Natural History Studies on the Comal Springs Riffle Beetle (Heterelmis comalensis)




Cooke, Maria

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The Comal Springs riffle beetle (Heterelmis comalensis) is an endangered species inhabiting springs of the Edwards Aquifer. It is only known to exist in Comal and San Marcos springs and is threatened by decreased water quality and quantity, and habitat destruction. We tested H comalensis' restriction to spring outlets and assessed preference for several habitat variables in an effort to describe the determinants of the species' narrow range and provide information for protection of water quality and habitat. Furthermore, we tested substrate requirements needed for the larvae of H comalensis to successfully pupate and investigated the number of larval instars. To test the association of beetles with the spring outlets we buried long cotton strips in the spring openings. After four weeks we collected adults and larvae from the strip and recorded their distance from the opening. We then set up preference experiments, testing five different water quality parameters (well water, flow, CO2, temperature, and light) to find out which factors restrict this species to the spring outlets. We conducted all preference experiments with H comalensis and H vulnerata, a more widespread species that occurs in shallow streams, to compare preferences between the two. To test substrate types for pupation we set up aquaria with pupation chambers containing seven different substrate types, supplied with constant flow and food resources. The number of larval instars was determined by peaks in a frequency distribution, based on measurements of larval head widths. We found a significant association of beetles with the springs. The preference testing showed significant preferences for well water, low flow, elevated CO2, temperatures around 23 °C, and darkness. We did not detect significant differences between substrate types in our pupation experiment, likely due to inadequacies of habitats provided. We found H comalensts to have seven instars, within the typical range of elmid species. We concluded that H comalensis is restricted to a very narrow habitat at the springs and prefers water quality associated with the Edwards Aquifer.



ecology, elmidae, insects


Cooke, M. (2012). Natural history studies on the Comal Springs riffle beetle (Heterelmis comalensis) (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.


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