From Water Tigers to Deadly Divers: Dytiscidae

Plop. After surfacing briefly for a respite of fresh air, the expert diver submerges into the murky deep. Soon spotting the fish of interest, he darts forward, his legs moving in strong, coordinated strokes. Stealthily approaching his target (which rivals him in size), he fearlessly charges for the kill. The prey writhes and wriggles helplessly as the diver sinks his prong-like mandibles into its flesh. The predacious diving beetle’s excursion was successful; dinner has been secured.

Predacious diving beetles belong to the family Dytiscidae, which is derived from the Greek root word dytikos, which means “able to dive,” aptly describing the impressive ability of these little aquatic predators. This family encompasses over 4,000 described species. Adults generally range in size from 1 to 2.5 cm (0.4–1.0 inches) long and are found in slow-moving, freshwater bodies of water around the globe, with only a sparse few species making their homes in leaf litters. Characteristic adult colorings include tan, dark brown, or black, often with gold markings. Specific patterning aids in characterizing subfamilies (Foster & Bilton).

Like the flippers of divers, adults have long, fanned hairs on their back tibiae, which move in synchrony when swimming (unlike other aquatic beetles that move their legs alternately). Additionally, the males of certain species have modified scaly pads on their front legs, which serve as suction cup-like endings that aid in attachments during mating.

Akin to a scuba oxygen tank, adult diving beetles are able to trap air under their elytra when they surface for air. Additionally, these diving beetles have their own form of “snorkeling” apparatus, with open spiracles at the highest point of their abdomen, also utilized when nearing the surface of the water to exchange oxygen. This allows them to remain partially or entirely beneath the water’s surface for extended periods of time (Marshall).

Water tiger

Like other Coleoptera families, Dytiscidae are holometabolous. They often lay their eggs upon laid on submerged vegetation, and upon hatching, the larval stage of the beetle emerges. Even from the larval stage, these divers are deadly to other aquatic creatures. Rather apropos, Dytiscidae larvae have gained the name "water tigers" as they hunt and pounce on other aquatic organisms, such as glassworms or tadpoles. Though more aggressive than adults, the larvae have the same sharp, yet slightly more defined mandibles (McGavin). However, as they develop into adults, these divers are also able to prey on small fish or even full-grown bullfrogs!

Interestingly, adult diving beetles have two pairs of compound eyes. When they rest upon water’s surface, one pair sits above the surface of the water and the other is submerged, enabling them to see both above and beneath the waterline. Furthering the diver analogy, like a pair of goggle lenses atop one’s eyes, the larvae of certain species, such as the sunburst diving beetles, Thermonectus marmoratus, possess one of the most unique principle eyes in the animal kingdom. They consist of bifocal lenses with two retinas, a distal and proximal one beneath it. Recent molecular studies of the first instar larvae have proposed the model of distal and proximal retina each absorbing different wavelengths of the light spectra, even portions of UV light. Though not entirely understood, this unique adaption is believed to contribute to enhanced sensory perception and a key contributor to the success of the larval stages (Maksimovic et. al).

Moreover, these diving beetles have cultural significance. They serve as a food source for various people groups, yet themselves may also be threatened by man’s contribution to water pollution. These tiny divers also dated back to the folklore of North America. According to traditions of the Native American Cherokee creation narrative, the water beetling, after being unable to find a place of rest in the "liquid chaos" of the water, elected to dive to the deep and bring up soft mud which was brought to the surface and spread out to create the dry land of the earth (Miller & Bergsten). Although aquatic in nature, these divers

Works Cited:

Foster, G.N., Bilton, D.T. (2014). "The Conservation of Predaceous Diving Beetles: Knowns, Unknowns and Anecdotes". In D.A. Yee (ed.). Ecology, Systematics, and the Natural History of Predaceous Diving Beetles (Coleoptera: Dytiscidae). pp. 437–462

Maksimovic, S., Layne, J. E., & Buschbeck, E. K. (2011). Spectral sensitivity of the principal eyes of sunburst diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae), larvae [Abstract]. Journal of Experimental Biology, 214(21), 3524-3531. doi:10.1242/jeb.058990

Marshall, S. (2017). Insects: Their Natural History and Diversity: With a Photographic Guide to Insects of Eastern North America. Firefly Books, Limited.

McGavin, G.C. (2010). Insects. pp. 86–87.

Miller, K.; Bergsten, J. (2016). Diving Beetles of the World: Systematics and Biology of the Dytiscidae. Baltimore: Johns Hopkins University Press. p. 20.

Voshell, J. R. (2003). A guide to common freshwater invertebrates of North America. McDonald & Woodward Pub.