There is a good deal of variation on the under side, but chiefly of a minor character, and most often unconnected with variation on the upper side. The following are more important varieties.
Var. tessellata, the Straw May Fritillary of Petiver, and figured by him in 1717 and by Stephens in 1827, has the under side of the hind wings entirely straw-coloured with black veins. There are three large squarish yellow spots on the basal area, outlined in black; a yellow central band, margined and traversed by black lines. On the outer margin there is a series of yellow crescents, outlined in black.
Var. eos of Haworth (the Dark Underwing Fritillary) is the pyronia of Hübner and Stephens, and a modification of var. corythalia, Hübn. On the under side the fore wings are fulvous, and have two black spots in the discal cell, and a black band, intersected by the veins, on the central area. On the hind wings the basal third is fulvous with eight black spots; the central area is whitish intersected by the black veins. On the yellow-tinged whitish outer area there is a series of black-margined orange crescents; a row of black lunules precedes a thin black line on the outer margin.
The egg is upright, ribbed, and pale whitish-green in colour. As the caterpillar matures the shell becomes greyish. The eggs were laid in a cluster on a leaf of cow-wheat (Melampyrum pratense) as shown in the figure, but failed to hatch.
The full-grown caterpillar is black on the back, becoming olive tinged on the sides and olive-brown underneath; the divisions between the rings are olive. The whole of the upper surface, except a line along the middle of the back, is dotted with white, and there are eleven white-tipped orange or yellowish spines on each ring, except the last two and the three nearest the head; the first and the last each have four spines, the third has eight, and the second and the eleventh have each ten spines. The head is black marked with white, and is clothed with short, stiff, black hair or bristles (Buckler). The chrysalis is pale whitish-ochreous, the markings on the wing-cases are black, and those on the other parts are orange and black.
Cow-wheat appears to be the chief food of the caterpillar, but it will also eat, and has been found on, foxglove (Digitalis purpurea) and woodsage (Teucrium scorodonia). Plantain is also said to be a food-plant, but Buckler says that his caterpillars would not eat this. The caterpillars are rather shy in their habits, and, except when the sun is shining brightly, require to be carefully looked for among their food-plant and the dead leaves, etc., around. They hatch from the egg in July, feed for a few weeks, and then hibernate in companies under a web. In April and May they become active again, feed up quickly, and appear as butterflies in June and early July.
The species is, unfortunately, becoming scarcer in England than it used to be. It seems quite to have disappeared from some of the districts in which it was formerly common. No doubt in one or two of its old and well-known localities the butterflies, and perhaps the caterpillars also, have been too freely taken, and its natural enemies have probably completed the business. Clearings in woods or heathy borders of woods are the kind of places this species appear to prefer. Its headquarters in any given locality seems to be changed from time to time, so that the exact spot where it will occur next year cannot be predicted from this year's observations.
The butterfly seems to be unknown in Scotland, and has only been recorded from Killarney in Ireland. In England it is to be found in the counties of Essex, Kent, Surrey, and Devonshire.
Its geographical distribution extends through Europe into Asia Minor, East Siberia, and Northern Amurland. In Corea and Japan it is represented by a larger form known as var. niphona.