The tau emerald (Hemicordulia tau) is a species of dragonfly in the family Corduliidae.[3] Corduliidae is the only family not a clade within the Infraorder Anisoptera. The species was first described by Edmond de Sélys Longchamps in 1871.[2]
The tau emerald is a medium-sized, black and yellow dragonfly with long legs,[4] reaching a body length of about 50 mm.[5] As is unique to dragonfly and mayfly species, H. tau has flight muscles attached directly to the wings,[6] and in both the males and females the inboard edge of the hindwing is rounded.[4] The 7th abdominal tergum is extensively black above and yellowish at the sides.[7] The top of the frons are yellow with a dark T-mark.[8] The species exhibits a linear relationship between head width and body length. [9]
Nearly all of the dragonfly's head is eye, allowing incredible vision that encompasses almost every angle except right behind them.[10] H. tau has a particularly large and well developed compound eye with two principal regions, the dorsal eye and the ventral eye.[11] The dorsal eye is principally sensitive to short wavelength light, while the ventral eye has 3 or more spectral types of photoreceptors.[11]
The length of time each larval stage of H. tau requires can decrease as the temperature of the water it inhabits increases.[9] It is capable of completing its life cycle from egg to adult in less than 6 weeks.[12] Nymphs reach a total length of 22–24 mm. No mid-dorsal abdominal spines are present unlike other Hemicordulia, but humps may be present on segments 4–9 and the lateral spines are very small.[13] The nymphs have a ladle-shaped labium with premental and palpal setae, and well-developed palpal dentations bearing groups of setae. The prementum has a distinct ventro-basal midline/groove.[8]
An analysis of head width frequencies of samples from Lake Eucumbene suggest there is at least 9 instars in the life history of H. tau, however the number of larval instars is known to vary between, and within, Odonata species.[9]
H. tau is found in all parts of Australia except northern Queensland and north-western Western Australia[8] and is one of Australia's most abundant dragonflies.[9] It is considered a dry-country dragon fly, often inhabiting the arid Australian inland.[14] The adults are vagrants[8] and highly opportunistic users of habitat for breeding.[9]
In addition to rivers, lakes and swamps[8] H. tau has the ability to breed in temporary waters due to its short larval stages.[15] Nymph have a high thermal growth coefficient and details of its life history may vary with their location.[9] The nymph are able to tolerate the high temperatures found in temporary shallow ponds.[9] The ability of H. tau to utilize temporary waters and reduce its larval life with increase in temperature contribute to its abundance and wide distribution.[9] They are also able to overwinter in cold permanent refuges,[9] and have been observed in water with a salinity between 7.8g/L to 13.9g/L.[16]
H. tau larva, similar to other Odonata species, are associated with clean water and it has been suggested they could be monitored to detect changes in water quality.[14][4] Larval habitat is typically leaf litter and weeds.[9] Many aquatic invertebrates with terrestrial adult stages require a snag extending above the water surface for emergence to the adult stage.[17]
Predators primarily include fish[9] and birds.[18]
The tau emerald is listed as least concern under the International Union for Conservation of Nature and Natural Resources (IUCN) red list of threatened species.[19] Population trend is classified as unknown and no conservation actions are recommended.
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: CS1 maint: url-status (link) The tau emerald (Hemicordulia tau) is a species of dragonfly in the family Corduliidae. Corduliidae is the only family not a clade within the Infraorder Anisoptera. The species was first described by Edmond de Sélys Longchamps in 1871.