This butterfly (Plate [26]) has the tips of the fore wings sharply pointed, and there is a rather acute angle about the middle of the outer margin of the hind wings. The colour of the male is bright sulphur yellow, with a central orange spot on each wing, that on the hind wings usually the largest; there is also a rusty dot at the outer end of the upper veins and along the front margin of the fore wings towards the tip. The female is greenish yellow, and is marked similarly to the male. In both sexes the horns (antennæ) are reddish, and the long silky hair on the thorax is a noticeable character. It is probably this insect to which the name "butter-coloured fly," contracted into butterfly, was first given; anyway, it is the only species to which the name applies so well.
The egg. If the under sides of the leaves of buckthorn (Rhamnus catharticus) or of the berry-bearing alder (R. frangula) are examined in May or June, the eggs of this butterfly may be found thereon. They are often placed on a rib of the leaf, but sometimes they are laid as shown in the illustration (Plate [25]). At first the colour is pale greenish and rather glossy, but it soon changes to yellowish, and later on, when the caterpillar has formed inside, to a dull purplish-grey.
The caterpillar when full grown is green, merging into bluish-green on the sides, thickly powdered with shining black specks. There is a pale line on each side below the spiracles. It feeds in June and July on both kinds of buckthorn, and will generally be found resting along the main rib of a leaf.
Larger Image
Pl. 22.
Clouded Yellow.
1, 3 male; 2, 4 female.