Description of Hypogastrura ellisi sp. n. with notes on H. tethyca Ellis and the trybomi group (Collembola, Hypogastruridae)

Abstract A new species, Hypogastrura ellisi, from Iran and Greece is described. It does not fit the definition of any known species group, but, as it has weakly differentiated blunt Ant. IV sensilla, one tenent hair on tibiotarsi, broad empodial lamellae, and dens with fine granulation and seven setae, it can be compared with some members of the trybomi and monticola groups and H. aterrima Yosii, 1972. It seems to be especially close to a representative of the trybomi group, H. tethyca Ellis, 1976. They differ mainly in the shape of apical papillae on the labrum, the size of anal spines, and the presence of setae m2 on Th. II. Notes on H. tethyca, the trybomi group, and a key to the species of the group are given.


Introduction
Hypogastrura Bourlet, 1839 currently comprising 168 species (Bellinger et al. 1996(Bellinger et al. -2017, is the most speciose genus within the family Hypogastruridae. Looking for phylogenetic relationships between them, and for practical reasons, the genus has been divided into some species groups based on morphology (Yosii 1960, Christiansen and Bellinger 1980, Babenko et al. 1994, Skarżyński 2009). Presently, nine groups are used in the taxonomy of the genus: crassaegranulata, manubrialis, monticola, nivicola, packardi, sahlbergi, socialis, trybomi, and viatica. This group system is not complete; numerous species do not belong to any of these groups due to their specific morphology or poor knowledge on their morphology. In the material collected in Iran, several specimens that resemble Hypogastrura tethyca Ellis, 1976, a member of the trybomi group, were found. Studies on the type material of this species made it possible to ascertain that the Iranian specimens represented a new species that does not fit the definition of any known species group. Its description, notes on H. tethyca and the trybomi group as a whole, as well as a key to all known species of the group are given below.

Materials and methods
Specimens of Hypogastrura ellisi sp. n. were cleared in Nesbitt's fluid (Wang et al. 2003), subsequently mounted on slides in Swan's medium (Swan 1936) and studied using a Nikon Eclipse E600 phase contrast microscope. Figures were drawn with the camera lucida. Photographs were made using a camera Nikon D5100 mounted on a microscope mentioned above. Photographs were stacked using Helicon Focus 6.7.1. and prepared for publication using Adobe Photoshop CS6.
Abbreviations used:

