All the species live in colonies, so that when a specimen is seen or captured others may be expected to occur on, or somewhere around, the same spot. The caterpillars bear a close resemblance to each other, and are not always easily distinguished.
Over thirty species of Zygæna are found in Europe, and about thirty-six more have been described from other parts of the Palæarctic Region. There are at least twenty-five Palæarctic species referred to the genus Ino, and about ten of these are European.
By most authors filipendulæ is regarded as the type of the genus Zygæna, Fabricius; but others refer this species, and its allies, to the genus Anthrocera, Scopoli, using the Fabrician genus for phegea, Linnæus. The latter species and its allies are perhaps more frequently referred to Syntomis, Ochsenheimer, the typical genus of the family Syntomidæ, the systematic position of which is near the Arctiidæ. It may be added that S. phegea, and also Naclia ancilla, have been reported as British. There does not seem, however, to be any reason to suppose that the occurrence of either species in Britain could be other than accidental.
The Transparent Burnet (Zygæna purpuralis).
Two Welsh specimens are depicted on Plate [146], Figs. 1 and 2; these are of the typical form. A rare aberration has the spots and the hind wings more or less suffused with blackish (ab. obscura, Tutt), but a still rarer variety has the crimson of spots and hind wings replaced by yellow (ab. lutescens, Tutt).
Newman in 1861 referred an Irish specimen to achilleæ, but a little later, after seeing other examples, in the same year he changed the name to nubiginea. Birchall (Ent. Mo. Mag., iii. pt. i.) figured four forms of the species from Ireland; his minos (Figs. 5a and 5b) seems to represent two modifications of ab. interrupta, Staudinger, in which form the red blotches are widely separated or interrupted by the ground colour; and his nubigena is made up of more or less typical purpuralis (Fig. 6a), and a variety (Fig. 6b), with red marks between the lower and central blotches.
The caterpillar (Plate [145], Fig. 1) is dark green inclining to olive above and paler below; the spots are black (outer row) and yellow (inner row); the line along the back is obscure whitish; hairs, whitish. It feeds on thyme and burnet saxifrage (Pimpinella). Trifolium and Lotus have also been given among other food plants. In late summer, and after hibernation, in the spring. The moth flies in June, and is locally common in Ireland (Clare and Galway), Wales (Abersoch), and Scotland (Oban, Loch Etive). It has been reported from Tintagel, Cornwall, and possibly, as suggested by Tutt, these Cornish specimens may turn out to be Z. achilleæ, the latest addition to our small band of Burnets. Perhaps the Scottish specimens recorded as purpuralis, or at least some of them, may prove to be achilleæ.
This species was figured by Brünnich, in 1763, as purpuralis, and authorities are now agreed that this name must be adopted in place of pilosellæ, Esper (1781), or minos, Fuessly (1782).