2010 SOUTHEASTERN NATURALIST 9(2):251–258 Emergence-site Selection by the Dragonfly Epitheca spinosa (Hagen) Wade B. Worthen* Abstract - Odonates are vulnerable during emergence, when they shed their larval skin (exuvia) to take flight as adults. Emergence-site selection should adapt to the local mortality risks. Here, I characterized emergence-site selection of Epitheca spinosa (Robust Baskettails) by noting the substrate, height, and distance from water of exuviae in a 300 m x 5 m plot at Weston Lake, Congaree National Park, Hopkins, SC. Of the 82 Robust Baskettail exuviae sampled, 52 (63.4%) were found on trees with corky bark (Nyssa aquatica [Water Tupelo], Nyssa biflora [Swamp Tupelo], Fraxinus pennsylvanica [Green Ash]), while no exuviae were found on the peeling, flaky trunks of Taxodium distichum (Bald Cypress) or the smooth, platy trunks of Acer rubrum (Red Maple). However, 26 (31.7%) exuviae were on T. distichum pneumatophores. This pattern was significantly different from the relative abundances of these substrate types (χ2 = 19.8, df = 3, P < 0.001). Most exuviae (93.9%) were on substrates touching the water, suggesting that larvae climb directly from the water to their emergence site. The mean height of exuviae on trees was 3.3 ± 1.37 m, with a range from 1.8–7.7 m. Highclimbing by Robust Baskettail larvae may be an adaptation to flooding at Weston Lake; major flood events (>3 m) are common (5 of the last 10 years) during their March–April emergence period. Introduction The corduliid dragonfly Epitheca (Tetragoneuria) spinosa (Hagen) (Robust Baskettail) ranges from New Jersey to Georgia and the Florida panhandle (Needham et al. 2000). The drab, hoary adults hover along the margin of ponds and lakes. Larvae live in swamps where there is slowmoving water (Dunkle and Westfall 1982). Although uncommon throughout the majority of its range (Tennessen 1994), it is locally abundant at Congaree National Park, 30 km southeast of Columbia, SC. This is not surprising, as the 10,700+ ha of Congaree National Park encompass the largest old-growth floodplain forest in the United States, including several ancient oxbow lakes that are ideal habitat for Robust Baskettails and congeners Epitheca cynosura (Say) (Common Baskettail) and Epitheca semiaquea (Burmeister) (Mantled Baskettail). Because Robust Baskettails are uncommon, little is known of their larval ecology. In odonate development, emergence is defined as the period when an individual clambers from the water, attaches to a substrate, climbs from its larval exoskeleton (exuvia), expands its abdomen and wings, and takes *Biology Department, Furman University, Greenville, SC 29613; wade.worthen@ furman.edu. 252 Southeastern Naturalist Vol. 9, No. 2 flight as an adult (Fig. 1a–c; Corbet 1999:237). Once they are attached to a substrate, they are immobile, soft-bodied, and vulnerable to damage and predation. This vulnerability is probably why emergence usually occurs quickly—in less than an hour under ideal conditions—and at night. Selection should also favor organisms that select emergence sites that reduce these risks (Cordero 1995). The presence of new, intact exuviae attached to a substrate allows easy characterization of odonate emergence sites. Here, I describe emergence-site selection by Robust Baskettail larvae by describing the distribution of exuviae at Weston Lake, Congaree National Park. Study Site Weston Lake (33°49'17.33"N, 80°49'05.31"W; 29 m elevation) is approximately 400 x 50 m, and is skirted by a bottomland forest dominated by Taxodium distichum (L.) L. C. Rich. (Bald Cypress), Nyssa aquatica L. (Water Tupelo), Fraxinus pennsylvanicus Marsh (Green Ash) and Acer rubrum L. (Red Maple). There is also an occasional Nyssa biflora Walt. (Swamp Tupelo), Alnus serrulata (Ait.) Willd. (Alder) Quercus pagoda Raf. (Cherrybark Oak), and Liquidambar styraciflua L. (Sweetgum). The first six species grow at the bank, and tree trunks and cypress “knees” (pneumatophores) are the only vertical surfaces available to climbing larvae in early spring. Methods I visited Weston Lake on March 20–22, 2009, and recorded the location of Robust Baskettail exuviae in a 300- x 5-m plot along the northwestern shore. Species emerging under cold conditions often favor this aspect, perhaps to be warmed by direct rays of early morning sunshine (Beynon 1995). The height, distance from the bank, and substrate type were noted. Substrates were characterized as trees with: 1) corky bark (Water Tupelo, Swamp Tupelo, or Green Ash), 2) peeling bark (Bald Cypress), 3) smooth, platy bark (Red Maple), and 4) pneumatophores (Bald Cypress). Results and Discussion I observed 82 Robust Baskettail exuviae in the plot, including 4 with individuals in the process of emergence (as in Fig. 1a–c). Robust Baskettail larvae