The addition of segments during the postembryonic ontogenesis of the Actinotrichida (Acarida) and its importance for the recog-nition of the primary subdivision of the body and the original segmentation
1963 - Volume: 5 Issue: 3 pages: 443-454
evolution and genetics
1. In the present paper it is demonstrated that the body of the Actinotrichid mites obviously must be regarded as primarily subdivided into proterosoma and opisthosoma, and not into proterosoma and hysterosoma. It is suggested to restrict the use of the terms gnathosoma and podosoma to practical purposes only, and to evitate at least the use of the term hysterosoma because of its having a misleading theoretical base. The name dorso-sejugal for the furrow separating prosoma and opisthosoma (in its present sense) is also misleading. It is suggested here to replace it by disjugal, which name can eventually be extended to the postpedal furrow. 2. Apparently, the number of segments constituting the Actinotrichid body, is 17 at the most (rarely met with; only present in three families of primitive Oribatid mites, and two families of Endeostigmata), whilst 16 is the normal number of adult Oribatid mites; 14 appears to be the number in Tydeus (and probably in many other Prostigmata). In these numbers the precheliceral segment is not included. 3. The genital orifice of the Actinotrichida is longitudinal, and apparently extends over three segments, viz., the nos. VIII-X. It is pointed out that it is extremely important to make a further study of the order in which the genital segments appear during ontogenesis. Sorne suggestions are made as to the relation of the genital hairs to the segments. 4. It appears that the ptychoid condition of part of the Oribatid mites, is the result of a mobility of the VIIth segment, the same which in other Arachnida provides of the movable juncture between prosoma and opisthosoma. The articulation can be named disjugal, in contradistinction to the so-called protero-hysterosomatic which is a sejugal articulation.
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