New species of Homidia (Collembola, Entomobryidae) from eastern China with description of the first instar larvae

Abstract Morphology of the first instar larvae of Collembola has considerably taxonomical and phylogenetic significance. We describe the first instar larvae for the first time in Homidia. External morphology of first instar larvae and adults of Homidia jordanai sp. n. is described based on observations under light and scanning electron microscopes. Most organs of adults bear considerably more setae than the first instar larvae; in addition, first instar larval Homidia lack labial seta R, seta on tenaculum, mucronal spine, and dental spines. The new species is characterized by weakly pigmented body, long antennae subequal to body in length, 1+1 inner macrochaetae on Abd. III, few inner macrochaetae on posterior Abd. IV, and spiny and short seta pi on dental base. Differences between new species and other two similar ones, taxonomical significance of the first instar larvae and the position of Homidia are also discussed.

Keywords new species, first instar, scanning electron microscope, chaetotaxy, China introduction In epimetabolic Collembola, the number of setae and pigmentation often change after moult (Christiansen 1992). Morphology of the first instar larvae has considerably taxonomical and phylogenetic significance (Szeptycki 1979), but it has been rarely studied. Homidia is the most widespread entomobryid genus in East Asia, particularly Japan, Korea and China. The genus is characterized by presence of spines on inner edge of dens, "eyebrow" macrochaetae on anterior part of Abd. IV of adults, ommatidia 8+8, mucro bidentate with subapical tooth much larger than the apical one, and absence of body scales. The chaetotaxy of first instar larvae (primary chaetotaxy) in Homidia has never been described.
In the present paper, the primary chaetotaxy of Homidia was studied for the first time based on H. jordanai sp. n., from East China. Many other morphological details are showed under scanning electron microscope.

Material and methods
Alcohol preserved young and adult specimens, were cleared in lactic acid, mounted on microscope slides in Marc André II solution, and studied using Leica DM2500 and Nikon 80i microscopes. Few specimens, coated with platimum under vacuum conditions, were observed under scanning electron microscope. Photographs were taken with Leica AL2 and Nikon SMZ1000 microscopes using a mounted Nikon DS-Fi1 camera and Hitachi S4800 scanning electron microscope, numbers and letters added with photoshop CS2 (Adobe Inc.), all length data measured with Nikon NIS-Elements Documentation 3.1. First instar larvae were determined by the stability of primary chaetotaxy (e.g. 19/18 common setae on meso-/metathorax, Szeptycki 1979). Cephalic dorsal chaetotaxy is designated after Jordana and Baquero (2005) and Soto-Adames (2008), interocular setae after Mari-Mutt (1979, 1986, labial palp setae after Fjellberg (1998), labial setae after Gisin (1964), dorsal chaetotaxy after Szeptycki (1979).
The first instar larva. Size. Body length up to 0.7 mm. Habitus. Ground colour pale white in alcohol. Eye patches dark. Distal antennae slightly pigmented (Figs 3, 4).
Remarks. The new species is characterized by weak pigment on dorsal body, long antennae subequal to body in length, labial basal seta L 1 ciliate, absence of setae m2i2 on Th. II, a1 on Abd. I, a2 on Abd. III, A4-5 on Abd. IV, and dental basal seta pi spiny and shorter than bs. It is similar to Homidia unichaeta Pan, Shi & Zhang, 2010    and Homidia tibetensis Chen & Zhong, 1998 in colour pattern, cephalic chaetotaxy, labrum, coxal formula, chaetotaxy of Abd. II and claw. However, it can be distinguished from them by the length of antennae, labial setal formula, chaetotaxy on Th. II, Abd. I, Abd. III-IV and seta pi on basal dens. Additional differences are listed in Table 1. Differences between the first instar larvae and adults. Some characters are principally same in the first instar larvae and adults: ommatidia, interocular setae, Ant. III organ, apical bulb on Ant. IV, labrum and labral papillae, labial palp, maxillary outer lobe, claw, bothriotricha and s-chaetoxic pattern on terga.
Characters that develop after the first instar: s on distal part of Ant. II, smooth setae on base of Ant. II, labial seta R, smooth spiny setae on trochanteral organ, mac on anterior face of ventral tube, seta on corpus of tenaculum, pseudopores on manubral plaque, basal spine on mucro and genital plate.
Chaetotaxy become more complicated during postembryonal development, detailed differences between the first instar larvae and adults (apart from chaetotaxy of body tergites) are listed in Table 2. Tergal chaetotaxy of adults becomes much more complicated than that of primary chaetotaxy. In addition to numerous secondary common mic and mac on terga, some primary mic are transformed into mac: m2, m5 and p4 on Th. II, a1, m5, p5 and p6 on Th. III, a2, a3 and a5 on Abd. I, a2 and a3 on Abd. II, am6 and p6 on Abd. III, A6 and B4 on Abd. IV (homology of lateral setae difficult to determine).

