The arachnids, Arachnida, are a class of joint-legged invertebrate animals in the subphylum Chelicerata. Arachnids are named for the mythological figure Arachne. They are chiefly terrestrial arthropods, some 65,000 to 73,000 species including the spiders, scorpions, harvestmen, ticks, and mites. The arachnids are easily distinguished from the insects by the fact that they have eight legs instead of six. The pedipalpi (leg-like mouthparts) of some species has instead been adapted for sensory, prey capture or reproductive functions. In solifugae the palpi are quite leg-like and make solifugae appear to have ten legs. Larval mites have only six legs: they grow their fourth pair when they molt into nymphs.
Arachnids are further distinguished by the fact they have no antennae and no wings. They have a two-segment body, made up merely of the thorax and abdomen.
Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Many are venomous - they secrete poison from specialized glands to kill prey or enemies. Others are parasites, some of which are carriers of disease. Arachnids usually lay eggs, which hatch into immature adults.
Physical characteristics
The scorpion body is divided into 2 main segments: the cephalothorax (also called the prosoma) and the abdomen/opisthosoma. The abdomen consists of the mesosoma and the metasoma.
Cephalothorax/prosoma: the “head” of the scorpion comprising the carapace, eyes, chelicerae (mouth parts), pedipalps ( claw) and the 4 pairs of walking legs.
Mesosoma: The first half of the abdomen and is made up of six segments. The first segment contains the sexual organs as well as a pair of vestigial and modified appendages forming a structure called the genital operculum. The second segment bears a pair of featherlike sensory organs known as the pectines. And the final four segments each contains a pair of book lungs. The mesosoma is armoured with chitinous plates, on the upper surface by the tergites and on the lower surface by the sternites.
Metasoma: The tail of the scorpion comprising 6 segments (the first tail segment looks like a last mesosoman segment), the last containing the anus of the scorpion and bearing the telson (the sting). The telson, in turn, consists of the vesicle, which holds a pair of venom (poison) glands, and the hypodermic aculeus, the barb used to inject the venom.
Cuticle: The cuticle makes a tough armour around the body. In some places it covered with hairs that acts like balance organs. An outer layer that makes them fluoresce in green under ultraviolet light is called the hyaline layer. Newly molted scorpions do not glow before after their cuticle has hardened. The hyaline layer is said to still be intact in hundreds of millions of years old fossil rocks, and it is still able to fluoresce.
Main scorpion physical features:
Pedipalps: The claws. Some believe these were originally modified legs that evolved into claws. The claws are used primarily for prey capture, but also play an important part in the scorpion mating dance. The pedipalps are made of up of 6 segments. The claw itself consists of the upper fixed finger ( tibia) and the lower movable finger ( tarsus). The size of the pedipalps is highly dependent on the species; however a general trend is that the more venomous the scorpion, the narrower the pedipalps, as scorpions with less potent venom use their larger pincers to subdue prey.
Pectines: These are a pair of feather-like sensory organs, consisting of a row of pegs, and are unique in scorpions. The pectines are located beneath the scorpion, attached to the sternum. They constantly touch the ground and play an important role in the scorpion's sensory equipment. The pectines are covered in thousands of chemosensors that allow the scorpion to detect minute chemical signals in their environment; they probably play an important role in social interaction, mating and hunting.
Eyes: Fossil scorpions had compund eyes, present scorpions in general have three sets of eyes. One pair located in the top centre of the carapace are called the median eyes. There are two more sets of lateral eyes on either side of the front of prosoma, the number varies between species. In total there are usually from three and up to six pairs of eyes present. Some species of cave- and litter-dwelling scorpions, however, have no eyes, relying purely on their other senses. Scorpions' eyes are primitive organs, only able to detect regions of strongly contrasting light intensity, such as horizons. The eyes are however very sensitive: some have theorised that they even allow the scorpion to navigate by star light, and they are important in regulating the scorpion's circadian rhythm. It is doubtful as to whether they are used in prey capture.
Sound: Scorpions make a characteristic sound similar to the rasping or chirping of cockroaches, but a bit higher pitched.
Scorpion venom
All species of scorpion possess venom. In general scorpion venom is described as neurotoxic in nature. It consists of a variety of small proteins as well as sodium and potassium cations, which serve to interfere with neurotransmission in the victim. Scorpions use their venom to kill or paralyze their prey so that it can be eaten; in general it is fast acting, allowing for effective prey capture.
