The British species belonging to this family, with one exception, have the tips of the fore wings pointed and curved downwards, forming a sort of hook, hence the English name Hook-tips. The exception is Cilix spinula, a round winged moth, not at all like other members of the family, but its caterpillar is very like others of the group.
The bristle and catch arrangement for locking the wings is present in all the species, but the tongue or proboscis is absent, or practically so. The caterpillars are not furnished with anal claspers, therefore have only fourteen legs, that is, six true legs and eight false legs (pro-legs). The last ring of the body is more or less tapered, sometimes terminating in a point; the back is roughened with raised spots and warts, or humped. They feed on the leaves of trees and bushes, usually exposed, and they pupate in a silken cocoon, spun up between leaves, or in a folded leaf, of the food plant.
Of the eleven species occurring in the Palæarctic Region, seven are European, and six of these are found in the British Isles.
The fore wings are brown, whity brown, or whitish; the central area is crossed by three blackish wavy lines, a blackish blotch in the third line and two blackish dots between it and the second line; beyond there is a dark brown, or reddish-brown curved line from the tip of the wing to the inner margin. Hind wings similar in colour to the fore wings, but paler on the front area; crossed by five wavy dusky lines, sometimes not well marked except on the inner margin; generally, there is a black central dot. The paler forms have a dusky shading on each side of the curved line on the fore wings.
The egg is yellow freckled with orange, chiefly at one end. Caterpillar green, the back reddish-brown, except towards the black-marked yellowish head; two conspicuous warts on rings two to five, and less noticeable raised spots on the other rings, all bearing hairs. In a younger stage it is blackish, with white marks on the fourth and seventh rings; later it becomes greenish below, and the markings on the back of rings four, seven, eight, and ten are whitish or creamy. Until nearly full grown it usually lives on the underside of a leaf, the edges of which are turned over and held down by silken threads; sometimes it may be seen on the upper side of a leaf under a slight web. It feeds chiefly on birch, but is occasionally found on alder, in June and July, and in September and October, and may be obtained by searching or by beating, but the former, although perhaps slower, is much the better method. The moth is shown on Plate 68, and the early stages on Plate 69.
The species is widely distributed, and seems to occur, sometimes commonly, wherever there are birches, especially of bush-like growth, in most English counties and also in Scotland. In Ireland it appears to be somewhat local and scarce.
The general colour of this species is brownish; the fore wings are slightly tinged with ochreous and speckled with minute violet-tinged silvery scales; between the first and second brown lines there is an irregular ochreous brown mark enclosing yellowish spots; the violet-tinted glistening scales are most in evidence on both sides of the black mark before the outer margin. Hind wings similar in colour to the fore wings; crossed by two brown lines, the second with an ochreous brown blotch above it (Plate 68).
Caterpillar, yellow freckled with brown; clouded with brown on first three rings; a reddish brown irregular stripe runs along the sides and upwards towards middle of the back on rings five, six, eight, and nine; a double-pointed hump on the back of ring three, the points tipped with yellow. Head notched on the crown, dotted and clouded with brown. It feeds on the small-leaved lime (Tilia parvifolia), and may be found from July to September and even later. I have not seen a living caterpillar of this species; the above short description has been drawn from an inflated skin (Plate 69).
The only British locality for this species is the Leigh Woods near Bristol, where it was first met with in 1837. It is, however, very rare and difficult to obtain. Abroad it ranges through Central Europe to Livonia, Southern Sweden, and to Northern Italy. On the Continent the caterpillar feeds on the foliage of other trees than lime, and there are two broods in the year.