2017 Southeastern Naturalist Notes Vol. 16, No. 3
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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 - mhodge@lsmsa.edu.
Manuscript Editor: Richard Brown
Notes of the Southeastern Naturalist, Issue 16/3, 2017
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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
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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.
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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.
Since males are often observed in such a position with adult (never penultimate) females
(Catley 1993; Fergusson 1972; M. Hodge, pers. observ.), such guarding may indicate that
male–male interactions will occur if another male approaches the guarded female. The haplogyne
structure of the female epyginum is often associated with a reproductive pattern of
last male sperm precedence in which, after mating, males guard females until they produce
an eggsac (Calbacho-Rosa and Peretti 2015). Given that females exhibit cannibalism and
intraguild predation (Hodge and Marshall 1996), the extreme length of the male’s first legs
may be involved surviving agonistic interactions with both females and competing males.
Acknowledgements. Elizabeth Ballenger, Sarah Elderkin, and Michelle Gray assisted with field
work. We thank 2 anonymous reviewers and V.W. Framenau for providing useful suggestions on an
earlier draft.
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