Water measurers (a.k.a. marsh treaders or, more properly, Hydrometridae) have always been one of my three favorite groups of true bugs. They’re so delicate and alien-looking, and they’re just rare enough, for me anyway, that coming across one carefully traversing the surface of some pond still elicits child-like glee. This week we’ll cover the southern water measurer, Hydrometra australis, a species for which we have 30 specimens.

Hydrometra australis was originally described as Hydrometra lateralis var. australis by Thomas Say in 1832, based on at least one specimen collected in New Orleans. [Note: Hydrometra lateralis Say is a junior homonym of H. lineata Eschscholtz, and so Say’s lateralis is now Hydrometra martini Kirkaldy, 1900] The original description is included below. There are two synonyms, Hydrometra myrae Bueno, 1926 and Hydrometra myrae Hungerford, 1934, and some biologists feel that martini itself is a junior synonym of australis (i.e., they are the same species). The type specimen(s) is now lost, but clearly more research needs to be done.

Smallish (<12 mm long), elongate, thin, and cylindrical, with long, thread-like legs. Antennae long, oriented anteriorly on the head; head about as long as thorax, with eyes located about mid-length; apices of tarsi, which are subdivided into 3 meres, with single tarsal claws (see below). Wings, if present, strap-like. Second antennal subdivision 2.5x times as long as the first; supracoxal lobes of middle leg with 1 pit on each side of median line.

This species, like all water measurers, is a predator of small arthropods and an opportunistic scavenger of other small animals. With their long, beak-like mouthparts these insects pierce their food and suck the juices out of their prey. Semiaquatic springtails (Collembola) are commonly consumed by Hydrometra, and some studies suggest that this bug specializes as a predator on these hexapods (e.g., see Taylor & Wood 2000). Food discovery is made via olfaction, rather than by sensing the vibrations of dying insects in the water, which is the common strategy found in other semiaquatic Heteroptera. Maier (1977) provides detailed natural history observations of a related species, H. championiana.

H. australis are semiaquatic and can be found walking slowly on the surfaces of relatively still water (hence heir common name), often in the company of aquatic plants and floating algal mats.

This species is apparently one of (if not the) most common water measurers in the south, especially in the Gulf states. Our specimens are mostly from the Piedmont and Coastal Plain of North Carolina, though we do have a specimen collected as far north as Monroe County Indiana by Tom Daggy in 1949. The species is also known from localities south of the US border, deep into Central America.

• C. J. Drake and F. C. Hottes. 1952. Distributional and synonymical data and descriptions of two new Hydrometra (Hemiptera: Hydrometridae). Journal of the Kansas Entomological Society 25 (3): 106-110.
• Hydrometra australis species page at GBIF. http://data.gbif.org/species/13721412/ (0 specimens as of 13 January 2011)
• Say, T. 1832. Descriptions of new species of heteropterous Hemiptera of North America. New Harmony, Indiana, 39 pp. (Fitch reprint, 1858: 755-812)
• Chris T. Maier. 1977. The behavior of Hydrometra championana (Hemiptera: Hydrometridae) and resource partitioning with Tenagogonus quadrilineatus (Hemiptera: Gerridae). Journal of the Kansas Entomological Society 50 (2): 263-271. http://www.jstor.org/pss/25082933