Sternum (scorpions)

Euscorpius avcii , female: above the sternum, in the middle the genital perculum, below the crest organ

Euscorpius avcii , male: the sternum is located between the coxae of the rear pairs of legs

The sternum (plural sterna, ancient Greek στέρνον sternon , breast ') is the breast plate of the scorpion's exoskeleton . In all recent scorpions, it is located on the underside of the prosoma between the coxes of the two rear pairs of legs. There is a great variety of shapes and locations in fossil scorpions relative to coxae. From the middle of the 19th to the beginning of the 21st century, the outer shape of the sterna was considered an important taxonomic feature for distinguishing the families of the scorpions.

Location and construction

All scorpions have a sternum. It is a part of the exoskeleton that in all recent scorpions is located under the prosoma between the coxae of the rear pairs of legs. In front of him lie the coxes of the second pair of legs, which meet in the middle of the prosoma, behind him the genital opculum . The sternum is firmly connected to the surrounding coxae and connected to the genital opculum by an interstitial membrane. While the position of the sternum relative to the coxes of the legs is uniform in recent scorpions, fossil scorpions show a very large variability in this regard.

In recent scorpions, the sternum is pentagonal, triangular or bar-shaped when viewed from the outside. The most common form is the pentagon, it occurs in all families, but only rarely in the Buthidae and Bothriuridae . The Buthiden usually have a triangular sternum, while the bar-shaped sternum occurs only in the Bothriuridae family.

The evolutionary origin of the sternum has not yet been conclusively clarified. It is generally believed that the scorpion's sternum was formed by a fusion of the sternites of the seventh and eighth somits. An alternative hypothesis assumes a fusion of appendages of the opisthosoma, which are homologous with the chilaria of the Xiphosura . Embryological examinations indicate an emergence from the first segment of the opisthosoma.

Types

The observation that the first stage of juvenile scorpions always has a pentagonal sternum led to the hypothesis that the pentagonal sternum is the most original shape. As early as 1916, Alexander Petrunkewitsch recognized that the bar-shaped sternum, which is often divided vertically in the middle and gives the impression of two separate segments, is actually a folded pentagonal sternum. The triangular sternum is also a pentagonal sternum, the two front sides of which are greatly shortened, giving the impression of a triangle.

In 2003 the American arachnologists Michael E. Soleglad and Victor Fet published a detailed study of the sterna of all recent families of the scorpions, taking into account 89 genera. Her analysis was the first to take into account not only the external appearance but also the internal structure of the sterna. For the first time, Soleglad and Fet provided detailed explanations of their development from the pentagonal sterna for the triangular and bar-shaped sterna. In doing so, they traced these and numerous intermediate forms back to varying degrees of vertical or horizontal compression.

Type 1

The type 1 sternum is pentagonal when viewed from the outside, with shapes ranging from an equilateral pentagon to an apparently almost triangular sternum with greatly shortened anterior sides. In the back of the sternum there is always a depression that never divides the base of the sternum in two. The recess is closed at the rear by a bulge that can be covered by the front edge of the genital opculum. The size and shape of the sterna can vary greatly. In the center of the sternum there may be a concavely arched area starting from the depression at the base, which sometimes merges forward into a median furrow. The anterior area of ​​the sternum can also be designed very differently. Sometimes there is a slight depression or a slight, sharply demarcated, round elevation that can be partially covered by the slightly overlapping coxae of the second pair of legs.

Looking at the inside of the sternum always shows a three-dimensional structure as a continuation of the depression at the rear edge, which extends obliquely backwards into the body cavity. It can be very different in size and shape, from a barely perceptible slight elevation to an appendage that protrudes into the body cavity behind the base of the sternum. The New World scorpions have particularly pronounced structures, while those of the Old World regularly remain smaller. The anterior and lateral edges of the sternum are folded inward and their edges protrude slightly into the body cavity. The sternum of type 1 is regarded by Soleglad and Fet as a primitive form, which occurs in addition to the fossil scorpions of the family Palaeopisthacanthidae in the recent families Pseudochactidae , Buthidae , Microcharmidae and Chaerilidae .

Type 2

Notched sternum of type 2 in tityus apozonalli

The type 2 sternum always has a notched posterior margin. Convex lateral lobes extend rearward on either side of the notch. At the anterior end of the sternum there is always a depression that varies in depth depending on the shape and size of the lateral lobes. It can be completely or partially covered by the coxes of the second pair of legs. The proportions are very variable, from a greatly elongated sternum to the short and very wide one that was formerly known as bar-shaped.

