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Dorsal scutum

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Dorsal scuta by Martens 1978

Scheme of dorsal scuta by Martens (1978) [1]

The dorsal scutum or dorsal shield is the single plate formed by the fusion of some or all the tergites.

Types of dorsal scutumEdit

Hadži (1942)[2], in a section called "Ist der Rückenschild (Scutum) oder sind die Einzelplättchen Primärbildungen?" provided a phylogenetic discussion on the different degrees of fusion of the tergites in Opiliones and created a terminology for them.

Scutum completumEdit

Hadži defined this (which he also called "Großschild") as a shield with preserved boundary lines between the 1st to 9th tergites, which are adjacent and firmly fused with each other. He also indicated that this is present in what he called "more primitive Opiliones", the Cyphophthalmi, and among the Laniatores, the Oncopodidae, but never in the Palpatores.

Scutum magnumEdit

Hadži defined this (which he also called "komplex Schild") as a shield formed by the fusion of the carapace with the 1st to 5th abdominal tergites.

Scutum parvumEdit

Carapace and abdominal scutum separated as two individual shields. Example are some Ischyropsalis.

Scutum laminatumEdit

Abdominal scutum subdivided into individual plates. Some Ischyropsalis.

Scutum tenueEdit

Abdominal scutum much reduced and soft, but shield not individualized into tergites.

Hadži exemplified further reduction of scuta:

  • In Nemastoma karamani Hadzi, an extremely specialized cave-dweller, one can no longer talk of the carapace or scutum; the entire body is soft and pasty.
  • Something similar can be seen in the family Travuniidae under the Laniatores, but in which the most extreme kind: Abasola hofferi Silhavy despite the extreme thinness of the cuticle was able to preserve the appearance of a shield.

Scutum complexumEdit

Defined by Kratochvíl as the fusion of carapace with 1st to 7th abdominal tergites.

Kratochvíl (1958)[3] added one more type of scutum, and discussed the difference between this and Hadži's scutum completum (actually he called both two stages of the "scutum complexum") [translation AB Kury]: Although in the family Oncopodidae the union of the first to ninth tergites with the carapace to a common "big shield" Scutum complexum is known as primary formation, but this association shows a completely different character than the Paralolidae. In the Oncopodiden each abdominal tergite is clearly marked and divided into two parts in the median-sagittal plane. A deep incision is located between the cephalothorax and the abdomen, so that by which the carapace and the abdominal scutum are to be distinguished. In the Paralolidae all traces of an association of the carapace with the abdominal tergites and all furrows between the first to fifth tergites disappeared. A short transverse furrow is present only between the 5th to 6th tergites and a slightly longer between the sixth and seventh tergites.

Particularities of scutum in DyspnoiEdit

As commented by Hadži:

The scutum of Trogulus is sui-generis both in its development and its furrows. The fusion of the partial plates, particularly the carapace with the scutum remains imperfect and occurs very late. In Nemastoma the scutum develops first as an entire very thin shield, then sclertotization progresses from individual centers.

Martens classificationEdit

Martens (1978)[1] in an important compilation book, which also contained a wealth of new information, presented a slightly modified version of Hadži's terminology and gave more extensive examples. He explained that tergites IX and X do not make part of scutum.

Scutum completumEdit

Scutum of cephalothorax (carapace) fused with abdominal scutum. Abdominal scutum formed by tergites I to VIII. Cyphophthalmi and Oncopodidae .

Scutum magnumEdit

Carapace fused with abdominal tergites I to V. Laniatores except Oncopodidae, Trogulidae , Dicranolasmatidae , Nemastomatidae , (except Ortholasma and in part Dendrolasma ) and except Hesperonemastoma .

Scutum parvumEdit

Carapace and abdominal scutum independent from each other. 

  • Carapace  either bears sclerites of segments I to VI (almost all Ischyropsalididae) or

Scutum tenueEdit

Organization of sclerites as in scutum parvum, plates thin, coriaceus, sometimes barely recognizable. Widespread in Phalangioidea, especially Gagrellinae .

Scutum laminatumEdit

Carapace separated from abdominal sclertites I to VI. These are independent from each other. Some Ischyropsalididae and Sabaconidae .

Scutum intermediumEdit

Similar to scutum laminatum. Sclerites I to V always separated from carapace, but variedly fused among them. Present only in some Ischyropsalididae.

Scutum dissectumEdit

Individual sclerites of scutum laminatum subdivided, forming a transverse row of small sclerites on each segment. Present in some species of Taracus and Sabacon.

Scutum compositumEdit

During the adult phase sclerotized Intersegmental membrane present between the abdominal segments V / VI and VI / VII, adding the sclerites VI and VII to the abdominal plate (segments I to V). Contrary to all other scutal formations, that are already finished by the adult molt, the S. compositum is formed only after the adult molt. Present in some genera of Nemastomatidae.

Phylogenetic interpretations of scutum developmentEdit

As commented by Hadži:

  • 1) The harvestmen probably originally had separate tergite plates, throughout areas of the opisthosoma. Among extant species, as it seems, there are no more in this condition.
  • 2) Then a fusion of individual tergite plates came about forming single shields, whether a more extensive complex (at least more than five anterior plates) are connected to the front carapace creating the full shield (Scutum completum) or a smaller unit (front 5 tergites + carapace) or large shield (Scutum magnum) finally, probably secondary, it came to the separation of the carapace from the head shield (scutum parvum). Of these, the scutum magnum is the most common.
  • 3) The third step was a strong tendency to reduce the shield, particularly strongly and earlier in females. This reduction was and is gradually in several stages and several ways: 
  • a) by separating the carapace from scutum;
  • b) by restriction to the five anterior abdominal segments;
  • c) by disintegration of the single small shield in secondary independent individual platelets (so far as is known, this happened in the Ischyropsalididae), whereby the Scutum laminatum arose, a condition that differs in principle from that of the original free tergites;
  • d) by a reduction of the shield as a whole, by way of softening the cuticle to form special microsculpture (scutum tenue).

See alsoEdit

Limbo posterior. Mesotergum.

ReferencesEdit

  1. 1.0 1.1 Martens, J. (1978b) Spinnentiere, Arachnida: Weberknechte, Opiliones. Die Tierwelt Deutschlands. Vol. 64. G. Fischer Verlag, Jena, 464 pp.
  2. Hadži, J. (1942) Raziskovanja o ishiropsalidih (Opiliones) [Untersuchungen über die Ischyropsaliden (Opiliones)]. Razprave Matematicno-Prirodoslovnega Razprave Akademija Znanosti in Umnetnosti, Ljubljana, 21, 5–114 (in Slovenian with German summary).
  3. Kratochvíl, J. (1958a) Die Höhlenweberknechte Bulgariens (Cyphophthalmi und Laniatores) [Jeskynní sekáči Bulharska (Cyphophthalmi a Laniatores)]. Práce Brněnské základny Československé akademie věd (= Acta Academiae Scientiarum Čechoslovenicae Basis Brunensis), Brno, 30(9), 371–396.

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