Scorpion venoms are geared towards activity in other arthropods and therefore most scorpions are relatively harmless to humans; stings produce only local effects (such as pain, numbness or swelling). However a few scorpions, mostly in the family Buthidae, can be dangerous to humans. Among the most dangerous are Leiurus quinquestriatus, which has the most potent venom in the family, and members of genus Parabuthus, Tityus and Androctonus whose venom is also strong. These scorpions and others in the family Buthidae have been responsible for many deaths, although scorpions are generally unable to deliver enough venom to kill healthy adults; deaths normally occur in the young, elderly or infirm. Unless molested, scorpions are generally harmless and timid, and only make use of the sting for the purpose of killing prey. Generally, they will run from danger, or remain very still. It is unknown whether scorpions are venom conservers, however often the venom is not used unless the prey cannot be subdued with the claws. Many species of scorpions such as Pandinus and Hadogenes rarely, if ever, use their venom, instead relying on their strong bulky pedipalps in prey capture.
Reproduction
Scorpions generally reproduce sexually and all species have male and female individuals. Reproduction is accomplished by the transfer of a spermatophore from the male to the female; scorpions have evolved a complex courtship and mating ritual to effect this transfer.
Mating starts by the male and female locating and identifying each other using a mixture of pheromones and vibronic communication, once they have satisfied each other that they are of opposite sex and of the correct species mating can commence.
The courtship starts with the male grasping the female’s pedipalps with his own; the pair then performs a “dance” called the “promenade à deux”. In reality this is the male leading the female around searching for a suitable place to deposit his spermatophore. The courtship ritual can involve several other behaviours such as juddering and a cheliceral kiss, probably as a meanings of pacifying the female.
When he has identified a suitable location he guides the female over the spermatophore allowing it to enter her genital opercula, this triggers release of the sperm; fertilising the female. The mating process can take from 1 to 25+ hours and depends on the ability of the male to find a suitable place to deposit his spermatophore, if mating goes on for too long females may eventually break off the process.
Once the mating is complete the male and female quickly separate: the male will generally retreat quickly to avoid being cannibalised by the female, although sexual cannibalism is infrequent with scorpions.
Birth and development
Compsobutus werneri female with youngUnlike the majority of Arachnida, scorpions are viviparous. The young are born one by one, and the brood is carried about on its mother's back until the young have under gone at least one moult. Before the first moult scorplings cannot survive naturally without the mother, depending on her for protection and to regulate their moisture levels. In some species the young/mother association can continue for an extended period of time especially in species which display more advanced sociability (e.g Pandinus spp.). The size of the litter depends on the species and can range from 3 to 100+ scorplings .
The young in a general way resemble their parents. Growth is accomplished by periodical shedding of the exoskeleton ( ecdysis). A scorpion's developmental progress is measured in instars (how many moults it has undergone). Generally scorpions require between 5-7 moults to reach maturity. Moulting is effected by means of a split in the old exoskeleton which takes place just below the edge of the carapace (at the front of the prosoma). The scorpion then emerges from this split, the pedipalps and legs are first removed from the old exoskeleton, followed eventually by the metasoma. When it emerges the scorpion’s new exoskeleton is soft, making the scorpion highly vulnerable to attack. The scorpion must constantly stretch while the new exoskeleton hardens to ensure that it can move when the hardening is complete. The process of hardening is called sclerotization. The new exoskeleton does not fluoresce, however as sclerotization occurs the fluorescence gradually returns.
Scorpions have very variable life spans and the actual life span of most species is not known, however the range is approximately 4-25 years (25 years being the maximum reported life span of the species H. arizonensis)
Origins
Scorpion remains are often found in coal deposits that are believed to be of the Carboniferous Period, and no essential structural difference has been discovered between these fossils and existing forms—suggesting that the group has existed without material structural modification for millions of years. These scorpions were believed to be preceded by others, now occurring in marine Silurian deposits, which apparently lived in the sea and exhibit some anatomical differences marking them off as a group distinct from their Carboniferous and recent descendants and attesting affinity with the still earlier marine Arachnida referred to the group Gigantostraca. Their legs were thought to be short, thick, tapering, and ended in a single strong claw, and were well adapted, it seems, like the legs of shore-crabs, for maintaining a secure hold upon rocks or seaweed against the wash of waves. The method of breathing of these ancient types is not certainly known; but probably respiration was effected by means of book gills attached to the ventral plates of the body. At all events no trace of respiratory stigmata has been detected even in well-preserved material. These Silurian scorpions, of which the best-known genus is Palaeophonus, were of small size, only two to five centimeters in length.
