The larvae of this species are orange upon hatching but as they mature they turn yellow green.[8] They are slightly paler on their underside.[8] When mature they form a cocoon from soil and silk on the surface of the ground underneath their host plants.[8]
Hudson described the adults of the species as follows:
The expansion of the wings of the male is 1+1⁄4 inches; of the female 1+3⁄8 inches. On the forewings the green colouring is more or less confined to the neighbourhood of the veins and is much less pronounced than in T. tipulata; the basal and subterminal areas are transversed by several narrow cream-coloured bands, which are especially evident in the male, and the black markings are more extensive and slightly tinged with dull reddish. The abdomen of the male is considerably shorter than in the same sex of T. tipulata and the anal lope of the hind-wings nearly twice as large.[7]
T. agrionata is very similar in appearance to Tatosoma tipulata and can be confused with that species.[7] This species can be distinguished T. tipulata as it lacks the Z-shaped basal line and has a conspicuous large pale patch near tornus. Robert Hoare has also stated that T. agrionata has is a very distinct black V on the forewing dorsum near the base that appears to be a distinguishing feature as it is not present on T. tipulata.[9]
Larvae of T. agrionata are slow moving.[8] Adults emerge from July to May and it has been hypothesised that there are two generations per year.[8] The adults of this species can be looked for resting on tree trunks.[7]
^Walker, Francis (1862). "XXV: Geometrites (continued)". List of the Specimens of Lepidopterous Insects in the Collection of the British Museum. pt. 25: 1281–1477 – via Biodiversity Heritage Library.
^Walker, Francis (1862). "XXV: Geometrites (continued)". List of the Specimens of Lepidopterous Insects in the Collection of the British Museum. pt. 25: 1281–1477 – via Biodiversity Heritage Library.
^ abcdefDe Lange, Peter J.; Norton, David A., eds. (1997). New Zealand's Loranthaceous Mistletoes: Proceedings of a Workshop Hosted by Threatened Species Unit, Department of Conservation, Cass, 17 -20 July 1995. Wellington, N.Z.: Department of Conservation, New Zealand. ISBN0478019068. OCLC154315406.