Heteroisotoma Stach, 1947

Jie, Ding, Potapov, Mikhail & Sokolova, Elena, 2011, Further study on the labial palp in the Isotomidae (Collembola) with reference to the genus Heteroisotoma Stach, Zootaxa 2991, pp. 49-61 : 50

Genus Heteroisotoma Stach, 1947

Type species: Isotoma andrei Mills, 1934

Diagnosis. White or with rare scattered pigmentation, with 0 + 0 to 4 + 4 ocelli. PAO boat-shaped: elongate, strongly protruding, with pointed ends, in extreme case subdivided into lobes. Maxillary outer lobe with trifurcate palp and 4 sublobal hairs. Labial palp with all papilla (A-E), 16 (more rarely 15) guards, and four processes: lateral one, as common for Collembola , medial at papilla A, and processes B' and D' on inner side of papillae B and D, respectively. The development of two last processes varies depending on species. Maxilla without expanded and elongated lamellae. Tibiotarsi without clavate tenent hairs, with 8–9 setae in distal whorl. Ventral tube with at least 30 setae. Tenaculum with more than 5 setae. Furca long, reaching ventral tube. Distal part of manubrium with 1 + 1 to 4 + 4 ventroapical spines or spiniform setae (excluding H. jacutica ). Dens with or without spines along inner side. Body macrosetae long. Tergal sensilla usually well differentiated, thin, often elongated on the last abdominal segments. Abd. V with 4 sensilla in characteristic position as 2 as-sensilla in front and 2 medial accp-sensilla in posterior position. Setal glands, in understanding of Deharveng (1981), present under manubrial and dental spines. For synonyms of the genus see Potapov and Stebaeva (1992).

Labial palp. The main part of labial palp, if not reduced, is strikingly similar in different families of Collembola ( Fjellberg 1999) and consists of several setae, five papillae (A–E) each armed with terminal sensillum and several guard setae ("guards"), and a lateral process (lost in few families). In Isotomidae , the palp is the most complete and the least differentiated. In the family at whole, the labial palp varies depending on species and genus and appears to be of great taxonomical value ( Fjellberg 1999). Unlike in other subfamilies, species of the subfamily Isotominae , however, show invariable labial palp having full set of labial papillae, 15–16 "guards", and a characteristic finger-like lateral process (with the genus Metisotoma Maynard as an exception).

In the genus Heteroisotoma , however, the labial palp undergoes the multiplication of un-armed (i.e. those which lacking guards) processes. It bears totally 4 such processes ( Figs 1–5 , 33 ): one common finger-like lateral process, one medial process of the same shape, and two more broad ones B' and D'. Such a structure has not before been noted before in Isotomidae indicating the monophyletic origin of the genus Heteroisotoma . Length and width of the processes vary depending on species. Heteroisotoma sinorossica sp. nov. has smaller B' and D' processes, so does H. stebaevae with B' (but more material is wanted to confirm the intraspecifiс variability of the character). The un-armed additional process on labial palp is known only in Desoria trispinata (MacGillivray) , which has socalled "a particular lobe" at the base of papilla D (Fig. 58 in Fjellberg 1999). The highly evolved labium of Archisotoma Linnaniemi (subfamily Anurophorinae s.l.) has numerous additional papillae but each is armed with associated guard (b1–4, d1–4, e1–6).

Dental spines. The generic position of the two new species described in the present paper is somewhat uncertain. Due to the presence of spines on dens H. sinorossica sp. nov. and H. heteronomica sp. nov. may formally belong to the genus Semicerura for which the modern generic diagnosis is unknown. Type species, Semicerura bishopi Maynard ( USA), has strong papillate spines on dens and a full set of ocelli. In the most characters, two new species resemble members of the genus Heteroisotoma Stach according to Potapov and Stebaeva (1992). They share the shape of PAO, general pattern of sensillar chaetotaxy of last abdominal segments, spines on manubrium, tridentate mucro, and complete or partial loss of ocelli. Because S. bishopi was not redescribed using modern taxonomical characters we place two our species to the genus Heteroisotoma . The labial palp of S. bishopi is unknown but two species studied by us in Eastern Asia, S. goryshini and S. sp. cf. bishopi , have neither medial process nor lobes B' and D' (see Fig. 36 for S. goryshini ). In addition, Semicerura has the two last abdominal segments fused (vs. separated in Heteroisotoma ), a simple PAO (vs. boat-shaped), and roundish (vs. sharp) apical labral ridges. A tridentate mucro, reduced apical whorl of setae on tibiotarsus (8–10 in Semicerura ), and spines on dens and manubrium, however, are shared characters of these genera. So far, Isotoma s.str, Pseudisotoma Handschin , Semicerura , Heteroisotoma , and possibly also Folsomotoma Bagnall form a natural group in Isotominae with a reduced whorl of distal setae on tibiotarsi and an independent tendency to form spines on different parts of furca. The taxonomical validity of Semicerura and Heteroisotoma needs further study, also their relation with "spined" southern taxa ( Greenslade 1989) remains even less understood. Misturasotoma Bernard & Christiansen, 2010, a large-sized blind genus from North America, formally resembles Heteroisotoma . The former genus, however, has 11 apical setae on tibiotarsi, quadridentate mucro, dense cover of equally short sensilla on two last abdominal segments, and have no additional processes on labium, that indicate a similarity with Desoria Nicolet and its allies.

Figures

FIGURES 1 – 5. Labial palp in Heteroisotoma spp.: 1, H. carpenteri; 2, H. sinorossica sp. nov.; 3, H. dilatosetosa; 4, H. stebajevae; 5, H. jacutica; A, B, C, D, E: papillae, m. p., l. p.: medial and lateral processes, B', D': additional processes, e 7: guard e 7, prx 1: proximal seta 1.

FIGURES 31 – 36. Heteroisotoma heteronomica sp. nov. (31 – 35) and Semicerura goryshini (36): 31, dorsal side of manubrium and proximal part of dens (not all spines shown on dens); 32, tip of antenna; 33, labial palp (partly, guards not shown); 34, postantennal organ, lateral view; 35, proximal part of dens; 36, labial palp (area associated with medial process marked); sm: subapical microsensillum, org: subapical organite, B', D': additional processes, m. p.: medial process.