We may, in general, readily distinguish this elegant insect from the last species—the females of which it rather resembles in its markings—by the difference in the ground tint of the wings, which in this vary from primrose or sulphur yellow to a greenish white.
There is, however, some risk of confounding this with the white variety of Edusa (Helice), a mistake often committed by young entomologists; so it will be well to point out the most prominent distinction between the two; and this is easily done, by observing that in Edusa the dark border of the upper wings is of nearly
equal breadth along the whole of the outer margin, and at the lower corner is continued inwards for a short distance; whilst in Hyale this border narrows rapidly, and disappears before reaching the lower corner of the wing. Also the dark border of the hind wings is much broader in Edusa than in Hyale. Here we have distinctive marks, quite independent of the ground colour of the wings.
The sexes of this butterfly are nearly alike in their markings, the chief difference being in the yellower ground tint of the males.
The same localities—viz. the south and south-east coast, and the adjacent district—that are most prolific in its near relative, Edusa, likewise furnish this species in the greatest plenty; but this is by far the rarer species of the two, and, either by coincidence, or in obedience to some direct law, several successive periods of its abundance have been septennial, or have occurred once in seven years. Thus the years 1821, '28, '35, '42, '49, and '56 are noted in entomological records as having produced it in great numbers.
On the coast of France, opposite to our own, it is one of the common butterflies, and it is not improbable that it frequently makes the passage of the Channel. The maritime habits of both this and Edusa are well known, and I have frequently seen the latter flying out to seawards, and coquetting with the waves, till the eye could follow the golden speck no longer. Taking advantage then of a favouring wind, its naturally strong
and rapid flight would quickly take it across the few miles of sea that separate us from the Gallic shore.
Hyale, whose flight is at least as strong as Edusa's, and whose salt-water tastes are similar, doubtless acts in the same manner.
The northward range of this species is more limited than that of Edusa, but it has been taken singly near York, Manchester, and a few other northern localities. In the lucerne fields near Brighton, a dozen or more have been sometimes captured in one day.
The caterpillar is of a sea-green colour, with four yellow lines, two along the back and one on each side; and is to be found, in June and July, feeding on lucerne and other plants of the same natural order.