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Sexual Dimorphism in the Lampshade Spider Hypochilus thorelli
Margaret Hodge and Samuel Marshall

Southeastern Naturalist, Volume 16, Issue 3 (2017): N24–N28

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2017 Southeastern Naturalist Notes Vol. 16, No. 3 N24 M. Hodge and S. Marshall Sexual Dimorphism in the Lampshade Spider Hypochilus thorelli (Araneae: Hypochilidae) Margaret Hodge1,* and Samuel Marshall2 Abstract - Recent comparative analyses of sexual size dimorphism in web-building spiders have not included data from the Hypochilidae, an ancient group of spiders in which half of the species have geographic distributions that are restricted to the Appalachian Mountains. Females are slightly larger than males as measured by cephalothorax width, but male leg 1 is much longer than that of females. We document the development of this dimorphism in the field and discuss the possible adaptive significance of this trait. Spiders are typically sexually dimorphic in body size, with females being generally larger than males (Foellmer and Moya-Larano 2007). In most species, the size dimorphism is usually slight, but some species exhibit extreme sexual size dimorphism (SSD) in which females are huge compared to their miniscule male counterparts (Coddington et al. 1997). Alternative hypotheses to explain SSD include selection for female gigantism (Coddington et al. 1997, Hormiga et al. 2000, Prenter et al. 1997) and selection for male dwarfism (Moya-Larano et al. 2002; Vollrath and Parker 1992, 1997). Another common sexually dimorphic feature is leg length, such that male legs are often relatively longer than female legs (Foellmer and Moya-Larano 2007). Hypotheses to explain this include avoidance of sexual cannibalism by courting males (Elgar et al. 1990), and selection for locomotor efficiency and increased female encounter rates in species in which males search for females (Foellmer and Fairbairn 2005, Framenau 2005). Comparative analysis, which controls for phylogenetic effects, has been the most common approach to testing the generality of hypotheses concerning sexual dimorphism (e.g., Coddington et al. 1997, Prenter et al. 1997, Ramos et al. 2005). The greater the taxonomic breadth of data available for such studies, the stronger the conclusions that can be drawn from them (Harvey and Pagel 1991). For this reason, we present data on the development of sexual dimorphism in the spider family Hypochilidae. The family Hypochilidae was considered to be the sister group to all other araneomorph (modern “non-tarantula”) spiders (Catley 1994, Forster et al. 1987, Hedin and Wood 2002). Recent phylogenomic analysis places them as a sister group to Filistatidae, the latter being a member of the paraphyletic Haplogynae (spiders with simple genitalia) that are basal to remaining araneomorphs (Bond et al 2014, Garrison et al. 2016). The genus Hypochilus includes 10 species, half of which have allopatric distributions in the southern Appalachian Mountains (Hedin 2001, Keith and Hedin 2012). The Hypochilidae exhibit a mixture of primitive and modern anatomical features and build a uniquely shaped web almost exclusively on rock outcrops, from which they get their common name “lampshade spiders” (Catley 1994). Here, we provide data on sexual dimorphism in Hypochilus thorelli Marx, which will contribute to an understanding of its reproductive biology as well as to the larger discussion concerning the adaptive significance of sexual dimorphism in spiders. 1Louisiana School for Math, Science, and the Arts, Science Department, Natchitoches, LA 71457. 2Northwestern State University of Louisiana, Department of Biological Sciences, Natchitoches, LA 71497. *Corresponding author - Manuscript Editor: Richard Brown Notes of the Southeastern Naturalist, Issue 16/3, 2017 N25 2017 Southeastern Naturalist Notes Vol. 16, No. 3 M. Hodge and S. Marshall During our field studies of aggregation and intraguild predation (June–August 1992– 1993) conducted in the Cumberland Mountains of East Tennessee (along the Cumberland Trail, Walden Ridge section, between Lake City and Oliver Springs, TN), H. thorelli were removed from webs, measured, individually marked on the abdomen with Testors® brand model paint, and returned to their webs (Hodge and Marshall 1996, Hodge and Storfer-Isser 1997). We mapped each rock outcrop and recorded the position of the captured spiders onto graph paper, and marked the position of webs with flagging tape upon which the identification mark of the spider was noted in waterproof ink (Hodge and Storfer-Isser 1997). We checked spiders daily or weekly for web location and measured and re-marked spiders that molted. We recognized penultimate females by a tuft of setae on the anterior ventral abdomen (which develops the female genital opening in mature individuals) and penultimate males by enlarged pedipalpal tarsi, which develop into the male copulatory organ, and darkened leg 1 (Coyle 1985; M. Hodge, pers. observ.). We measured the patella–tibia length of leg 1 with dial calipers to the nearest 0.01 mm each time a spider was marked. We scored spiders as having molted from penultimate to adult if the spider occupying the web had a pale, soft exoskeleton that had not completely hardened and the caste-off exoskeleton with the original mark was hanging from the web. Cephalothorax width is a commonly used measure of overall body size in spiders since it is independent of recent feeding history, unlike abdomen length (Foellmer and Moya-Larano 2007). Carapace width was not measured in the field because live spiders draw their legs over the cephalothorax when handled, making it difficult for accurate measurement. Therefore, to control