Proteonematalycus wagneri Kethley reveals where the opisthosoma begins in acariform mites.

Samuel J Bolton,Michael Scott Brewer

doi:10.1371/journal.pone.0264358

Abstract

It is generally thought that the anterior border of the opisthosoma of acariform mites is delineated by the disjugal furrow, but there is no evidence to support this other than the superficial appearance of tagmosis in some oribatids. It is proposed herein that the disjugal furrow is an apomorphic feature that does not correspond with any segmental borders. Although the disjugal furrow is absent from Proteonematalycus wagneri Kethley, the visible body segments of this species indicate that this furrow, when present, intersects the metapodosoma. Therefore, the disjugal furrow does not delineate the anterior border of the opisthosoma. Instead, this border is between segments D and E (segments VI and VII for all arachnids). This hypothesis can be accommodated by a new model in which the proterosoma warps upwards relative to the main body axis. This model, which is applicable to all Acariformes, if not all arachnids, explains the following phenomena: 1) the location of the gnathosomal neuromeres within the idiosoma; 2) the relatively posterior position of the paired eyes; 3) the shape of the synganglion; 4) the uneven distribution of legs in most species of acariform mites with elongate bodies.

Highlights

Mites comprise two superorders: Parasitiformes and Acariformes
scanning electron microscope (SEM) reveals that each hysterosomal segment is clearly delimited by intersegmental furrows, which fall on either side of a transverse row of setae
By revealing furrows that correspond with the borders of all the hysterosomal segments, SEM removes any remaining ambiguity concerning the homology of the furrows with segmental borders

Summary

Introduction

Almost all mites have lost the furrows that delineate their body segments. The dorsal setae of acariform mites are arranged in transverse rows, which correspond with the underlying body segments [1]. There are conflicting interpretations concerning the homology of two of these segments, C and D (bearing setae c and d). Van der Hammen [4] instead homologized segments C and D with the first two segments of the opisthosoma (segments VII and VIII) (Fig 1C and 1D), which was based on his hypothesis that the anterior border of the opisthosoma is represented by the disjugal furrow (Fig 1E). If not most, acarologists accord with van der Hammen’s hypothesis by referring to the structures associated with segments C and D (principally setae and plates) as ‘opisthosomal’.

Methods

Results

Conclusion