Appendisotoma bisetosa Martynova, 1970

Figs 1–15, 61

Material. Winter form: Holotype of Appendisotoma bisetosa. Russia, Orenburg Region, Novosergievsky District, Barabanovka, ~ N 52.21°, E 53.48°, on snow, 24.11.1965, collection of Zoological Institute, St.-Petersburg.

Voronez Region, Khopyor Nature Reserve, ~ N 51.2°, E 41.7°, on snow in mass, 20– 30.01.1990, leg. E.Timonov.

Penza Region, Zemetchinsky District, Chernoyar, ~ N 53.81°, E 42.24°, 20– 28.02.2009, on snow, leg. Y.Shveenkova.

* after Fjellberg (2007) and our own material from European part of Russia.

** data of Fjellberg (1999, 2007) and our materials differs: proximal field of labium with 3 (Fjellberg 1999) or 4 (our data) chaetae, Ant. I with ca.14 common chaetae after our data (vs. 20 after Fjellberg 2007).

Orenburg Region, Abdulino, ~ N 53.67°, E 53.66°, on snow, 22.11.1967.

Summer form: Russia, Orenburg Region, Orenburg–Orsk highway, 69 km, N 51.648°, E 55.970°, arable land, rotten straw, 18.04.2023, leg. M.P. and N. Kuznetsova.

Orenburg Region, near Kuvandyk, Ibragimovo, N 51.534°, E 57.446°, soil and turf in steppe, 22.04.2023, leg. M.P. and N. Kuznetsova.

Bashkiria, Orenburg – Ufa highway, Staraya Otrada, N 52.558°, E 55.842°, small wood, litter, 19.04.2023, leg. M.P. and N. Kuznetsova.

Volgograd Region, Pallasovsky District, near El'ton Lake, right bank of Khara River, ~ 3.5 km upstream from the mouth (Malus, Rosa, Prunus), 19.04.2007, leg. M.P.

Penza Region, Privolzskaya Lesostep NR (Kuncherovskaya Steppe cluster), ~ N 52.69°, E 46.28°, steppe, different dates (June: 6.06.2000, 4.06.2001, 9.06.2001; September: 27.09. 2000, 18.09. 2001, October: 11.10. 2000), leg. Y. Shveenkova.

Description. Body size 1.0– 1.3 mm. Corpus slender (Fig. 8). Dark purple including appendages. Abd. V separated from Abd. IV and fused with Abd.VI. Cuticle with orthogonal-hexagonal primary granulation. Ocelli 8+8 (Figs 11, 15). Ratio PAO length: ocellus = 1.3–2.9 (affected by cyclomorphosis). PAO 0.4–0.6 as long as width of Ant. I and much shorter than inner unguis length (0.6–0.9). Maxillary head with unmodified lamellae. Maxillary outer lobe with 4 sublobal hairs, maxillary palp simple. Labral formula as 4/5,5,4. Labium with 5 usual papillae (А–Е) and labial formula A1B4C0D4E6 (15 guards at whole), guard chaetae e7 absent, 3 proximal, 4 basomedian and 5 basolateral chaetae. Ventral side of head with 4(5)+4(5) chaetae. Ant. I with 13–15 common chaetae (11 of basal set and 2–4 additional ones), 2 s-chaetae and 2 bms-chaetae, 1 dorsal and 1 ventral. Ant. II with 1 latero-distal s and 5 bms (2 dorsal set together, 2 ventral and 1 additional). Additional bms larger and denoted as "?bms" on Fig. 5. Ant.III with 1 bms and 6 distal s (including 2 lateral), without additional s-chaetae (Fig. 5). Proximal s of lateral group shorter than outer s of AO. S-chaetae on Ant.IV weakly differentiated. Organite small, micro s-chaeta of common shape. Males with spurs on Ant.II and III.

Common chaetae smooth. S-formula as 3,3/2,2,2,4,4 (s), 1,0/0,0,0 (ms) (Figs 1,2). Tergal s-chaetae short (Fig. 2). All s-chaetae on Abd.I and IV and medial s-chaetae on Abd.II and Abd.III situated in p-row, lateral s-chaetae on Abd.II and Abd.III in more anterior position. All s-chaetae on Abd. V set in one posterior transversal row (Fig. 1). Macrochaetae 2,2/3,3,3 in number, medial macrochaetae on Th.II-Abd.II shorter. Macrochaetae on Abd.V 1.4–2.1 times longer than dens (affected by cyclomorphosis) and 2.6–3.8 times longer than mucro. Foil chaetae at the tip of abdomen absent. Axial chaetotaxy of Th.II–Abd.III as 6–7(8),5(4,6)/3,3–4,3–4,5–6. Axial group of Abd.III often with 3+3 paired and an unpaired chaeta in anterior position. Thoracic segments without ventral chaetae.

