Journal 
INTRODUCTION
The Family Isostomidae belongs to order Entomobryomorpha in class Collembola and approximately 1,472 species in 113 genera have been known to be distributed all over the world (Bellinger et al. 1996-2018). From Korea, only 37 species of 14 genera have been recorded (Shon 1963;Yosii and Lee 1963;Yosii 1966;Lee 1973, 1977;Lee et al. 1993;Lee and Kim 1994;Thibaud and Kim 1995;Kim and Lee 2000;Park and Lee 2000;Potapow 1997, 2001;Lim and Park 2011, 2013;Dányi and Park 2016;Park 2018;Kim and Park 2018).
The genus Folsomia has the following features: 1) three last abdominal segments fused to a single segment, 2) no apical bulb on the antenna, 3) presence of PAO, 4) 4 sublobal hairs on outer maxillary lobe, 5) presence of ventroapical setae on manubrium, and 6) no ventral setae on Th I and II (Potapow 2001;Fjellberg 2007). There are 189 species in the genus Folsomia over the world, comprising 13% of all described species of the family Isotomidae (Bellinger et al. 1996-2018). However, only 8 species in Folsomia have been known from Korea (Kwak 2011;Dányi and Park 2016), and there are a limited number of researches on descriptions and ecology of them in Korea.
For ecological study, collembolan species were investigated in six mountains of Korea from 2013 to 2015, to understand an interaction of Collembola community occurred in the forest soil in Korea with their environmental factors. In the study, Folsomia quadrioculata or F. octoculata was the dominant species in 5 sampling sites among 6 sampling sites (Table 1). F. quadrioculata and another species, F. octoculata, were found in litter of deciduous and pine forest of Korea, respectively. According to the result of this study, the major tree species seems to be one of the important factors determining their habitat. F. quadrioculata is a newly recorded species from the Korean Peninsula. F. octoculata was reported in South Korea by Lee (1973, 1977), with the brief remark about color pattern and the chaetotaxy of tenaculum, manubrium and dens in only second report. Therefore, F. octoculata is redescribed using recent important characters with illustrations based on the new materials from South Korea. As a result of this study, Korean faunal list of Isotomidae consists of 38 species in 14 genera. This study will be important information on morphology as well as ecology of the two dominant collembolan species in forest soil in Korea.
MATERIALS AND METHODS
Collecting and imaging
The sampling was conducted at six mountains in Korea and details including their location, elevation, and major species of tree were given in Table 1. Three soil sampling cores (diameter: 5 cm, height: 10 cm) were taken at each sampling site and the collembolan species in the sampled soil and leaf litter were extracted by Tullgren funnels. The dominant species was selected as the most abundant species in three sampled soil by sampling core. Specimens were preserved in 95% ethanol and taken photograph with a 3.1-megapixel digital camera (Toupcam, Zhejiang, China) mounted on a stereo microscope.
Morphology
For microscopic study of entire individuals, specimens were cleared in 5% KOH, then mounted in polyvinyl alcohol to prepare permanent slides (Downs 1943). Slides were dried and hardened in the 60°C oven for 3 days. Specimens were examined with bright-field or differential interference- contrast microscopes.
Morphological abbreviations used in this paper are as follows: [Abd. I-VI] abdominal segments, I-VI, [Ant. I-IV] antennal segments I-IV, [Ant. III O] Third antennal organ, [bms] basal microsensilla, [Cl] Claw, [Man] manubrium, [PAO] post-antennal organ, [Th. I-III] thoracic segments I-III, [Ret] retinaculum, [VT] ventral tube.
Voucher specimens are deposited in the National Institute of Biological Resources (NIBR), Incheon, Korea and the Insect Collection, Department of Biology Education, Chonbuk National University, Jeonju, Korea.
TAXONOMIC ACCOUNT
Family Isotomidae Schäffer
Genus Folsomia Willem
Folsomia quadrioculata (Tullberg 1871) (Figs. 1A & 2)
네눈마디톡토기 (국명신칭)
Isotoma quadri-oculata Tullberg, 1871: 152 (type locality: Sweden, “Uppsala”)
Redescription. Body length 1.0 to 1.5 mm. Body slender and elongate, abdomen thicker towards tip (Fig. 1A). Ground color pale yellow overlaid with mottled grayish pigment except intersegmental margins; antennae pale; legs grayish on coxae, otherwise pale; furcula pale yellow. Eye patches black, ocelli 2+2, arranged far from each other on each side. PAO narrow, elongated, 1.2 times longer than width of Ant I. Antenna about 1.5 times of head diagonal. Ant I with 3 small basal microsensilla (2 dorsal and 1 ventral, bms) and 2 ventroapical sensilla (Fig. 2A, B); Ant II with 3 bms (1 dosal, 1 ventral, and 1 latero-distal) (Fig. 2C, D), lateroapical setaceous sensillum present; Ant III with 1 bms and 5 sensilla (Fig. 2E), the setaceous sensillum absent; Ant IV without apical bulb, with subapical knob in a pit and 6 slender, curved differentiated setae (Fig. 2F). Maxillary palp bifurcate, outer maxillary lobe with 4 sublobal hairs. Labrum 4/5, 5, 4. Cl without teeth. Tibiotarsi III with many setae (up to 28) (Fig. 2G). Sensillary chaetotaxy of Th. II to Abd. IV-VI as 4, 3 / 2, 2, 2, 8, including 1, 0 / 1, 0, 0, 0 microsensilla (Fig. 2H). VT with 3+3 laterodistal setae. Ret quadridentate and 1 seta. Man with 1+1 ventroapical setae. Dens normally with 8 anterior and 3 posterior setae. Mucro bidentate.
