Telson (scorpions)

Telson of Euscorpius avcii , male above, female below

Metasoma of an emperor scorpion with the membrane between the 4th and 5th segment ( A ), 5th segment ( B ), setae ( C ), anus ( D ), telson ( E ), poisonous bladder ( F ) and poisonous sting ( G )

The telson (from ancient Greek τέλσον telson , German 'border' ) is the last part of the scorpion's body. It has several peculiarities compared to the telson of other arthropods , including that of all other recent jaw-claw carriers , and consists of the poisonous bladder ( vesicle ) with the poisonous glands inside and the poisonous sting ( aculeus ).

External structure

From right: last segment of the metasoma, poisonous bladder with subacular tubercle and poisonous sting with poisonous drops

The telson is the last section of the scorpion's metasoma and is not considered a segment (although there are researchers who believe that the scorpion's telson is actually a merger of the original telson with an abdominal segment). Deviating from the blueprint of the other arthropods , in which the last section of the body with the anus is called the telson , the anus of the scorpions is located ventrally at the end of the fifth and last segment of the metasoma, immediately in front of the telson. The telson consists of the poison bladder with the two poison glands, muscles and sensory organs and the poison sting .

In many scorpions there is a lump or thorn-shaped process, the subacular tubercle , ventrally at the transition from the poisonous bladder to the sting. It can, where present, be an important taxonomic trait. The absence of the subacular tubercle distinguished the Scorpionidae family from the Diplocentridae family, which was repealed in 2003 by the arachnologists Michael E. Soleglad and Victor Fet . When measuring the lengths of the telson or venomous sting, measurements are taken from the subacular tubercle. Length specifications for the metasoma apply to its five segments and never include the telson.

Like almost all organs of the scorpions, the telson can also be designed differently in the sexes. In most species, the poisonous bladders of the female scorpion are larger and more rounded. The opposite is the case with the genera Anuroctonus and Euscorpius . In Anuroctonus phaiodactylus , male scorpions have a thickening at the base of the venomous spine, and in the genus Hemiscorpius the male telson is elongated and divided into two parts.

In many cases, the external appearance of the Telsons is influenced by adaptations to the way of life of a species of scorpion. For example, some psammophilic species, such as Vejovoidus longiunguis , have streamlined metasomes, including the telson, that help them move about in the sand. Species that dig burrows and lurk for prey at their entrances often have powerful chelae , which are more often used than the poison sting when catching prey and defending against predators . Their telson is therefore often reduced in size, as in the species of the genera Heterometrus and Opistophthalmus in the family Scorpionidae . In contrast, species of the Buthidae family , in which the poison sting is often used, often have a particularly large telson.

internal structure

Historical representation of the anatomy of the scorpions: Metasoma, fourth and fifth segment and telson from the side ( 2 ), cross section of the telson with the two poison glands and surrounding muscles ( 6 ); Longitudinal section of the telson ( 5 ).

The inside of the telson is occupied by two poison glands , which are located to the left and right of the body axis. They are surrounded by muscles in the middle and at the top and separated from one another by a layer of muscle. A hose-shaped exit leads from each of the poison glands to the poison sting. When the scorpion venom is used, the muscles contract, the venom glands are pressed against the exoskeleton and the venom reaches the sting through the tubes. Several species of the genus Parabuthus are able to spray a mist or jet of their poison up to half a meter high and a meter wide.

The poison glands are a sac-like structure made of a membrane with a layer of connective tissue inside. The actual glandular tissue is a monolayered glandular epithelium , which in most cases is folded several times and to different degrees in the different families of the scorpions. It partially fills the cavity of the gland. In the part of the poison glands that is not covered by the epithelium, the poison produced is held ready for use.

There are three methods used to extract scorpion poisons for the production of antivenins . The macerating separated Telsone in water or saline solutions resulting contaminated strong, because in addition to the poison other substances are dissolved. Nevertheless, the method is used to obtain sufficiently large quantities of injectable poison quickly in the manufacture of antivenins. The manual compression of the poisonous bladders is extremely time-consuming, but results in up to ten times more poison than electrostimulation. With electrical stimulation, an electric current is applied to the telson, which releases the poison. Here, too, the poison can be contaminated by other tissue fluids. In addition, after a few poison withdrawals, a habituation effect occurs, so that the stimulation of the muscles with electricity remains ineffective. Nevertheless, electrical stimulation is the predominantly used method today.

Sensory and nerves

The telson, including the venomous sting, is a highly developed organ with strong sensory abilities. Compared to other parts of the scorpion's body, it is densely covered with sensory hair and has cleft sensory organs . While the sensory hairs at the end of the poison sting are missing, there are also cleft sensory organs and sensors retracted into cup-like depressions.

In the telson and the fifth segment of the metasoma there are receptors that respond to light stimuli. In addition, it could be proven that the sensitivity to heat stimuli is particularly high on the pedipalps and on the telson. However, it is unclear whether there are special thermosensors or whether scorpions have internal heat receptors throughout the body. The telson lacks receptors for determining its own movement; these are recorded by the corresponding receptors in the last segment of the metasoma.

The telson is controlled by a double nerve cord that originates in an abdominal ganglion of the fourth metasomal segment. It is functionally connected to other nerve cords that control the movements of the pedipalps and legs to coordinate attack or defense movements.