Unguis with lateral and weak inner teeth (Figs 3, 4). Empodial appendage 0.5–0.7 as long as unguis. Tibiotarsi with 4 T-chaetae (11 chaetae in apical whorl). Tibiotarsi I and II usually with 1–2 additional chaetae (21+4T+1–2 = totally 25–26 chaetae), tibiotarsi III with a few additional chaetae (>26 chaetae at whole). Adult males without differentiated spurs on Leg III. Tibiotarsal tenent hairs pointed or with blunt tips, 0.6–1.2 as long as length of inner edge of unguis (affected by cyclomorphosis). Ventral tube with 4+4 (rarely 5+5) laterodistal and 4–6 posterior chaetae (usually 2 in distal transversal row and 3 proximal), anteriorly without chaetae. Tenaculum with 4+4 (sometimes 3+ 3 in winter form) teeth and 2 chaeta. Anterior furcal subcoxa with 7–9, posterior one with 7–8 chaetae. Anterior side of manubrium with 1+1 chaetae (Figs 6, 7), posterior side with 3+3 laterobasal and many chaetae on main part, usually with 2+2 (rarely 1+1 or 3+3) latero-central chaetae (Fig. 6). Dens stout, almost smooth or with notches on posterior side (affected by cyclomorphosis), with 3 (rarely 2 on one side) rigid and short anterior chaetae (Figs 6, 7). Posterior side with 5 chaetae, one of which sometimes lost. External lobe near apex of dens present or absent (cyclomorphosis). Mucro with 4 teeth. Ratio manubrium: dens: mucro = 3.1–5.7: 1.4–2.5: 1 (affected by cyclomorphosis).

Males present, without any specific modification in reproductive stage.

Cyclomorphosis. Winter (W) specimens differ from summer (S) ones by the presence of external lobe on dens, more compact mucro, longer tenent hairs on tibiotarsi, and more developed cornea of ocelli (Figs 8–15). Basal rods of furcal apparatus are more chitinized, and dens is thicker and it has posterior notches in winter form. Paired chitinized processes are usually seen at base of manubrium (denoted as ch.p. in Fig. 12). We have not recorded small specimens of winter form, its size varies between 0.8 and 1.2 mm (Fig. 63, upper row of observations)

The following morphological ratios are affected by cyclomorphosis:

- PAO length: ocellus = 1.3–1.6(W) vs. 1.8–2.9(S) (ocelli larger in W)

- Mac Abd.V: dens = 1.4–1.7(W) vs. 1.7–2.1(S) (dens longer in W)

- Tibia t.h.: inner edge of U = 0.9–1.2(W) vs. 0.6–0.8(S) (tibiotarsal tenent hairs longer in W)

- Man: dens: mucro = 4.9–5.7: 1.9–2.5: 1(W) vs. 3.1–3.9: 1.4–1.8: 1(S) (mucro shorter in W)

Remarks. Appendisotoma bisetosa is a well defined species due to short furca with few anterior chaetae. Complete apical whorl of tibiotarsi (11 chaetae) is shared with many species of Appendisotoma. See also the Discussion part to A. monomorpha sp. nov.

Distribution and ecology. The species is distributed in eastern part of steppe zone of the European Plain (Fig. 63). It prefers different open habitats, mostly steppes, and it is often observed on snow in mass occurrence. Because of this A. bisetosa is usually seen and called "snow flea" by people.

Life cycle. We have no representative winter data on age classes based on funnel extraction. Body size of specimens active on snow, which can be only a part of a over-wintering population, is shown in Fig. 63 (observations in November, January and February). They belong to a large although not reproducing preadult "winter" form. In spring they moult to the summer form and April population consequently consists of largest individuals which reproduce (Fig. 63, April). A considerable part is represented by males with fully developed ejaculatory duct and females with open genital aperture. In June instars with different age occur, with predomination of early stages. Middle-age individuals predominate in September. Presence of early age instars in September indicates either late summer reproduction (although we have not recorded the reproducing individuals) or belated hatching of diapausing eggs. The latter phenomenon is known for Collembola (Leinaas & Bleken 1983). Based on our data, number of generations in this species is uncertain. Like the most univoltine species, A. bisetosa actively reproduce in spring although its age class distribution in summer doesn't show two distinct generations as known for Desoria active on snow: D. hiemalis, D. blekeni and D. olivacea (Fjellberg 1975 a, 1975b; Leinaas 1980). So far, there are two studies describing the annual dynamics of of Appendisotoma. Tamura et al. (1969) showed that A. mitra Uchida & Tamura, 1968, the species with apical lobe on dens, was strictly a winter animal in south Hokkaido (Japan) and "disappeared" in warm period of the year. Hart (1979) investigated A. vesiculata (Folsom, 1937) in Indiana (U.S.A.) and presented the life history data which suggested that the apical lobe was only observed during October, November and December, while all individuals collected starting from March were without the lobe. The data of J. Hart correspond to our observations on A. bisetosa.