Taxonomical significance of the first instar larvae and the position of the genus Homidia
The adult chaetotaxy of Entomobryidae exhibits marked differences among genera or species. Szeptycki (1972) found that the primary chaetotaxy of Entomobryomorpha was almost identical in number and position. Later, he (1979) studied the ontogeny of tergal chaetotaxy of the representative Entomobryidae genera and its preliminary phylogenetic significance at higher level of hierarchy with four subfamilies included in Entomobryidae. Subsequent authors (Rusek 2002;Deharveng 2004) emphasized again the systematic significance of the first instar larval chaetotaxy at higher level.
We compare the primary dorsal chaetotaxy of the body among H. jordanai sp. n. and another five species of family Entomobryidae. The morphology of some primary homologous setae under Szeptycki's nomenclature exhibits stable differentiation (mac, mic or absent) at the first instar (Table 3), though number and position of them are apparently similar. Among six species, the primary tergal chaetotaxy of H. jordanai sp. n. is closest to Entomobryoides myrmecophila, almost identical in number and arrangement except s-chaetae on Abd. IV. Integrating its adult features, such as  Th. II  m1  mac  mac  mic  mac  mac  mac  m2  mic  mac  mic  mic  scale  mac  p5  mac  mac  mic  mic  mac  mac  Th. III  a2  mac  mic  mic  mic  mic  mac  a3  mac  mic  mic  mic  mic  mic  a4  mac  mac  mic  mic  mac  mac  m1  mic  mic  mic  mic  -mic  m2 - elongated Abd. IV, four antennal segments and absence of scales, the genus Homidia apparently belongs Entomobryini sensu . It differs from Orchesellinae in 4 antennal segments (5-6 in the latter), elongated Abd. IV, as well as more primary setae on Abd. IV. It also can be distinguished from Seira and Lepidocyrid species by presence of p4 and absence of p3 (reverse in the latter) on Abd. III, and absence of E2 on Abd. IV. Among genera of Entomobryini (Entomobrya, Drepanura, Sinella, Coecobrya), adults of Homidia is easily separated from others in presence of dental spines, "eyebrow" on Abd. IV, and larger subapical mucronal tooth; as for first instar larvae (no dental spines), we also separate it from others by below characters: smooth labial seta E, abundant elongated s-chaetae on Abd. IV (much more than primary setae), and longer distance between area aM and pM on Abd. IV. The origin of peculiar "eyebrow" on anterior part of Abd. IV of adults couldn't be traced by primary chaetotaxy; postembryonic development may provide key evidences to homology of enigmatic "eyebrow".
It is still a long way to achieve the correct homology of setae in Entomobryidae. Further exploration at species level could be studied by thorough survey of more species and more characters of first instar, although first instar larvae are usually more difficult to collect than adults and subadults.

Morphology of the "smooth" setae under SEM
Spiny setae on antennae are smooth under LM and SEM. S and ms on dorsal body smooth under LM, but not smooth under SEM (Figs 53-62, 68, 69, 72). Guard setae on labial palp, proximal setae, "smooth" setae on ventral tube, tenent hair and supraempodial seta on Tita III are also smooth under LM, but weakly ciliate under SEM (Figs 50,51,(64)(65)(66)74,75). We have to carefully describe "smooth" setae in future (Chen and Christiansen 1993). Descriptions of some details (e. g. setal surface) based on LM are incomplete, and may bring confusion. SEM observation could provide fine details as a better supplementary tool for species diagnosis.