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:
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.
Image Links:
isfba.bugpeople.org/sites/CANF050000/html/2009-0421-NF050002-COL00824-Dytiscidae[2004h28s,T,A,aquarium]%7BEXD%7D-G.htm
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