Acta Arachnologica, 68(1): 31–33, June 30, 2019

[Short Communication] this species currently belongs to the family , the morphology of male individuals suggests that it should be included in the family Araneidae (Yoshida 2003). Discovery of the web of Phoroncidia Many species of the family Araneidae are orb-weavers; their webs are composed of non-viscid radial threads and altiventris (Araneae: Theridiidae) and viscid ecribellate spiral threads (Eberhard 1982). This basic structure has also been found in families Tetragnathidae, implications for its taxonomic position Theridiosomatidae, Anapidae, Mysmenidae and Symphy- tognathidae (Griswold et al. 1998; Eberhard 1987; Shinkai Yuya Suzuki1 & Tomoyuki Yokoi2* & Shinkai 1988; Shinkai 1990; Hiramatsu & Shinkai 1993) but has never been documented in Theridiidae. The web of Theridiid is characterized by a three-dimensional Laboratory of Conservation Ecology, Graduate School of Life tangled structure, as well as mesh web and sheet web varia- and Environmental Science, University of Tsukuba, 1-1-1, Tenno- tions (Arnedo et al. 2004; Eberhard et al. 2008). The web of dai, Tsukuba-shi, Ibaraki, 305-8572 . Phoroncidia is one of these unique variations, and is com- 1E-mail: [email protected] posed of a single line of thread with an array of adhesives 2E-mail: [email protected] (Eberhard 1981; Shinkai 1988a). Therefore, the identifica- *Corresponding author tion of web structure should contribute to understanding the taxonomic position of P. altiventris. However, the presence of a web had never been reported. The previously reported Abstract ― The web of Phoroncidia altiventris (Ara- foraging observations lacked detailed information on web neae: Theridiidae), which had been unknown since its structure and capture behavior (Hagimoto & Hagimoto taxonomic description, was recently discovered and 2002). Recently, we found a juvenile P. altiventris in its confirmed to form an orb-web structure. This result natural habitat that was hanging on its web (Figs. 1A, B). In supports the thought that this species should be reas- this paper, we describe the web structure of P. altiventris and sessed and placed in the family Araneidae. emphasize the need to reconsider its taxonomic position. The nocturnal observation was conducted in Tsukuba Ex- Key words ― Araneidae, hub, orb web, radial thread, spi- perimental Forest (36.115593N, 140.100733E), Tennodai, ral thread, Tsukuba, Ibaraki Prefecture, Japan, on April 24, 2019. Al- though the individual that wove a web was not matured, we identified it as P. altiventris on the basis of its external mor- Phoroncidia altiventris Yoshida 1985 (Araneae, Theridii- phology and the fact that several adults of the species were dae) is a small, lumpy-shaped that is located in Hon- collected at the same location. The web was found at 23:31 shu and Kyushu areas of Japan (Yoshida 2003). Although PM. It was stretched on the underside of a Eurya japonica

Fig. 1. A, a juvenile of Phoroncidia altiventris hanging on its web; B, posterior-lateral view of the web. Scales = 2 mm (A), 10 mm (B). 32 Y. Suzuki & T. Yokoi

Fig. 2. The web structure of Phoroncidia altiventris. A, a view from the undersurface of a leaf; B, posterior-lateral view; C, a hub. Solid lines and dashed lines indicate non sticky threads (radial threads) and sticky threads (viscid spiral threads), respectively. Thick grey parts and thin grey parts indicate the upper surface and under surface of the leaf, respectively. The correspondence of each radial thread in the figures is indicated by numbers (no. 1-6). Scales =10 mm (A, B), 1 mm (C).

(Thunb 1783) leaf and the web was at a height of 1.5 m from 1959 (Shinkai 1988b)). In contrast, other Araneid families the ground (Fig. 1B). The detail of the web structure is pre- that weave modified orb webs (e.g. Theridiosomatidae and sented in Fig. 2. The web was 38 mm in length and 21 mm Anapidae) commonly lack frame threads, which lead to ra- in width. It was composed of six radial threads and two spi- dial threads stretching in various directions (Eberhard 1987; ral viscid threads (Fig. 2). The radial threads were connected Shinkai & Shinkai 1988; Shinkai 1990). This trait seems to the edge of the leaf and intersected with the viscid threads to occur independently within different groups; therefore, excluding thread no. 6, which was directly connected to the it is difficult to determine the taxonomic position of P. al- undersurface of the leaf (Figs. 2A, B). Foraging behavior tiventris based only on its web morphology. However, the was not observed; however, the viscidity of the spiral thread characteristics of the observed web does not conflict with was confirmed based on the presence of a sticky glue array the thought that P. altiventris should belong to Araneidae and the fact that it trapped a gnat fly. Viscid threads diverged (Yoshida 2003). It also strongly suggests the low validity of at the intersection of radial threads no. 3, 4, and 5 (Figs. the current taxonomic position of this species, belonging to 2A, B), indicating that the number of spirals were origi- Theridiidae. Further phylogenetic studies using both mor- nally three and had reduced to two due to a fusion between phological and molecular approaches should be conducted the outer and median spirals. The web lacked framework to verify the taxonomic position of this species. threads and seemed to be tangled rather than plain (Fig. 2B). Phoroncidia altiventris is often found hanging under The shape of the hub was distorted compared to the regular leaves (Shinkai 2006). However, it is not clear what factors hexagonal formation (Fig. 2C). may trigger (i.e. humidity, time or growth phase and so on) The web of P. altiventris, composed of radial threads or are responsible for web construction. Further observations and ecribellate spiral viscid threads are similar to the ba- on the web structure, web constructing process, and foraging sic structure of Araneid’s web (Eberhard 1982) rather than behavior are necessary. that of Theridiid’s (Arnedo et al. 2004). The observed hub Acknowledgements structure resembles Araneid’s “closed hub”, whereas is clearly different from Tetragnathid’s “opened hub” (Ikeda We would like to thank Mr. Akira Shinkai for his valuable com- 2003) (Fig. 2C). The lack of frame threads rarely occurs in ments and interpretations on the web structure, Dr. Akio Tanikawa (University of Tokyo) for providing the information on the taxonomic Araneidae (e.g. spiders in the Cyrtarachne (Suginaga issues in this species, Tsukuba Experimental Forest, Mountain Science 1963)) and Tetragnathidae (e.g. Tetragnatha lauta Yaginuma Center, University of Tsukuba for providing us the opportunity to