Arachnids are further distinguished by the fact they have no antennae and no wings. They have a two-segment body, made up merely of the thorax and abdomen.
Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Many are venomous - they secrete poison from specialized glands to kill prey or enemies. Others are parasites, some of which are carriers of disease. Arachnids usually lay eggs, which hatch into immature adults.
Physical characteristics
The scorpion body is divided into 2 main segments: the cephalothorax (also called the prosoma) and the abdomen/opisthosoma. The abdomen consists of the mesosoma and the metasoma.
Cephalothorax/prosoma: the “head” of the scorpion comprising the carapace, eyes, chelicerae (mouth parts), pedipalps ( claw) and the 4 pairs of walking legs.
Mesosoma: The first half of the abdomen and is made up of six segments. The first segment contains the sexual organs as well as a pair of vestigial and modified appendages forming a structure called the genital operculum. The second segment bears a pair of featherlike sensory organs known as the pectines. And the final four segments each contains a pair of book lungs. The mesosoma is armoured with chitinous plates, on the upper surface by the tergites and on the lower surface by the sternites.
Metasoma: The tail of the scorpion comprising 6 segments (the first tail segment looks like a last mesosoman segment), the last containing the anus of the scorpion and bearing the telson (the sting). The telson, in turn, consists of the vesicle, which holds a pair of venom (poison) glands, and the hypodermic aculeus, the barb used to inject the venom.
Cuticle: The cuticle makes a tough armour around the body. In some places it covered with hairs that acts like balance organs. An outer layer that makes them fluoresce in green under ultraviolet light is called the hyaline layer. Newly molted scorpions do not glow before after their cuticle has hardened. The hyaline layer is said to still be intact in hundreds of millions of years old fossil rocks, and it is still able to fluoresce.
Main scorpion physical features:
Pedipalps: The claws. Some believe these were originally modified legs that evolved into claws. The claws are used primarily for prey capture, but also play an important part in the scorpion mating dance. The pedipalps are made of up of 6 segments. The claw itself consists of the upper fixed finger ( tibia) and the lower movable finger ( tarsus). The size of the pedipalps is highly dependent on the species; however a general trend is that the more venomous the scorpion, the narrower the pedipalps, as scorpions with less potent venom use their larger pincers to subdue prey.
Pectines: These are a pair of feather-like sensory organs, consisting of a row of pegs, and are unique in scorpions. The pectines are located beneath the scorpion, attached to the sternum. They constantly touch the ground and play an important role in the scorpion's sensory equipment. The pectines are covered in thousands of chemosensors that allow the scorpion to detect minute chemical signals in their environment; they probably play an important role in social interaction, mating and hunting.
Eyes: Fossil scorpions had compund eyes, present scorpions in general have three sets of eyes. One pair located in the top centre of the carapace are called the median eyes. There are two more sets of lateral eyes on either side of the front of prosoma, the number varies between species. In total there are usually from three and up to six pairs of eyes present. Some species of cave- and litter-dwelling scorpions, however, have no eyes, relying purely on their other senses. Scorpions' eyes are primitive organs, only able to detect regions of strongly contrasting light intensity, such as horizons. The eyes are however very sensitive: some have theorised that they even allow the scorpion to navigate by star light, and they are important in regulating the scorpion's circadian rhythm. It is doubtful as to whether they are used in prey capture.
Sound: Scorpions make a characteristic sound similar to the rasping or chirping of cockroaches, but a bit higher pitched.
Scorpion venom
All species of scorpion possess venom. In general scorpion venom is described as neurotoxic in nature. It consists of a variety of small proteins as well as sodium and potassium cations, which serve to interfere with neurotransmission in the victim. Scorpions use their venom to kill or paralyze their prey so that it can be eaten; in general it is fast acting, allowing for effective prey capture.
