PRAUNUS FLEXUOSUS

The first and second thoracopods (Thp1 and Thp2) are modified in comparison to the third to eighth pairs (Thp3– 8) of pediform thoracopods. The following descriptions relate only to Thp1, based on a detailed examination of the right Thp1 of five male individuals of P. flexuosus .

Cuticle and skeletal structures of Thp 1 in P. flexuosus

The more or less rectangular protopod (more than twice as wide as long) consists of coxa and basis. The thorax cuticle proximal to the coxa is robust medially, anteriorly and posteriorly, but membranous laterally.

The coxa is formed as a narrow, incomplete proximal ring of moderate robustness, open (i.e. membranous) anteromedially. The ends of the open ring are in contact with the thorax posteromedially and anteriorly, potentially establishing two articulation points, which would form a bicondylar articulation, mostly for adduction/abduction. Anteriorly, the narrow coxa is almost entirely hidden by the thorax. Posteriorly, the coxa forms a transversal invagination from its most medial to its most lateral extent (pCo in Fig. 13B, C), where it forms an articulation with the epipod. Proximally to that articulation, the coxa continues by curving around the lateral side into the membranous area of the thorax and towards anterior. From its most lateral point, a sclerite reaches dorsad and forms another articulation with the epipod. Distal to the posterior invagination of the coxa, a wide membrane (mem in Fig. 13B) connects the coxa to the basis. Posteromedially within this membrane sits a sclerite (presumably a fragment of the coxa), apparently articulating the basis with the thorax (Sc in Fig. 13C).

Muscles are characterized by their proximal and distal attachment sites. Relative size (compared with one another, based on circumference) is specified by one (/ = ‘small’) to three (/// = ‘large’) slashes.

Laterally attached to the coxa, a lamellar epipod is present (Ep in Fig. 13A, B). Dorsally, it forms an articulation via a sclerite with the lateral coxa, while posteriorly, it articulates with the lateral extent of the coxa invagination. An anterior contact point with the lateral basis (lBa in Fig. 13A) might constitute a third articulation point.

The basis appears subdivided by two longitudinal constrictions (from anterior over ventral to posterior) into three distinguishable portions: a medial, a central and a lateral portion. The medial portion (mBa in Fig. 13A–C) is fitted closely to the thoracic sternum (Fig. 13B, C) and carries an endite anteromedially that extends in a distomedial direction (Fig. 13A, B). Distally, the medial portion articulates via an anterior and a posterior articulation point (+ in Fig. 13A, B) with the endopod (i.e. the ischium). The constriction at the transition of the medial to the central portion forms a medial–central apodeme, serving as attachment site for one extrinsic exopod muscle (BEx1). The central and the lateral portions together form the distal (or ventral) wall of the basis (cBa and lBa in Fig. 13A, B). The proximal anterior margin of these portions forms a transversal invagination (aBa in Fig. 13A), serving as attachment site for two muscles (TB3 and BEx3). This invagination is most pronounced at its most medial and most lateral extent, respectively. The posterolateral margin of the lateral portion of the basis extends proximally towards the direction of the coxa, as a narrow cuticular beam; the basis and coxa are, in this area, separated by a wide interpodomeral membrane (Fig. 13B). Anterolaterally, the lateral portion of the basis curves proximolaterally, establishing a contact point (and possibly an articulation) with the epipod (Ep in Fig. 13A).

Distolaterally within the membrane of the lateral portion of the basis, the exopod (Ex in Fig. 13A, B) articulates via an anterior and a posterior hinge point with the basis. The peduncle of the exopod comprises a short stem and a much longer distal portion, followed by a flagellum. The peduncle is broad and anteroposteriorly flattened, with a ventral bulge and a prominent distodorsal pointed protrusion. The peduncle articulates with the flagellum via an anteroventral articulation point. The cuticle of the first annulus is slightly more robust around this joint. All individuals examined exhibited a total of eight annuli (Fig. 13A).

The crescent-shaped endopod consists of the ischium, merus, carpus, propodus and dactylus. Anteriorly, the endopod describes a strong concave curvature (Fig. 13A). The ischium (Is in Fig. 13A, B) is about as long as the proximodistal extent of the basis and carries medially a more or less distinct endite. A bicondylar articulation with the merus is realized by an anteromedial and a lateral articulation point, respectively (+ in Fig. 13A, B).

The merus (Me in Fig. 13A, B) is about as long as the ischium and carries medially a more or less distinct endite. A bicondylar articulation with the carpus is formed by an inwardly folded posterior edge and an anterior articulation point (+ in Fig. 13A, B).

The carpus (Ca in Fig. 13A, B) is about twice as long as the merus. A bicondylar articulation with the propodus is formed by an anterior and a posterior articulation point, respectively (+ in Fig. 13A, B).

The propodus (Pr in Fig. 13A, B) is about half as long as the carpus. Its distal opening is not oriented distally but is ~90° displaced in a medial direction. A bicondylar articulation with the dactylus is formed by a medial and a lateral articulation point, respectively (+ in Fig. 13A, B).

The dactylus is relatively short and rounded (Da in Fig. 13A, B), with its distal setose tip facing a proximomedial direction.