Material examined. 1 specimen on slide: Hyeolgusan (mountain), 126° 26ʹ 22ʺ E, 37° 42ʹ 32ʺ N, Gocheon-ri, Naega-myeon, Ganghwa-gun, Incheon. 6 specimens on slide: Joggyesan (mountain), 127° 19ʹ 14ʺ E, 35° 25ʹ 46ʺ N, Jukhak-ri, Seungju-eup, Suncheon-si, Jeollanam-do Provinve. 3 specimens on slide: Gyemyeongsan (mountain), 127° 58ʹ 58ʺ E, 36° 58ʹ 47ʺ N, Jongmin-dong, Chungju-si, Chungcheongbuk-do Province.
Distribution. Widely distributed Holarctic species. Newly recorded from the Korean Peninsula in this study.
Biology. All the specimens in the present study were found in litter of deciduous forests.
Remarks. The present materials agreed well with the previous studies by Deharveng (1982) and Fjellberg (2007), and easily identified by 2+2 ocelli which are arranged far from each other on each side, 1+1 ventroapical manubrial setae and 8 anterior setae on dens. According to some authors, the color of this species are variable, but the specimens of this study were covered with mottled grayish pigmentation without variation in body color.
Folsomia octoculata (Handschin 1925)
Folsomia octoculata Handschin, 1925: 226, Figs. 1-5 (Type locality: Java, “Tjibodas”)
Folsomia uenoea Uchida, 1943: 5, Pl. 3C-H.
Redescription. Body length 1.0 to 1.4 mm. Body slender and elongate, abdomen slightly expanded towards tip (Fig. 1B). Ground color pale yellow overlaid with scattered deep bluish-gray pigmentation except intersegmental margins; antennae bluish-gray; legs bluish-gray on coxae, otherwise
pale; furcula pale yellow. Eye patches black, ocelli 4+4, three of the four in the anterior and one in the posterior on each side. PAO narrow, constricted, about 1.5 times longer than the width of Ant I. Labrum 4 / 5,5,4. Ant. I with 2 small basal microsensilla (dorsal, bms) and 2 ventral sensilla (s) (Fig. 3A, B), Ant II with 2 bms, 1 dorsal and 1 ventral (Fig. 3C, D), Ant III with 1 bms, Ant III O with two inner sens (Fig. 3E), Ant IV without apical bulb, with subapical knob in a pit and 6 slender, curved differentiated setae (Fig. 3F). Sensillary chaetotaxy of Th. II to Abd. IV-VI as 4, 3 / 2, 2, 2, 8, including 1, 1 / 1, 1, 1, 0 microsensilla (Fig. 3G). Ret with 4+4 teeth and 1 seta. Man with 2+2 ventroapical setae. Dens with 9 anterior and 5 posterior setae. Mucro bidentate. Mucro : dens : Man=7 : 28 : 30.
Material examined. 4 specimens on slide: Youngmunsan (mountain), 127° 31ʹ 20ʺ E, 37° 30ʹ 55ʺ N, Yongmun-myeon, Yangpyeong-gun, Gyeonggi-do Province. 3 specimens on slide: Gyoryongsan (mountain), 127° 21ʹ 9ʺ E, 36° 22ʹ 8ʺ N, Sangok-dong, Namwon-si, Jeollabuk-do Province. 2 specimens on slide: Ansan (mountain), 126° 56ʹ 48ʺ E, 37° 34ʹ 36ʺ N, Bongwon-dong, Seodaemun-gu, Seoul.
Distribution. East Asia (Indonesia, Malaya, India, Pakistan, Nepal), Far East Asia (China, Japan, Korea), Russia (Far East), Hawaii
Biology. All the specimens in the present study were found in litter of pine forest.
Remarks. The present materials agree well with the previous studies by Handchin (1925) and Yosii (1956) from Indonesia. The ratio of the mucro, dens, and manubrium in the present study is closer to the original description of Handchin (1925; 7 : 25 : 30) than to the record of Yossi (1956; 1 : 4 : 3.5). According to the report by Lee (1977), the specimens from South Korea show variation from 1 to 3 in the number of setae on tenaculum. However, all the specimens under our study had 1 seta.
DISCUSSION
Folsomia quadrioculata and Folsomia octoculata were dominant species in this study, but they have been very few studies in Korea. F. quadrioculata, the specimens examined showed a considerable correspondence with that reported by Deharveng (1982) and Fjellberg (2007). F. octoculata which found in this study have 2 bms on each of Ant. I and II. This is a new morphological feature that has not been reported in previous literature on F. octoculata. The appearance of these two species looks very similar, but they can be easily distinguished by showing the difference in body color (F. octoculata is darker than F. quadrioculata), number of ommatidium, number of anterior setae on manubrium, and microsensillar formula on thorax II to abdomen III.
Despite the morphological similarity between F. quadrioculata and F. octoculata, their habitats seem to be very different. The major tree species and two Folsomia species were closely related. F. quadrioculata and F. octoculata were found at the sites where deciduous and pine forests are presence, respectively (Table 1). In the previous studies, F. octoculata was mostly found in a pine forests (Potapow 2001), and F. quadrioculata was reported as indicator species of the deciduous forests (Slawska et al. 2017). Although this relationship cannot be concluded in this study because of the small number of sampling sites, the major tree species seem to be one of the important factors determining their habitat.