and exuviae are easily distinguished from co-occurring congeners because the lateral spines on abdominal segment 9 do not extend beyond the tip of the abdomen, as they do in the other species (Fig. 2; Needham et al. 2000). I found only two exuviae of other species, both belonging to Basiaeschna janata Say (Springtime Darner). Dragonfly larvae can walk remarkable distances overland to find a suitable emergence site. Several species walk more than 10 m from water (Corbet 1999:630–631), and larvae of Plathemis lydia (Drury) (Common 2010 W.B. Worthen 253 Figure 1. (a–c) Emergence of Epitheca spinosa (stages 2–4, respectively). Whitetail) have walked more than 45 m from water (Jacobs 1955). At Weston Lake, however, Robust Baskettails did not travel far from water to emerge; most (93.9%) were found on supports in direct contact with water, and the rest were found on supports within 2 m of water. I returned to the plot on July 24–25, 2009, to determine the relative abundances of the substrate types within 2 m of water and in contact with the water. No account was taken of the bark texture below the water surface or the size of the trees. Most Robust Baskettail exuviae were on trees with corky bark (63.4%, Table 1). No exuviae were found on trunks of Bald Cypress or Red Maple, but 26 (31.7%) were found on Bald Cypress pneumatophores and 4 (4.9%) were found in one cluster of three Alder saplings (Table 1). To determine whether this distribution represented a selective substrate preference, I compared the observed distribution of exuviae across substrate types with an expected distribution based on the Figure 2. Exuvia showing diagnostic short lateral spines on segment 9. 254 Southeastern Naturalist Vol. 9, No. 2 relative abundances of these substrates. I limited this analysis to exuviae and trees found at the waterline, because this was where the vast majority of larvae (93.9%) selected a substrate to climb. Alder was both included and excluded from analyses because, with exuviae in the single cluster of saplings, the “replicated” selection of Alder by larvae is debatable. The distribution of exuviae across substrate types was significantly different from the relative abundances of substrates at the water line; regardless of whether pneumatophores are excluded from the comparison (χ2 = 20.4, df = 2, P < 0.001), are included with Bald Cypress (χ2 = 14.9, df = 2, P < 0.001), or are included as their own category (χ2 = 19.8, df = 3, P < 0.001). Including Alder as a separate category only made each of the above comparisons more significant. Trees with corky bark would seem to provide an ideal surface for grasping tarsi. The gnarled, flaky pneumatophores of Bald Cypress might also provide an adequate surface for climbing and attachment. In contrast, Bald Cypress trunks have shaggy, peeling, outer bark that could trap or impede climbing larvae. Where the outer bark has shed, the inner bark may be too smooth for larvae to grip. The smooth (sometimes plating) bark of Red Maple may account for the absence of exuviae on this species, as well. The exuviae found on mature trees (not Alder saplings or pneumatophores), where climbing height was not limited by the height of the substrate, were found at an average height of 3.3 ± 1.37 m (Fig. 3), with a range from 1.8–7.7 m. Exuviae could be high in trees if larvae float to the surface of flood waters and then cling to trees they encounter, or mean climbing height could be skewed if a flood removed low exuviae from supports. However, 1) the only flood before my sampling period (on March 4; 3.2 m) receded well before any emergences occurred during this cold year, 2) there were 26 exuviae on low pneumatophores, and 3) I observed a Robust Baskettail complete emergence on a tree trunk at a height of 5.0 m on March 22. So, I believe the exuviae sampled on March 21–22 are representative indicators of emergence-site selection by actively climbing Robust Baskettail larvae at Weston Lake. Table 1. The distribution of Robust Baskettail (Epitheca spinosa) exuviae on different substrates at Weston Lake, Congaree National Park, Richland County, SC, including all exuviae within the 5- x 300-m plot, and the subset (93.9%) found on substrates in direct contact with water. All exuviae Exuviae, waterline Species (n) Substrate type (n) (n = 82) (n = 75) Water Tupelo (108) Green Ash (31) Corky bark (153) 52 (63.4%) 47 (62.7%) Swamp Blackgum (14) Red Maple (41) Platy bark (41) 0 (0%) 0 (0%) Bald Cypress (25) Peeling bark (25) 0 (0%) 0 (0%) Pneumatophore (103) 26 (31.7%) 24 (32.0%) Alder (3) 4 (4.9%) 4 (5.3%) 2010 W.B. Worthen 255 Although it is remarkable to see dragonfly exuviae more than 5 m up tree trunks, it is not unprecedented. Larvae of other Epitheca species (either E. cynosura or E. costalis) were found at a median height