Pauropygus, Potapov, Mikhail, Gao, Yan & Deharveng, Louis, 2013

Potapov, Mikhail, Gao, Yan & Deharveng, Louis, 2013, Taxonomy of the Cryptopygus complex. I. Pauropygus - a new worldwide littoral genus (Collembola, Isotomidae), ZooKeys 304, pp. 1-16 : 1-3

Pauropygus gen. n.

Type species:

Pauropygus projectus sp. n.

Diagnosis.

Blind small-sized Anurophorinae with two last abdominal segment fused, modified mouthparts including remarkably modified pleural fold, and first segments of antenna set together on frontal part of head.

Description.

Without pigment and eyes, Abd.V and VI fused. Body size small, with primary granulation only. Antennal bases set close together on frontal side of the head, almost touching each other (Figs 1, 2). Sensilla on three first antennal segments thickened. Sensilla of Ant.IV moderately thickened. Maxillary palp simple, with 3 sublobal hairs set together (Fig. 3). Pleural fold with two high projections (Figs 3, 20, 23). Basolateral field of the labium with 6 chaetae (Fig. 20). Labium with three papilla projected forward, number of guards not reduced. Papillate sensilla reduced in size. Labrum swollen in central part, labral chaetae set on wide papilla. Two prelabral chaetae. Maxillary head with four enlarged lamellae, three of them ciliated; claw reduced to small finger-like process, not dentate. Mandible head reduced and thin, molar plate with 2 strong basal teeth. Tergal sensilla on abdomen situated in p-row of chaetae, their number reduced (2-3,0-1/0-1,0-1,1-2,1-2,1-3, depending on species), number of microsensilla 00/100 or 00/000 (Figs 8, 15, 21). Th.I-III without ventral chaetae. Body macrochaetae differentiated. Tibiotarsal tenent chaetae present (1-2-2), not clavate. VT with 4+4 laterodistal chaetae. Tenaculum with 4+4 teeth. Furca slender, manubrium with a pair of chaetae on anterior side, dens with crenulation and wide swelling on posterior side, mucro bidentate.

Name derivation.

The name is derived from the Greek suffix -pygus which points to the fusion of the abdominal segments and from the Greek prefix pauro- which refers to the reduced chaetotaxy, particularly reduced number of sensilla on the body.

Representatives.

Apart from the type species Pauropygus projectus sp. n., the new genusincludes Cryptopygus caussaneli Thibaud, 1996 and Pauropygus pacificus sp. n.

Affinity.

The new genus is established mainly due to the unusual position of antennae on head and the strongly modified mouthparts. In mouthparts, the V-shaped pleural folds and the presence of 6 chaetae on basolateral fields of the labium are especially important; these characters were never seen in the family before, except for the latter one that was mentioned for part of the Algerian population of Isotominella geophila sensu Jordana et al., 2009; normally, the pleural fold looks like a weak swelling proximal to the maxillary outer lobe, and basolateral field of mouth cone has 5 chaetae in species of the Isotomidae family ( Fjellberg 1984, 1999). Other modifications of mouthparts, like swollen labrum, unequal labial papilla, reduced plate of outer lobe of maxilla, are more common features. The sixth chaeta of basolateral field has unclear derivation; since it has weaker socket than the other five, it is probably one of the sublobal hairs which has migrated from the sublobal plate to more posterior area of head and grouped together with basolateral chaetae. Together with two finger-like extensions of pleural folds this chaeta probably makes lateral parts of head more functionally important. The projected position of antennae and modified mouthparts are probably adaptations to an interstitial life between small sand grains on the beach and to feeding on particles suspended in water. Projections on different parts of body are well known among species living in contact with salt water in genera Archisotoma Linnaniemi, 1912, Anuridella Willem, 1906, Xenylla Tullberg, 1869, Hypogastrura Bourlet, 1839, Friesea Dalla Torre, 1895, and others.

As an unusual feature for the group, the species of the new genus show considerable reduction of sensillar chaetom. In Pauropygus projectus sp. n. all medial sensilla on Th.II-Abd.IV and lateral sensilla on Th.III-Abd.II are lost, while in Pauropygus caussaneli and Pauropygus pacificus sp. n. it is the posterior and lateral parts of body that lost sensilla. In spite of differences between number of sensilla on body (2,0/0,0,1,1,3) and (3,1/1,1,2,2,3) among Pauropygus species, the general pattern of their distribution and differentiation is kept.

Pauropygus is closely related to Isotominella Delamare Deboutteville, 1948 after the redescription of Jordana et al. (2009). The identity of the type specimens of Isotominella geophila Delamare Deboutteville, 1948 (Ivory Coast) which were not seen by Jordana and specimens from Algeria on which the redefinition was based remains somewhat doubtful. The two genera share simple maxillary palp, two prelabral chaetae, posterior position of sensilla on tergites, and general appearance of furca. Other shared characters (blindness, absence of foil chaetae, sensillar equipment of antennae, microsensillar set 10/100) are less significant. Apart from the two characters mentioned above, Pauropygus sp. n. differs from Isotominella in more differentiated tibiotarsal chaetotaxy (presence of tenent and spiny chaetae), shape of PAO (flat and broad vs. oval), number of sublobal hairs and e-guards (3 vs. 2, and 7 vs. 5, respectively). Isotominella also has a rather common set of sensilla on body (33/22223) while it is reduced in Pauropygus . At last, the new genus is strictly restricted to seashore sands, while Isotominella geophila prefers soil. We also examined specimens of Isotominella geophila from Algeria kindly provided for us by Jordana and surprisingly concluded that females and males also differ in antennae on head which are positioned almost like in Pauropygus in males and set apart in females. The crenulation of basal part of dens was stressed by both Delamare Debouteville (1948) and Jordana et al. (2009) as one, if not the main diagnostic characters of the genus Isotominella . We consider that this character is of low taxonomical value since it strongly depends on mounting of the animal on slide. We have also seen other specimens of Isotominella from Eurasia (Ukraine and China). They did not show the dimorphism of Algerian populations and represent at least one more species of the genus. The comparison of Pauropygus gen. n. with other genera of the Cryptopygus complex is given below.