Scutisotoma titusi ( Folsom, 1937 )

Potapov, Mikhail, Babenko, Anatoly & Fjellberg, Arne, 2006, Taxonomy of the Proisotoma complex. Redefinition of genera and description of new species of Scutisotoma and Weberacantha (Collembola, Isotomidae), Zootaxa 1382 (1), pp. 1-74 : 17-19

Scutisotoma titusi ( Folsom, 1937)

Figs. 31–35

Proisotoma (Proisotoma) titusi Folsom, 1937: 49

Material studied. 3 exx. originally labeled as cotypes (slides, sex unknown), one of them was selected as lectotype, others as paralectotypes from “ Utah, E.G.Titus ” ( MNHN). 2 ♂♂, USA, Canby , Modoc Co., California, on ground, 04.v.1957, leg. H.R. Lathau. 1♀, 1♂, USA, Ravendale , Lassen Co., California, on rock in meadow, 18.ii.1970, leg. F.L. Evans, det. R.F. Wilkey (K. Christiansen coll.) .

Redescription. Size 1.5–1.8 mm. Colour dark purple with many pale spots. Antennae, legs, and manubrium pale purple ( Christiansen & Bellinger 1998). Cuticle with visible fine granulation, strongly rugose on Abd.V. Ocelli 8+8, subequal, G and H only slightly smaller. PAO elliptical, not constricted, twice as long as diameter of nearest ocellus and 0.6–0.8 as long as inner length of unguis. Maxillary outer lobe with 4 sublobal hairs, maxillary palp bifurcate. Labral formula 4/554. Labium with 5 papillae (A–E) and a full set of guards (a 1, b 1 –b 4, d 1– d 4, e 1– e 7). Some of the e-guards (not e 7) may be lost on one side. Proximal chaetae 3. Basomedial field (submentum) with 4 (rarely 5) chaetae, basolateral field (mentum) usually with 5 chaetae. Ventral side of a head usually with 4+4 postlabial chaetae. Ant.1 with 1 dorsobasal and 1 ventrobasal microsensillum (bms) and 2 ventral sensilla (s). Ant.2 with 3 bms and one laterodistal s. Ant.3 with 1 bms and 6 distal s (including two lateral s, clearly differing in size). Both males and females have additional sensilla in the middle part of Ant.3. Sensilla on Ant.4 poorly differentiated, subapical organite minute.

All abdominal tergites clearly separated. Tergal sensilla well differentiated, thinner and shorter than ordinary chaetae. Apart from the common accp-sensilla, each tergite has some additional sensilla. The number of sensilla is not constant and usually asymmetrical. Sensillary formula 4–6,4–5/2–3,3–5,3–6,7–8,8–12 (s) and 11/111 (ms). Two variants of their distribution are shown on Figs. 31, 32 . On Abd.I–III the lateral accp-sensilla set well in front of the p-row (1/3–1/2 of tergal length). Macrochaetae hardly differentiated, not even on Abd.V (0.17–0.27 as long as tergite). Axial chaetom of Th.II–Abd.III as 9–10,8–9/6,7,7–8. Thorax without ventral chaetae.

Unguis long and narrow, with small inner tooth. Upper subcoxa of Leg 3 with a single chaeta. Tibiotarsi 1–3 with additional proximal chaetae (with more than 21-21-25 chaetae), B-row complete, T-chaetae absent. Tibiotarsal tenent chaetae clavate (1-2-2), about as long as inner edge of unguis. Chaetae x and B 5 on Ti. 3 in males clearly thinner and shorter than other chaetae. Ventral tube on each side with 5–7 laterodistal chaetae, posterior chaetae 5. Tenaculum with 4+4 (rarely 3+3) teeth and one chaeta. Anterior furcal subcoxae with 18–22 chaetae, posterior ones with 10–13. Anterior side of manubrium with a pair of distal chaetae, its posterior side with numerous setae. Laterobasal lobes have more than 5+5 chaetae. Dens with 22–24 anterior chaetae extending almost to base, only the basal 1/10 free of chaetae ( Fig. 33 ). Posterior side of dens crenulated, with 14–15 chaetae, of which 6–7 are in the basal group, usually 3 (2–4) in the outer row and 4 in inner row. In addition a subapical chaeta is present near mucro ( Fig. 35 ). Mucro strong, lamellate, with 3 subequal teeth ( Fig.34 ). Ratio of manubrium: dens: mucro as 6–9: 6–8: 1.

Taxonomic remarks. In the most recent description of the species Christiansen & Bellinger (1998) give a lower number of anterior chaetae on dens (16–18) compared to our observations (22–24). They also give a wider range of posterior chaetae (9–15). The latter is probably connected with the presence of several related North American species with an identical modification of the Abd.V cuticle. We have seen an undescribed species in material kindly sent to us by Prof. K. Christiansen (leg. T.R. Haig). That species differs from typical S. titusi by a shorter furca with 6–7 anterior and 8–9 posterior chaetae on dens and a lower number of macrosensilla on tergites (4,3–4/2,2,2,2–3,4–7).

Scutisotoma titusi differs significantly from most known species of the genus by an increased number of sensilla on tergites. The same characteristic was found in Rhodanella minos (Denis) [ Côte d'Ivoire, leg. C. Girard (MNHN)] which differs from Scutisotoma in other characters.

Distribution. The species has been recorded in several western states in the USA (California, Idaho, Utah).

Figures

FIGURES 31–35. Scutisotoma titusi. 31–32, general distribution of sensilla and microsensilla (two different specimens), 33, dens, anterior side; 34, mucro; 35, dens, posterior side.