Scorpion venoms are geared towards activity in other arthropods and therefore most scorpions are relatively harmless to humans; stings produce only local effects (such as pain, numbness or swelling). However a few scorpions, mostly in the family Buthidae, can be dangerous to humans. Among the most dangerous are Leiurus quinquestriatus, which has the most potent venom in the family, and members of genus Parabuthus, Tityus and Androctonus whose venom is also strong. These scorpions and others in the family Buthidae have been responsible for many deaths, although scorpions are generally unable to deliver enough venom to kill healthy adults; deaths normally occur in the young, elderly or infirm. Unless molested, scorpions are generally harmless and timid, and only make use of the sting for the purpose of killing prey. Generally, they will run from danger, or remain very still. It is unknown whether scorpions are venom conservers, however often the venom is not used unless the prey cannot be subdued with the claws. Many species of scorpions such as Pandinus and Hadogenes rarely, if ever, use their venom, instead relying on their strong bulky pedipalps in prey capture.
Reproduction
Scorpions generally reproduce sexually and all species have male and female individuals. Reproduction is accomplished by the transfer of a spermatophore from the male to the female; scorpions have evolved a complex courtship and mating ritual to effect this transfer.
Mating starts by the male and female locating and identifying each other using a mixture of pheromones and vibronic communication, once they have satisfied each other that they are of opposite sex and of the correct species mating can commence.
The courtship starts with the male grasping the female’s pedipalps with his own; the pair then performs a “dance” called the “promenade à deux”. In reality this is the male leading the female around searching for a suitable place to deposit his spermatophore. The courtship ritual can involve several other behaviours such as juddering and a cheliceral kiss, probably as a meanings of pacifying the female.
When he has identified a suitable location he guides the female over the spermatophore allowing it to enter her genital opercula, this triggers release of the sperm; fertilising the female. The mating process can take from 1 to 25+ hours and depends on the ability of the male to find a suitable place to deposit his spermatophore, if mating goes on for too long females may eventually break off the process.
Once the mating is complete the male and female quickly separate: the male will generally retreat quickly to avoid being cannibalised by the female, although sexual cannibalism is infrequent with scorpions.
Birth and development
Compsobutus werneri female with youngUnlike the majority of Arachnida, scorpions are viviparous. The young are born one by one, and the brood is carried about on its mother's back until the young have under gone at least one moult. Before the first moult scorplings cannot survive naturally without the mother, depending on her for protection and to regulate their moisture levels. In some species the young/mother association can continue for an extended period of time especially in species which display more advanced sociability (e.g Pandinus spp.). The size of the litter depends on the species and can range from 3 to 100+ scorplings .
The young in a general way resemble their parents. Growth is accomplished by periodical shedding of the exoskeleton ( ecdysis). A scorpion's developmental progress is measured in instars (how many moults it has undergone). Generally scorpions require between 5-7 moults to reach maturity. Moulting is effected by means of a split in the old exoskeleton which takes place just below the edge of the carapace (at the front of the prosoma). The scorpion then emerges from this split, the pedipalps and legs are first removed from the old exoskeleton, followed eventually by the metasoma. When it emerges the scorpion’s new exoskeleton is soft, making the scorpion highly vulnerable to attack. The scorpion must constantly stretch while the new exoskeleton hardens to ensure that it can move when the hardening is complete. The process of hardening is called sclerotization. The new exoskeleton does not fluoresce, however as sclerotization occurs the fluorescence gradually returns.
Scorpions have very variable life spans and the actual life span of most species is not known, however the range is approximately 4-25 years (25 years being the maximum reported life span of the species H. arizonensis)
Origins
Scorpion remains are often found in coal deposits that are believed to be of the Carboniferous Period, and no essential structural difference has been discovered between these fossils and existing forms—suggesting that the group has existed without material structural modification for millions of years. These scorpions were believed to be preceded by others, now occurring in marine Silurian deposits, which apparently lived in the sea and exhibit some anatomical differences marking them off as a group distinct from their Carboniferous and recent descendants and attesting affinity with the still earlier marine Arachnida referred to the group Gigantostraca. Their legs were thought to be short, thick, tapering, and ended in a single strong claw, and were well adapted, it seems, like the legs of shore-crabs, for maintaining a secure hold upon rocks or seaweed against the wash of waves. The method of breathing of these ancient types is not certainly known; but probably respiration was effected by means of book gills attached to the ventral plates of the body. At all events no trace of respiratory stigmata has been detected even in well-preserved material. These Silurian scorpions, of which the best-known genus is Palaeophonus, were of small size, only two to five centimeters in length.
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