[4][2] Also in 1864 Walker, thinking he was describing a new species, named this moth Tinea bisignella.

[5] In 1891 Edward Meyrick, also thinking he was describing a new species, named this moth Decadarchis monastra.

[10] George Hudson discussed and illustrated this species in his 1928 book The butterflies and moths of New Zealand under the name Erechthias externella.

[2] Walker's assertion that the collector of the type specimen was Colonel Bolton was regarded by Dugdale as erroneous.

[2] The male holotype specimen was collected by Thomas R. Oxley in Nelson and is held at the Natural History Museum, London.

The fore-wings are elongate-oblong with the costa slightly arched and the tornus rounded; very deep purplish-black with coppery reflections; there are two indistinct darker transverse bands; a coppery crescentic mark near the apex, enclosing a paler, black-centred, eye-like spot which occupies the whole of the apical lappet; the entire wing is also irregularly strewn with bluish-white scales.

[13] E. externella is a sun-lover, and appears in early New Zealand spring, flying actively in the afternoon.