for body size in our examination of developmental allometry in leg length, we measured cephalothorax width (measured across the bases of the coxae of leg II, approximately the widest point of the cephalothorax) as well as the patella–tibia length of all 4 legs from preserved adult male and female specimens. Voucher specimens are in the invertebrate collection at the Department of Biological Sciences, Northwestern State University of Louisiana. Measurements of preserved mature specimens showed that the cephalothoraces of adult females (n = 8) were significantly wider than those of males (n = 6) (t = 5.40, df = 12, P = 0.0002). Ramos et al. (2005) used the mean female:male cephalothorax width ratio as a measure of the degree of sexual size dimorphism (SSD). Within the broad range of values found in their comparative study, an SSD ratio such as that of H. thorelli (1.229, calculated from the data in Fig. 1) falls within their classification scheme as a sexually monomorphic species with respect to overall body size. Figure 1. Average cephalothorax widths (CW) and patella–tibia lengths (PT) of legs 1–4 of adult male (n = 6) and female (n = 8) Hypochilus thorelli. Spider legs are numbered from anterior to posterior. All measures are significantly different between sexes. Data from preserved specimens. 2017 Southeastern Naturalist Notes Vol. 16, No. 3 N26 M. Hodge and S. Marshall All 4 male legs were longer than female legs (Fig. 1; ANOVA F1, 4 = 19.8, df = 1, P = 0.0001) as measured from preserved specimens. Our measurements from live spiders in the field showed that males (n = 16) and females (n = 11) overlapped in patella–tibia length of leg 1 in the penultimate instar, but male patella–tibia length increased by over 10 mm more than that of females in the adult instar (Fig. 2). A 2-way ANOVA for repeated measures found a significant effect of sex (F1, 24 = 84.53, P < 0.0001), instar (F1, 24 = 30.80, P less than 0.0001) and interaction of sex and instar (F1, 24 = 4.82, P < 0.0001). This result indicates that the sexes exhibited differential growth between instars. The proportional increase in patella–tibia 1 length was almost twice as great for males as for females (mean proportional increase ± 1 sd: males = 0.72 ± 0.12, females = 0.37 ± 0.09). The difference in the proportional increase in patella–tibia length between males and females was significant (arcsin square root transformed data: t = -5.39, df = 25, P < 0.0001). To further quantify the magnitude of the sexual leg dimorphism in H. thorelli, we compared the ratio of the average male:female patella–tibia 1 length, and found it to be 1.38. We calculated this ratio from morphological data found in the literature for 3 other Hypochilus species: H. bernardino: 1.38 (Catley 1994); H. jemez: 1.32 (Catley 1994); H. petrunkevitchi: 1.69 (Gertsch 1958). Though these data are from only the type specimens for each species, the values are similar to our data on H. thorelli, indicating this trait is probably shared by all members of the genus. Comparing the average of all 4 Hypochilus species to information in the literature on relative male:female leg lengths for other groups of spider species that are size monomorphic, we found that the leg dimorphism in the Hypochilidae is much greater than for other groups of web-building spiders for which information is available (e.g., Araneidae: 1.02, Tetragnathidae: 1.15 [Elgar et al. 1990]; Pholcidae: 1.28 [Huber 2005]; Filistatidae: 1.18 [Zonstein and Marusik 2016]; Hypochilidae: 1.44 [this study]). In sessile species, such as web-building spiders, males usually abandon web-building upon maturing, and spend the rest of their lives searching for females. It has been suggested that mate searching may select for males with longer legs to increase vagility, and thus their encounter rates with females, as well as to increase the surface area available for detecting female cues (e.g., chemical, vibratory) (Foellmer and Fairbairn 2005, Prenter et al. 1995). Figure 2. The increase in the length of the patella– tibia of leg 1 of Hypochilus thorelli males (n = 16) and females (n = 11) measured in the field before and after molting to the adult instar. The proportional increase in males in the adult instar is significantly greater than that of females. N27 2017 Southeastern Naturalist Notes Vol. 16, No. 3 M. Hodge and S. Marshall Other forms of sexual selection on male leg length may involve agonistic interactions between males (male–male competition for females) and/or reducing the risk of cannibalism by being able to keep a greater distance while courting the female (Elgar et al. 1990). What little we know about the reproductive biology of Hypochilus suggests that all of the above may be important selective pressures on leg length of males. Male Hypochilus abandon their web upon maturation (Catley 1993, Coyle 1985, Fergusson 1972), and in 1 observation a male relocated 6 m in 24 h over open ground to non-adjacent rock outcrops to the web of a mature female (M. Hodge, pers. observ.). The fact that these spiders are essentially blind (Gertsch 1958) suggests pheromonal communication, and sensory receptors may exist on the long first pair of legs. Hypochilus pococki is the only species in the genus for which observations of courtship have been published (Catley 1993). In that report, a male initiated courtship by pulling on the webbing with his first pair of legs, which resulted in a series of aggressive attacks by the female. After several attempts, copulation was achieved, ending with female pursuit of the male. The male then returned to the female’s web and assumed a “guarding” posture, extending his first 3 pair of legs over the female. 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