Hypogastrura ellisi
Description. Body length 1.1-1.6 mm. Habitus typical of the genus. Color in alcohol pale brown dorsally and yellowish ventrally, eye-patches dark. Granulation fine and uniform, 12-20 granules between setae p 1 on Abd. V.
Chaetotaxy of head typical of the genus, with complete set of v-setae ( Fig. 1). Setae slightly differentiated in length, especially on last abdominal segments, smooth and rather thick and stiff. Body sensilla (s) about 2-3 times longer than ordinary setae, fine and smooth. Dorsal chaetotaxy of Th. I-III and Abd. III-VI as in Figs 1-3. Th. I with 3 + 3 setae. Th. II with setae m 2 absent, m 3 present or absent and m 4, m 6 present. Th. III with setae m 2 and m 3 absent and setae m 4 and m 6 present. Abd. IV with setae p 3 present, p 7 absent and increased number of m-setae. On Abd. V setae p 2 present and m-setae absent. Subcoxae I, II, III with 1, 3, 3 setae respectively. Microsensillum on Th. II present.
Remarks. It is difficult to find a right place for H. ellisi sp. n. within the genus. This species does not key to any of the groups in Skarżyński (2009). However, having weakly differentiated blunt Ant. IV sensilla, one tenent hair on the tibiotarsi, broad empodial lamellae, dens without tooth-like granules and ventro-apical swelling and a mucro without a distinct subapical tooth, it can be compared with some representatives of the trybomi or monticola groups as well as H. aterrima Yosii, 1972, which has an isolated position within the genus.
Undoubtedly, H. ellisi sp. n. is the most similar to H. tethyca, considered as a member of the trybomi group. Most noticeably they differ in the shape of labral apical papillae (H. ellisi sp. n. -convex, strong, well visible, Figs 9-10; H. tethyca -flat, delicate, hardly visible, Figs 11-13). Apart from this H. ellisi sp. n. lacks setae m 2 on Th. II (present in H. tethyca) and possesses smaller anal spines (the ratio of anal spine + basal papilla/inner edge of claws III 0.5-0.6 in H. ellisi sp. n. vs 0.75-1.1 in H. tethyca), 5-10 short pointed setae in the ventral file on Ant. IV (Fig. 5) (H. tethyca -approx. ten short and stiff sensilla, truncate at apex, Fig. 14), and a mucro with a relatively high outer lamella (both inner and outer lamellae are low in H. tethyca).
Both species have similar general ranges of distribution (H. ellisi sp. n.: Iran, Greece -Lesbos; H. tethyca: Greece -Crete, Lesbos and Azerbaijan), but on a local scale they co-occur only in Lesbos. In Ellis's material from this island, numerous H. ellisi sp. n. from two localities and rare H. tethyca individuals from five sites were found. Nevertheless, they were isolated spatially. Unfortunately, due to incomplete collecting data, we do not know whether these populations differ in habitat preferences.
The new species is easy to distinguish from the members of the monticola group by the absence of m-setae on Abd. V (vs present) and the size of the postantennal organ, which is equal to, or slightly larger than, the neighboring ocellus (vs 1.5-2 times larger than ocellus). H. aterrima can also be easily separated from H. ellisi sp. n. due to tridentate retinaculum (vs quadridentate), minute anal spines, slightly larger than surrounding granules (vs large, the ratio of anal spine + basal papilla/inner edge of claws III 0.5-0.6 in H. ellisi sp. n.), the presence of setae m 2 on Th. II, and the absence of setae m 6 on Th. II-III.   (20978, 20979, 20980), 7 males (20983, 20984, 20985, 20986, 20987, 20988, 20989) Note. The description (Ellis 1976) and redescription (Babenko et al. 1994) of H. tethyca are highly informative; however, the examination of the types and other material allow us to add some more details. Hypogastrura tethyca has 10-18 granules between setae p 1 on Abd. V, Ant. IV with three lateral and one dorsal long thin and curved blunt sensilla (sometimes 1-2 more in the dorsal group, longer and less curved, in arrangement as in H. ellisi sp. n., Fig. 4) and approximately ten short stiff sensilla truncate at the apex in the ventral file (Fig. 14), the postantennal organ 1.0-1.5 (usually 1.2) as long as the nearest ocellus, the labrum with delicate flat and hardly visible apical papilla (Figs 11-13), a maxillary outer lobe with two sublobal hairs, and large anal spines on high papillae (the ratio anal spine + basal papilla/inner edge of claws III 0.75-1.1). Ellis (1976), looking for H. tethyca affinities, pointed out difficulties with its placement within the genus and compared it with a wide spectrum of species, including H. monticola Stach, 1946, H. aterrima, andalso H. trybomi (Schött, 1893). Then, Babenko et al. (1994), based on specimens from Azerbaijan, put this species into the trybomi group. Although this concept seems well justified, H. tethyca, having a labrum with delicate apical papillae and tibiotarsi with long and clavate tenent hair, occupies a rather isolated position within the group. Undoubtedly, further research is needed to establish its relationships. The characteristics of the trybomi group and a key to the known species of the group are given below. H. tethyca is also similar to H. ellisi sp. n. They differ in the characters mentioned above.
These species have fine cuticular granulation of the body (7-18 cuticular granules between setae p 1 on Abd. V), long and thin blunt Ant. IV sensilla arranged in two groups: 2-3 lateral and 1-8 dorsal (often difficult to distinguish from ordinary setae), a labrum without distinct apical papillae, a postantennal organ from slightly smaller to slightly larger than the neighboring ocellus, one usually short pointed tenent hair on the tibiotarsi (only in H. tethyca clavate), a broad basal empodial lamella, a quadridentate retinaculum, dens with 6-7 setae and without tooth-like granules and ventro-apical swelling, a mucro without distinct subapical tooth, setae m 6 on Th. II-III present, m-setae on Abd. V absent, and usually 4 + 4 setae on the ventral tube (only in H. trybomi 7-9 + 7-9). Moreover, some of them have the head of the maxilla with prolonged lamellae, a maxillary outer lobe with only one sublobal hair, and Ant. IV with a developed ventral file of sensilla. Members of this group differ in the characters summarized in Table 1 and a key.
Considering their morphology, one can conclude that three species: H. tethyca, H. trybomi, and H. hargrovei, occupy rather isolated positions. Especially the first one due to the reasons mentioned above, the second one because of the ventral tube with numerous setae and tibiotarsi with relatively long, but pointed, tenent hair (judging from fig. 111 in Fjellberg (1985) and fig. 17.5 in Babenko et al. (1994)), and the third one due to highly modified mouthparts: the labrum elongated, the head of the maxilla with only two teeth and prolonged lamellae, the labial palp without papilla C, with hypostomal setae set on a narrow long projection, and a weakly developed ventral file of Ant. IV sensilla.