, are one of only two major animal groups that have adapted to life in dry environments. One arthropod sub-group, insects, is the most species-rich member of all ecological guilds in land and freshwater environments; the lightest insects weigh less than 25 micrograms. Some living crustaceans are much larger; the embryos of all arthropods are segmented, built from a series of repeated modules. The last common ancestor of living arthropods consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However, all known living and fossil arthropods have grouped segments into tagmata in which segments and their limbs are specialized in various ways; the three-