Polyxenus aff. lagurus Linnaeus, 1758

Polyxenus aff. lagurus Linnaeus, 1758 View in CoL

( Figures. 1A–C View Fig , 2 View Fig ).

Material examined: Adult female (SMF Be 1855a), in Baltic amber. Te amber is stored in the SMF amber collection.

Description:

General features: Five ommatidia on each side of the head, close to the ommatidia are three trichobothria, arranged in a triangle. Te closest trichobothrium to the posterior vertex is immediately adjacent to the last seta and smaller than the other two. Posterior vertex with two groups of setae, each group with 18 setae in two rows; the six most distal ones of the anterior row closely arranged. Lateral protuberances each with three setae. Approximately 25± 2 ornamental setae arranged into a fan at the terminal end. Tergites with two rows of setae posteriorly connecting with rosettes of setae laterally, however that is not the case for the terminal tergite, bearing the caudal bundles.

Measurements: Body length of ~ 2.6 mm without the caudal bundles, with caudal bundles ~ 3.5 mm. Head measuring 310 µm, collum measuring 140 µm. Tergite width measurements from the most anterior to posterior, 300 µm, 250 µm, 260 µm, 300 µm, 300 µm, 340 µm, 325 µm, 310 µm and 155 µm. Te length of the legs are as follows: 475 µm, 545 µm, 585 µm, 575 µm, 580 µm, 580 µm, 585 µm, 620 µm, 585 µm, 610 µm, 615 µm, 650 µm and 600 µm.

Head ( Fig. 2A, E, F View Fig ): Five ommatidia are located on each side of the head, with three positioned laterally. On the head are inserted two well separated tufts of 18 setae each on the posterior vertex, each group are made of an anterior row of 12 setae and posterior row of six setae. The six anterior distal setae are inserted closely to one another. A single pair of setae are present medially, posterior to the posterior vertex. Three trichobothria are located on each side of the head (not visible on the right side, as the specimen is badly preserved), and are arranged into a triangle with the small trichobothrium closely inserted to the most distal setae of the posterior vertex. The two remaining trichobothria are inserted on large cylindrical funiculi, slightly elevated from other setae insertions ( Fig. 2A View Fig ). The lack of details due to the preservation of both the labrum and clypeo-labrum limit the visibility of setae and papillae. Gnathochilarium with lateral palps 160 µm long, with at least 13 sensilla. The proportions of the antennae are as follows, from proximal to distal elements: 92 µm, 45 µm, 35 µm, 60 µm, 56 µm, 78 µm, 52 µm and 16 µm ( Fig. 1G View Fig ). The preservation does not allow for the observation of the fine antennal sensilla. A possible single sensillum is visible on the last antennal element, but not depicted here due to uncertainty.

Trunk ( Figs. 1A, B View Fig ; 2A, B View Fig ): Te collum has two rows of inserted setae (anterior/posterior), and an additional two rows of setae are inserted laterally (i.e., rosette, Fig. 1A View Fig ); in total 49 setae are inserted on the collum. Te lateral protuberances on either side of the collum with three setae each. Te insertions of the setae and spine on the legs are as shown in Fig. 1H View Fig . Due to the poor preservation and slight decay of the cuticle, round protrusions are found along the legs of this specimen (see Suppl. Figure 1 View Fig ).

Terminal end ( Figs. 1A, B View Fig ; 2C, D View Fig ): A relatively small number of ornamental setae are forming the fan; between 25–27 setae ( Fig. 2F View Fig ) are present dorso-medial to the widely separated caudal bundles.

Remarks:

Several extinct species of Polyxenus have been already diagnosed by Koch and Berendt (1854) from fossil material, namely P. ovalis , P. conformis , P. caudatus , P. colurus Menge, 1854 and P. lophurus Menge, 1854 . From these, Polyxenus aff. lagurus when compared to the diagnosis of P. conformis is similar in body length, but the proper measure is questionable as Koch and Berendt (1854) used the “Line”. Te “Line” is an old unit of measurement based on a certain proportion of an inch; however, it varies considerably between countries. If we refer to the Anglo-Saxon value P. conformis measures ~ 3.1 mm in length. Te length including the caudal bundles is not available, as in the original diagnosis their specimen was lacking complete bundles ( cf. p. 11 Koch & Berendt, 1854). Proper comparison is also very limited with these historical species as the descriptions lack important characteristics such as the number of ommatidia, the setae arrangement, shape of the terminal hooked setae, just to cite a few important diagnostic features (see “Discussion”).