Australian Emerald and Tau Emerald Dragonfly: A Comparison

Tau Emerald Dragonfly
Hemicordulia australiaeBrownish eyes
Inverted T mark on the frons
Orangish leading veins on coastal margin
Brown pterostigma
Yellow mark on 10th abdominal segment
Black 7th abdominal segment

Location: South Western Australia

Season: Summer, December

Order: Odonata 

Family: Corduliidae 

Genus: Hemicordulia 

Hemicordulia Tau

Hemicordulia is a common species of dragonflies in Australia. Australian Emerald Dragonfly, H. australiae and Tau Emerald, H. tau are a bit tricky to distinguish in the field. However, I have learnt a few pointers towards identification. 

Similarities:

Size: Both are medium sized dragonflies, about 5 to 6 cms in length.

Perching pattern: H. tau and H. australiae both perch similarly. They hang on to the tip of the branches perpendicular to the ground. This feature makes it easy to distinguish the Hemicordulia dragonflies from other yellow dragonflies such as female Blue Skimmers, Yellow Striped Hunters which rest parallel to the ground. 

Hemicodulia species perch on higher branches and settle there for longer periods.

 Abdominal markings: Both have metallic dark green pattern over orangish yellow colour. This colour shines brilliantly in sunlight giving the name Emerald. The thorax is yellowish grey. 

Differences between H. australiae and H. tau 

Eyes: Mature H. australiae has bright greenish blue eyes that dazzle in the sunlight. H. tau has brownish eyes. 

Abdomen: H. tau has a yellow spot on the 10th abdominal segment (tip of the tail)

The seventh abdominal segment is darkly coloured in H. tau whereas the yellow marking is present in H. asutraliae.

Head: Inverted T shaped mark on H. tau is the telltale feature, hence the name. This is not present on H. australiae.

Wings: H. australiae has clear wings. The veins are black.

H. tau has brownish red to orange leading viens. The pterostigma is brown. 

Again, I present you with a not-so-spectacular image of H. australiae. It is about time I got a better picture of the Emerald, but some features are still remarkable in the photo below.

Australian Emerald
Hemicordulia asutraliae
Bright greenish blue eyes
No T mark on the frons
Clear wings with black veins.
Black/clear pterostigma
7th abdominal segment has yellow/orange marking

Reference: Wiki, Ecology Solutions, Esperance Fauna

Blue Skimmer Dragonfly: Orthetrum caledonicum

Blue Skimmer Dragonfly
Orthetrum caledonicum

Location: Western Australia, South West
Season: Summer, December
Order: Odonata 

Suborder: Anisoptera 

Family: Libellulidae 

Genus: Orthetrum 

 Orthetrum caledonicum

Blue Skimmer Dragonfly is commonly found in Australia. It is powder blue in colour, hence the name Sky Blue Dragonfly. The fly's body has a waxy coating and when light passes through the colloidal wax, all light is absorbed but blue light is scattered by the particles, making the insect look blue (Tyndall Effect). The process of colour change is called Pruinescence.

Blue Skimmer Dragonfly with remnants of yellow still on the abdomen undergoing pruinescence

The young males, which are freshly moulted from the larval stages are yellow in colour. This is before they are coated with the waxy substance which results in the blue colour. The colour changes from yellow to blue can be seen in maturing males. Males and females look different. The female is not blue at all, even the mature ones.

In the two photos below, the dragonfly is yellow, which could be a young teneral male or a female. 

Young Teneral Male or Female Blue Skimmer Dragonfly 

Young Teneral Male or Female Blue Skimmer Dragonfly 

When I first saw this yellow dragonfly which is actually a Blue Skimmer, I confused it with the Australian Emerald Dragonfly (and misquoted it on Instagram). The description and pictures in the field guides I read had stark similarities. Two things confirmed for me that these are Blue Skimmers. One, I actually saw the process of Pruinescence as demonstrated in the second photo. Two, I actually spotted an Australian Emerald Dragonfly! Please pardon the quality of the picture below, I must get a good shot next time. But the metallic green coloured markings on the abdomen can be seen. The eyes are bright greenish blue. 

Australian Emerald Dragonfly
Could be a Tau Emerald as T mark is not visible in the photo
Hemicordulia australiae