Acta Arachnologica, 68(1), June 2019 Ⓒ Arachnological Society of Japan Discovery of the web of Phoroncidia altiventris 33 conduct the field survey. We express our gratitude to late Mr. Hajime havior of Patu sp. (Araneae: Symphytognathidae). Acta Arachnol., Yoshida, a specialist in Theridiidae taxonomy, who first noticed the 42: 181–185. (In Japanese with English summary) taxonomic issues of P. altiventris, for providing his taxonomic papers Ikeda, H. 2003. Box: types of hub. Pp. 152–153. In: Ikeda, H. (ed.) without any charge. We would like to thank Editage (www.editage.jp) Mystery in spider’s nests and webs. Bunyousha, Tokyo, 183pp. (In for English language editing. Japanese) Shinkai, A. & Shinkai, E. 1988. Web Structure of Conoculus lyugadi- References nus Komatsu (Araneae: Anapidae). Acta Arachnol., 37: 1–12. (In Japanese with English summary) Arnedo, M. A., Coddington, J., Agnarsson, I., & Gillespie, R. 2004. From a comb to a tree: phylogenetic relationships of the comb-foot- Shinkai, A. 1988a. Single line web of Phoroncidia pilula (Karsh), and ed spiders (Araneae, Theridiidae) inferred from nuclear and mito- its prey insects. Atypus, 92: 37–39. (In Japanese with English sum- chondrial genes. Mol. Phylogenet. Evol., 31: 225–245. mary) Eberhard, W. G. 1981. The single line web of Phoroncidia studo Levi Shinkai, A. 1988b. The web structure of Tetragnatha lauta Yaginuma. (Araneae: Theridiidae): A prey attractant? J. Arachnol., 9: 229–232. Kishidaia, 56: 15–18. (In Japanese) Eberhard, W. G. 1982. Behavioral characters for the higher classifica- Shinkai, A. 1990. Observations on the web structure and predatory tion of orb-weaving spiders. Evolution, 36: 1067–1095. behavior in Ogulnius pullus Bös. et Str. Atypus, 96: 19–24. (In Jap- Eberhard, W. G. 1987. Web-building behavior of Anapid, Symphytog- anese with English summary) nathid and Mysmenid spiders (Araneae). J. Arachnol., 14: 339–356. Shinkai, E. 2006. Picture book of Japanese spiders. Bun-ichi Co., Ltd., Eberhard, W. G., Agnarsson, I. & Levi, H. W. 2008. Web forms and Tokyo, Japan. 336 pp. (In Japanese) the phylogeny of theridiid spiders (Araneae: Theridiidae) : chaos Suginaga, A. 1963. Web-building and predation behavior of Cyrta- from order. Syst. Biodivers. 6: 415–475. rachne bufo and C. inaequalis. Atypus 31:13–15. (In Japanese) Griswold, C. E., Coddington, J. A., Hormiga, G. & Scharff, N. (1998) Yoshida, H. 2003. The spider family Theridiidae (Arachnida: Araneae) Phylogeny of the orb-web building spiders (Araneae, Orbicularinae: from Japan. Arachnological Society of Japan, Osaka. 223 pp. (In Deinopoidea, Araneoidea). Zool. J. Linn. Soc., 123: 1-99. Japanese) Hagimoto, S. & Hagimoto, K. 2002. Phoroncidia altiventris ate a spi- der. Kishidaia, 83: 53–54. (In Japanese) Received May 17, 2019 / Accepted May 22, 2019 Hiramatsu T. & Shinkai A. 1993. Web structure and web-building be-

Acta Arachnologica, 68(1), June 2019 Ⓒ Arachnological Society of Japan