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Moth Attraction by Cyrtarachne, Related to Bolas Spiders Yoshitaka Sakamaki2, & Akira Shinkai3

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Moth Attraction by Cyrtarachne, Related to Bolas Spiders Yoshitaka Sakamaki2, & Akira Shinkai3 3274-29-603 , Otsuka, Hachioji-city, Tokyo, 192-0352 Japan Acta Arachnologica, 50(1): 1-4, September 28, 2001 Evidence against moth attraction by Cyrtarachne, a genus related to bolas spiders Tadashi Miyashita', Yoshitaka Sakamaki2, & Akira Shinkai3 Laboratory of Biodiversity Science, School of Agriculture & Life Sciences, University of Tokyo, Bunkyo-ku, Tokyo, 113-8656 Japan E-mail: [email protected]. u-tkyo. ac jp 2Laboratory of Plant Pathology and Entomology , Faculty of Agriculture, Kagoshima University, Kagoshima, 890-0065 Japan Abstract - Spiders of the genus Cyrtarachne are known to feed mainly on moths. To determine whether these spiders attract male moths of a particular group like bolas spiders, we collected prey caught by Cyrtarachne and moths flying in the field. Contrary to our expectation, prey moths comprised both sexes of various species, with the sex ratio of 0.77 in favor of females. Furthermore, sex ratio of prey and that of moth flying in the field did not differ significantly. These results suggest that Cyrtarachne does not seem to attract male moths. Key words - chemical mimicry, prey, moth, Cyrtarachne Cyrtarachne, Poecilopachys, Pasilobus, and Mastopho- Introduction ra to webless Celaenia (Robinson & Robinson 1975; Eberhard 1980; Cartan & Miyashita 2000). Most araneid spiders construct vertical orb-webs to Mastophora is famous for attracting male moths by capture prey. However, webs of Cyrtarachne have sev- emitting chemical substances mimicking sex pheromone eral unique characteristics, e.g., 1) horizontal orb-web, components of female moths (Stowe et al. 1987; 2) widely spaced spiral thread, 3) spiral thread with low Gemeno et al. 2000), hence prey comprised only males shear joint (one end of a spiral connected to a radius of particular species (Eberhard 1977; Yeargan 1988). breaks easily), 4) strong stickiness (e.g., Suginaga 1963; Robinson and Robinson (1975) stated that the main prey Shinkai et al. 1985; Shinkai 1992). Recently, Cartan and of Pasilobus is also moths, and suggested the possibility Miyashita (2000) found that diameter, breaking strength, of attraction of moths, although no evidence was pro- and energy absorption ability of spiral silk in vided. Because Cyrtarachne belongs to the same Glade Cyrtarachne are much larger than those in other mem- as Mastophora and Pasilobus, and feeds mainly on bers of Araneidae with similar body size. They consid- moths (Shinkai et al. 1985), it is interesting to examine ered these silk characteristics to be an adaptation for the possibility of male moth attraction. capturing flying moth by a single spiral thread, because In this paper we collected moths caught by kinetic energy of flying moths is approximately equiva- Cyrtarachne in order to determine whether they are bi- lent to the energy single thread can absorb. ased to males of a particular group. In addition, we cap- Another interesting point is that this genus is thought tured moths flying in the field where Cyrtarachne is to form a Gladewith Mastophora and Pasilobus (Scharff living to know the difference between prey composition & Coddington 1997) due to the two synapomorphies: and potential prey composition. strong stickiness of spirals and extensive combing action of 4th legs. Pasilobus makes a triangular web with three Study site and methods radii, and Mastophora is known as a bolas spider using a single thread with a large droplet at the end of it. we collected prey of Cyrtarachne in Itsukaichi and Several authors speculated that there has been an evolu- Hachioji located in the western part of Tokyo from 1990 tionary tendency for web reduction in this Glade from to 1998, and in Matsudo located in the northern Chiba 2 T. Mi y ashita et al. prefecture in 1998. The vegetations of the study sites Collections were made from 7 pm to 9 pm, which cone- mostly consisted of tall grasses (such as Pleioblastus sponded to the period for collecting prey of Cyrtara- chino and Miscanthus sinensis) and herbs (such as chne. Solidago altissima and Pueraria lobata). We searched webs of the three species of Cyrtarachne, i.e., C. bufo Results (Bosenberg & Strand), C. inaequalis Thorell, and C. nagasakiensis Strand, from July to September, and Prey of Cyrtarachne collected moths being fed upon by spiders or those A total of 21 prey of C. inaequalis were collected hanging on their webs. Although small Nematocera were (Table 1). Noctuidae was most abundant (62%) followed often observed to be stuck to viscid threads, they were by Pyralidae (24%). Among Noctuidae, various species not collected. belonging to different subfamilies were included. The In 1999, we caught moths flying in the study sites by sex ratio across all species of moths was 0.76 in favor of using insect net form July to September. Large moths females. whose body