Semicerura Maynard 1951

Semicerura Maynard, 1951

Figs 1–39

Remarks to the genus Semicerura. The genus Semicerura and the species S. bishopi were described from the New York State and were characterized with presence of two rows of spines on dens (vs. four rows in the “austral” genus Procerura Salmon, 1941), 8 +8 ocelli, Abd. V and VI separated, and tridentate mucro (Maynard 1951). Maynard’s diagnosis of the genus was repeated by Martynova (1969) and Christiansen and Bellinger (1998); however, the fusion of two last abdominal segments was pointed out in the latter publication.

After the morphology of three surveyed species the genus is characterized also with the following particular characteristics:

- Sensillar chaetotaxy of Semicerura undergoes moderate reduction and strong differentiation with rather long dorsal (very long in S. draconis sp. nov. and S. goryshini) and short lateral s-chaetae (Figs 4, 20) as compared to most other genera of the subfamily Isotominae. Two short lateral s-chaetae on Abd. V are arranged in longitudinal row. This pattern is remarkable and unknown for the subfamily.

- Labial palp is specific due to 5 proximal chaetae of which 2 chaetae (instead 1) are placed at median line (Fig. 3). For the family, the basal set consists of 3 (Anurophorinae, Börner, 1906) or 4 (Isotominae, Schäffer, 1896) proximal chaetae. Only in hygrophilous genera (Agrenia Börner, 1906, Archisotoma Linnaniemi, 1912, Isotomurus Börner, 1903, Hydroisotoma Stach, 1947) their number strongly increased (>7) and varies within the species (Fjellberg 1999). The similar chaetotaxy of proximal part of labium is known only for the remarkable species Skadisotoma inpericulosa Greenslade & Fjellberg, 2015 and Mucronia fjellbergi Potapov & Babenko, 2014 which have 5 and 6 proximal chaetae, respectively. These two forms possess outstanding morphological features and strongly differ from Semicerura.

- Incomplete number of apical chaetae on tibiotarsi. This character indicates the possible relation to genera Heteroisotoma Stach, 1947, Pseudisotoma Handschin, 1924, Isotoma Bourlet, 1839 and Parisotoma Bagnall, 1940.

Considering the taxonomic value of spines on the dens, the relation of Semicerura and several “spined genera” of the South Hemisphere call for further study. However, morphological data on most “austral” representatives of Isotomidae are insufficient. For now, Semicerura can be reliably compared with the genera Folsomotoma Bagnall, 1949 and Heteroisotoma. Both taxa are better understood and tend to replace the chaetae on the furca with spines, although the presence of spines on posterior side of dens (the generic character of Semicerura) was shown neither for Heteroisotoma nor for Folsomotoma. Semicerura appears to be closer to the Holarctic genus Heteroisotoma by sharing distinct quadruplet of long s-chaetae on Abd. V consisting of two as-s-chaetae (as1 and as2) in anterior position and two accp-s-chaetae (accp1 and accp3) at posterior margin (Ding at al. 2011). In contrast, the “austral” genus Folsomotoma has quadruplet formed by one as-s-chaeta (as1) and three accp-s-chaetae (accp1, accp2 and accp3) (Potapov & Stebaeva 1992; Deharveng 1981; Fanciulli et al. 2018).

All species of Semicerura, including S. bishopi and S. multispinata (James, 1933), appear to have two last abdominal segments fused, unlike given by Maynard (1951) in the first diagnosis of the genus.