Above the muscles surrounding the venom glands is a structure called the dorsal gland, the function of which is unknown. It was found to resemble the ganglia of the nervous system and no further research has been done on it.

evolution

Last segment of the metasoma , poisonous bladder and venomous sting of Gondwanascorpio emzantsiensis

In addition to the scorpions, only two families of sea scorpions , the Carcinosomatidae and the Mixopteridae, had a metasoma with five segments and a telson . It is not known whether these fossil groups had poison glands. The telson of these sea scorpions is seen as an expression of convergent evolution , so that the telson of the scorpions represents an independent development. During organogenesis , the segments of the metasoma and the telson emerge together from a rudimentary tail segment.

Individual evidence

↑ Hannes F. Paulus (2004): Some about the tribal history of the arachnids (Arthropoda, Chelicerata). Denisia 12: 547-574.
↑ ^a ^b ^c ^d ^e ^f ^g John T. Hjelle: Anatomy and Morphology. In: Gary A. Polis (Ed.): The biology of scorpions. Stanford University Press, Stanford 1990, pp. 9-63, ISBN 0-8047-1249-2
↑ ^a ^b ^c W. David Sissom: Systematics, biogeography, and paleontology. In: Gary A. Polis (Ed.): The biology of scorpions. Stanford University Press, Stanford 1990, pp. 64-160, ISBN 0-8047-1249-2
^ ^A ^b ^c W. David Sissom, Gary A. Polis, and Dean D. Watt: Field and Laboratory Methods. In: Gary A. Polis (Ed.): The biology of scorpions. Stanford University Press, Stanford 1990, pp. 445-461, ISBN 0-8047-1249-2 .
^ ^A ^b Gary A. Polis and W. David Sissom: Life History. In: Gary A. Polis (Ed.): The biology of scorpions. Stanford University Press, Stanford 1990, pp. 161-223, ISBN 0-8047-1249-2 .
^ Gary A. Polis : Ecology. In: Gary A. Polis (Ed.): The biology of scorpions. Stanford University Press, Stanford 1990, pp. 247-293, ISBN 0-8047-1249-2 .
↑ Jean Joyeux-Laffuie: Appareil et venimeux venin du Scorpion . In: Archives de zoologie expérimentale et générale 1883, Volume 11, 733–783, Plate XXX, digitized .
^ Herbert L. Stahnke : The Genus Centruroides (Buthidae) and Its Venom . In: Sergio Bettini (Ed.): Arthropod Venoms (= Handbook of Experimental Pharmacology, Volume 48). Springer, Berlin, Heidelberg, New York 1978, pp. 277-307, ISBN 978-3-642-45503-2 .
^ Rainer Foelix, Bruno Erb and Matt Braunwalder: Fine structure of the stinger (aculeus) in Euscorpius . In: Journal of Arachnology 2014, Volume 42, No. 1, pp. 119-122, doi : 10.1636 / B13-64.1 .
^ ^A ^b Thomas M. Root: Neurobiology . In: Gary A. Polis (Ed.): The biology of scorpions . Stanford University Press, Stanford 1990, pp. 341-413, ISBN 0-8047-1249-2 .

[Paulus-1] Hannes F. Paulus (2004): Some about the tribal history of the arachnids (Arthropoda, Chelicerata). Denisia 12: 547-574.

[Hjelle-2] ↑ ^a ^b ^c ^d ^e ^f ^g John T. Hjelle: Anatomy and Morphology. In: Gary A. Polis (Ed.): The biology of scorpions. Stanford University Press, Stanford 1990, pp. 9-63, ISBN 0-8047-1249-2

[Sissom-3] W. David Sissom: Systematics, biogeography, and paleontology. In: Gary A. Polis (Ed.): The biology of scorpions. Stanford University Press, Stanford 1990, pp. 64-160, ISBN 0-8047-1249-2

[SissomPolisWatt-4] A ^b ^c W. David Sissom, Gary A. Polis, and Dean D. Watt: Field and Laboratory Methods. In: Gary A. Polis (Ed.): The biology of scorpions. Stanford University Press, Stanford 1990, pp. 445-461, ISBN 0-8047-1249-2 .

[PolisSissom-5] A ^b Gary A. Polis and W. David Sissom: Life History. In: Gary A. Polis (Ed.): The biology of scorpions. Stanford University Press, Stanford 1990, pp. 161-223, ISBN 0-8047-1249-2 .

[Polis-6] Gary A. Polis : Ecology. In: Gary A. Polis (Ed.): The biology of scorpions. Stanford University Press, Stanford 1990, pp. 247-293, ISBN 0-8047-1249-2 .

[Joyeux-Laffuie-7] Jean Joyeux-Laffuie: Appareil et venimeux venin du Scorpion . In: Archives de zoologie expérimentale et générale 1883, Volume 11, 733–783, Plate XXX, digitized .

[Stahnke-8] Herbert L. Stahnke : The Genus Centruroides (Buthidae) and Its Venom . In: Sergio Bettini (Ed.): Arthropod Venoms (= Handbook of Experimental Pharmacology, Volume 48). Springer, Berlin, Heidelberg, New York 1978, pp. 277-307, ISBN 978-3-642-45503-2 .

[Foelix-9] Rainer Foelix, Bruno Erb and Matt Braunwalder: Fine structure of the stinger (aculeus) in Euscorpius . In: Journal of Arachnology 2014, Volume 42, No. 1, pp. 119-122, doi : 10.1636 / B13-64.1 .

[Root-10] A ^b Thomas M. Root: Neurobiology . In: Gary A. Polis (Ed.): The biology of scorpions . Stanford University Press, Stanford 1990, pp. 341-413, ISBN 0-8047-1249-2 .