length exceeded 3 cm were not captured be- In C. nagasakiensis, 50% of prey (316) was cause they were not included in the prey of Cyrtarachne. Noctuidae, and the overall sex ratio was 0.83 in favor of Table 1. Taxonomic composition of prey moths caught by three species of Cyrtarachne, m: male, f: female. Evidence against moth attraction by Cyrtarachne 3 females. We found no difference between the sex ratio of prey In C. bufo, 50% of prey (4/8) was Pyralidae, and un- moths (three species of Cyrtarachne combined) and that like other species no Noctuidae was included. The sex of moths flying in the fields (x2 =2.33, df=1, p ratio of all moths was 0.75 in favor of females. =0 .127). When prey for the three species of Cyrtarachne were combined, Noctuidae was most abundant (46%), fol- Discussion lowed by Pyralidae (29%) and Tortricidae (11%), and the sex ratio was 0.77 in favor of females. Eberhard (1977) examined prey of Mastophora dizzydeani Eberhard and found that only males of two Moths flying in the fields species of moths were captured. Yeargan (1988) also re- We captured 54 moths including Pyralidae (37%), vealed that prey of M hutchinsoni comprised males of Noctuidae (31%), Geometridae (29%), and Uraniidae four moth species. However, prey of Cyrtarachne com- (4%) (Table 2). The sex ratio of each family was 0.58 prised various species of moths as well as males and fe- for Pyralidae, 0.63 for Noctuidae, and 0.67 for males. Therefore Cyrtarachne does not seem to attract Geometridae, all in favor of females. When all families male moths by chemical substance mimicking sex were combined, the sex ratio was 0.62. pheromone of female moths. The sampling of flying Table 2. Taxonomic composition of flying moths captured by using insect net. m: male, f: female. Acta Arachnologica, 50(1), September 2001 ©Arachnological Society of Japan 4 T. Miyashita et al. moths in the fields further supported this inference, be- cause sex ratios did not differ significantly between prey References moths and flying moths. One major discrepancy in taxo- nomic compositions between prey moths and flying Cartan, C. K. & Miyashita, T. 2000. Extraordinary web and moths is that no Geometridae was included in prey silk properties of Cyrtarachne: a possible link between whereas nearly 30% of Geometridae was found in flying orb-webs and bolas. Biol. J. Linn Soc., 71: 219-235. Eberhard, W. G. 1977. Aggressive chemical mimicry by a moths. The reason is not clear but there might have been bolas spider. Science, 198: 1173-1175. a subtle difference between microhabitat for flying Eberhard, W. G. 1980. The natural history and behaviour Geometridae and web-site in Cyrtarachne, or Geome- of the bolas spider Mastophora dizzydeani. Psyche, 87: tridae might have been captured more easily due to their 143-169. lower flying speed. Gemeno, C., Yeargan, K. V., & Haynes, K. F. 2000. Aggressive chemical mimicry by the bolas spider Cartan and Miyashita (2000) found extraordinarily Mastophora hutchinsoni: identification and quantifica- unique silk properties of Cyrtarachne and argued that tion of a major prey's sex pheromone components in the this may be an adaptation for capturing flying moths by spider's volatile emissions. J. Chem. Ecol., 26: 1235- single spiral thread. Other species of spiders belonging 1243. to the sub-family Cyrtarachninae such as Pasilobus, Robinson, M. H. & Robinson, B. 1975. Evolution beyond Mastophora, and Celaenia are thought to specialize on the orb-web: the web of the araneid spider Pasilobus sp., its structure, operation and construction. Zoo!. J. Linn. moths (Robinson & Robinson 1975; Eberhard 1977, Soc., 56: 301-314. 1980; Stowe 1986; Stowe et al. 1987). However, Scharff, N. & Coddington, J. A. 1997. A phylogenetic Mastophora and probably Celaenia developed chemical analysis of the orb-weaving spider family Araneidae mimicry for attracting male moths, while Cyrtarachne (Arachnida, Araneae). Zoo!. J. Linn. Soc., 120: 355- did not. How do we explain this discrepancy? The great- 434. Shinkai, A., Sadamoto, M., Suzuki, K. & Shinkai, A. 1985. est difference in prey capture method is that Cyr- Notes on the web and prey of Cyrtarachne. Atypus, 86: tarachne constructs a large horizontal orb-webs but 9-15. (In Japanese) Mastophora uses a simple "bolas" and Celaenia uses no Shinkai, A. 1992. Web structure and construction behavior thread. The investment of silk materials for webs in of Cyrtarachne yunoharuensis Strand: Is Cyrtarachne Cyrtarachne is equivalent to or slightly larger than that web an ordinary orb-web? Atypus, 100: 4-12. (In in other members of Araneidae that construct ordinal Japanese) Suginaga, A. 1963. Web weaving and foraging behaviour orb-webs (Cartan & Miyashita 2000). Obviously, the of Cyrtarachne bufo and C. inaequalis. Atypus, 31: 13- silk investment in Mastophora must be much smaller, 15. (In Japanese) and that in Celaenia zero. Accordingly, it seems unnec- Stowe, M. K. 1986. Prey specialization in Araneidae. pp. essary for Cyrtarachne to develop chemical mimicry. 101-131. In: Shear, W.
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