Tau emerald

Tau emerald
	Male
	Female
Conservation status
	; Least Concern (IUCN 3.1)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Odonata
Infraorder:	Anisoptera
Family:	Corduliidae
Genus:	Hemicordulia
Species:	H. tau
Binomial name
	Hemicordulia tau; (Selys, 1871)

The tau emerald (Hemicordulia tau) is a species of dragonfly in the family Corduliidae.^[3] The species was first described by Edmond de Sélys Longchamps in 1871.^[2]

Description

Adult

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]

Larval stages

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]

Ecology

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]

Conservation status

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.