THE NEOICHNOLOGY OF AQUATIC INSECTS AND A POSSIBLE FOSSIL ANALOG

There are numerous groups of aquatic insects within the orders Hemiptera, or true bugs, and the Coleoptera, or beetles. Some of these aquatic insects have body fossil records extending back to the Late Triassic or Early Jurassic. Despite this long stratigraphic range, however, trace fossils of these groups are not well known. We conducted experiments with two hemipteran and two coleopteran species to characterize the trackways these animals would make while swimming in shallow water conditions. The hemipterans included Giant Water Bugs (Belostomatidae) and Water Boatmen (Corixidae), whereas coleopterans included Whirligig Beetles (Gyrinidae) and Predacious Diving Beetles (Dytiscidae). All of these insects swim with in-phase strokes of their thoracic legs, the distal portions of which are covered with hair-like structures producing a large paddle. Giant Water Bugs and Whirligig Beetles use both the mesothoracic and metathoracic legs during swimming, whereas the Water Boatmen and Predaceous Diving Beetles swim with only their metathoracic legs. All of the insects created trackways that were composed of two rows of lunate impressions with opposite symmetry, and individual lunate imprints that were concave in the direction of travel. There are some differences in the trackways, however. Giant Water Bugs and Whirligig Beetles, for example, typically produced trackways in which the lunate imprints were paired within rows, whereas the trackways made by Water Boatmen and Predaceous Diving Beetles lacked paired imprints. The trackways made by the Water Boatmen and the Predaceous Diving Beetles are similar to a fossilized trackway called Lunula obscura, which was described by Edward Hitchcock from Early Jurassic deposits of Massachusetts. Lunula, as the name implies, is composed of two rows of lunate imprints arranged one in front of the other and in opposite symmetry. The midline of the fossil trackway is occasionally marked by a medial drag. Hitchcock proposed that L. obscura was made by a myriapod. Based on the similarities with the modern trackways, however, we consider it more likely that the fossil was made by an aquatic insect.