The inner side of the sternum shows on both sides of the notch two protruding structures that extend backwards into the body cavity and that have their starting point at the inner edges of the lateral lobes. These processes can also vary greatly, from barely recognizable bulges along the inner edges of the lateral lobes to extensive three-dimensional structures that extend far forward, backward to beyond the base of the sternum and laterally far towards the edges of the sternum . As with the Type 1 sternum, the anterior and lateral edges of the sternum are folded inward. Soleglad and Fet hypothesize that all more highly developed scorpions with type C orthobothriotaxia have a type 2 sternum.

Taxonomic meaning

The shape of the sterna was first used in 1861 by the German naturalist Wilhelm Peters as a taxonomic feature to differentiate the families of the scorpions from one another. Until then, the number of lateral ocelles was considered to be the most important characteristic, but it can vary even within a species. Peter's findings were an almost undisputed basis for the higher systematics of the scorpions into the 21st century. Taxa were repeatedly classified into certain families, although this was only justified by the shape of the sterna, and numerous other morphological features suggested other relationships.

In 2003, Soleglad and Fet determined in the course of their investigation of the sterna of the recent scorpions that the interpretation of the shape of the sterna, which has been common since the middle of the 19th century, as an essential taxonomic characteristic, does not have a sufficient basis. They developed, limited to the partial order Orthosterni with all recent scorpions and the fossil family Palaeopisthacanthidae , a system that takes into account both the external and the internal structure of the sternum.

literature

• 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 .
• 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 .
• Michael E. Soleglad and Victor Fet: The scorpion sternum: structure and phylogeny (Scorpiones: Orthosterni) . In: Euscorpius 2003, Volume 5, pp. 1–34, Online PDFhttp: //vorlage_digitalisat.test/1%3Dhttp%3A%2F%2Fwww.science.marshall.edu%2Ffet%2Feuscorpius%2Fp_2003_05.pdf~GB%3D~IA%3D~MDZ%3D%0A~SZ%3D~ double-sided% 3D ~ LT% 3DOnline% 20PDF ~ PUR% 3D , 5.5 MB.

Individual evidence

1. ↑ ^{a b c} John T. Hjelle: Anatomy and Morphology , p. 12.
2. ↑ ^{a b c d} Michael E. Soleglad and Victor Fet: The scorpion sternum: structure and phylogeny , pp. 1-2.
3. ↑ W. David Sissom: Systematics, biogeography, and paleontology , S. 142nd
4. ↑ ^{a b} Michael E. Soleglad and Victor Fet: The scorpion sternum: structure and phylogeny , pp. 2-3.
5. ↑ W. David Sissom: Systematics, biogeography, and paleontology , S. 66th
6. ↑ ^{a b} Alexander I. Petrunkewitsch: The Shape of the Sternum in Scorpions as a Systematic and a Phylogenetic Character . In: The American Naturalist 1916, Volume 50, No. 598, pp. 600-608, JSTOR 2456162 .
7. ↑ Alexander I. Petrunkewitsch: Paleozoic and Mesozoic Arachnida of Europe . In: Memoirs of the Geological Society of America 1953, Volume 53, pp. 1–128, here p. 39, doi : 10.1130 / MEM53 .
8. ↑ Michael E. Soleglad and Victor Fet: The scorpion sternum: structure and phylogeny , p. 3.
9. ↑ Michael E. Soleglad and Victor Fet: The scorpion sternum: structure and phylogeny , pp. 22-24.
10. ↑ ^{a b c d} Michael E. Soleglad and Victor Fet: The scorpion sternum: structure and phylogeny , p. 9.
11. ↑ ^{a b c d} Michael E. Soleglad and Victor Fet: The scorpion sternum: structure and phylogeny , p. 19.
12. ↑ Wilhelm Peters: About a new division of scorpions and about the species of scorpions he collected in Mossambique, from which an extract is given here. In: Monthly reports of the Royal Prussian Academy of Sciences in Berlin 1861, first half, pp. 507-516, here pp. 509-513, digitizedhttp: //vorlage_digitalisat.test/1%3D~GB%3D~IA%3Dmonatsberichtede18611knig~MDZ%3D%0A~SZ%3Dn539~doppelseiten%3Dja~LT%3D~PUR%3D
13. ↑ Michael E. Soleglad and Victor Fet: The scorpion sternum: structure and phylogeny , p. 28.