Emergence-site Selection by the Dragonfly Epitheca spinosa (Hagen)

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 (chi(2) = 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. High-climbing 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.