of 5.5 m at a lake in Alabama, and an average distance of 4.73 m from shore (Tennessen 1979). Several other species have been found above 5 m (Corbet 1999:630–631), and Brachythemis contaminata (Fabricius) and Pantala flavescens (Fabricius) exuviae have been found at heights of 12.5 m during the monsoon season in India (Mathavan and Pandian 1977). Such an extreme behavior begs an adaptive explanation, and several have been proposed. High climbing may be a response to high larval densities—i.e., it spaces individuals out and reduces the damage they could cause one another during the soft-bodied molting process (Bennett and Mill 1993, Corbet 1957). It may also reduce density-dependent predation by birds or other dragonflies (Coppa 1991, Miller 1964), and may reduce interspecific competition for emergence sites (Cordero 1995). But climbing height also correlates with physical factors like temperature and humidity. Anax junius (Drury) (Common Green Darner), for example, climb higher when the water and air are warm and humidity is low; so climbing may be a response to desiccation stress (Trottier 1973). At Weston Lake, high climbing may be an adaptation to the frequent and dramatic flooding that occurs at Congaree National Figure 3. Location of exuviae (circled) at the top of a 3.4-m snag, with close-up (inset). 256 Southeastern Naturalist Vol. 9, No. 2 Park. The Congaree River inundates the Park an average of 10 times a year (National Park Service 2006). One of the two peak flooding periods is March–April, when Robust Baskettails are emerging. There is a USGS gauging station 0.87 km from Weston Lake, on a tributary of the Congaree River called Cedar Creek (USGS gauging station #02169672; 33°49'0.47"N, 80°49'39.06"W, 29 m elevation). In 5 of the last 10 years, there has been at least one flood event over 3 m on Cedar Creek during the March–April emergence period (Table 2; US Geological Survey 2009). The exact relationship between gauge height on Cedar Creek and flood levels at Weston Lake has not been described explicitly, but anecdotal evidence (photos of Weston Lake at flood stage compared with contemporaneous gauge height readings at Cedar Creek) suggests that a 3-m gauge height at the Cedar Creek station corresponds to a 2-m flood at Weston Lake—roughly the minimum climbing height of Robust Baskettails on trees. There are also direct observations of 3-m floods at Weston Lake (F. Rametta, NPS Ranger, Congaree National Park, SC, pers. comm.). Floods at Congaree National Park are not only frequent, they can be severe and prolonged. Flood waters crested at the Cedar Creek station at 4.6 m and 4.1 m in 2003 and 2007, respectively, and gauge height exceeded 3 m for 26 of 62 days in March–April 2003 (Table 2). As such, it seems that flooding could be a primary factor selecting for high climbing by Robust Baskettail nymphs. This explanation may also apply to high climbing by tropical species during monsoon season (Mathavan and Pandian 1977). Climbing high may be doubly important for Robust Baskettails; low temperatures in early spring can prolong emergence and further increase the risk of damage from rising flood waters. For example, March 21, 2009, was sunny but cool (16 °C). It took one individual three hours (22:30–1:30 local solar time) to complete emergence from “hanging” stage 2 to its virgin flight (end of stage 4) in the “heat” of mid-day (Fig. 1a–c). This was twice as long as it took two dragonflies to complete emergence from stage 2–4 on March 22, at 21 °C at local solar noon. Most Robust Baskettails emerge Table 2. Flooding frequency and severity at Cedar Creek, Congaree National Park, during the months of March and April, from 2000–2009. Data compiled from USGS gauging station 02169672 (http://waterdata.usgs.gov). Year Days >3 m (events) Maximum (m) 2000 2 (1) 3.1 2001 0 2.8 2002 0 1.1 2003 26 (4) 4.6 2004 0 1.8 2005 5 (1) 3.6 2006 0 1.4 2007 6 (1) 4.1 2008 0 1.9 2009 4 (2) 3.2 2010 W.B. Worthen 257 before dawn when the temperatures are even cooler. When climbing time (stage 1) is included, the entire emergence process probably takes many hours to complete. High climbing would reduce the chance that the emerging dragonfly would be caught by rising water during this protracted process. It seems likely that emergence-site selection by these Robust Baskettails is an adaptation to avoid the frequent, predictable, and occasionally extreme spring flooding at Congaree National Park. However, comparisons of climbing heights at habitats with different flooding patterns are necessary to test this hypothesis. Acknowledgments I thank the staff at Congaree National Park, particularly Dr. Theresa Thom and Ranger Fran Rametta, for their pictures and recollections of flooding at Weston Lake. 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