Callunæ was not recognized as British until the year 1847, when it was introduced as a species distinct from quercus. The late Richard Weaver, who gave it the English name of the "Scotch Eggar," took specimens of the moth at Rannoch in 1845, and he found caterpillars in that year, as well as in 1844 and 1846. It is now well known to occur not only in Scotland, including the Hebrides and Orkneys, but also on the moors of Northern England, and in Ireland and Wales. In North Devonshire it is found not uncommonly in the Exmoor district, and it has been recorded from various parts of the New Forest in Hants.
The egg of callunæ is figured on Plate [55]. It appears rather polished, and in colour is pale brown mottled with darker brown. The eggs are stated to be deposited whilst the female is on the wing, and consequently they fall to the ground or are arrested in their descent by the herbage over which they are scattered.
The full-grown caterpillar of quercus, beneath the brownish fur with which the body is clothed, is dark brown on the back and rather violet brown on the sides; the ring divisions are velvety black; there is a white stripe along each side and below the stripe some reddish marks; the ring nearest the head is edged with reddish, and the next two rings each have two reddish centred white spots. The dull purplish brown chrysalis is enclosed in a hard oval-shaped cocoon which is spun up on or near the ground in a flimsy web among herbage, dead leaves, etc. Sometimes it is placed among the twigs of the food plant.
In Southern England the caterpillars hatch from the egg in August and usually hibernate when quite small. They feed up during the following spring and early summer, perhaps in June or July, and the moth appears in July or August. Occasionally, however, a few individuals depart from the general habit and complete their growth the same year, hibernate in the pupal stage, and produce moths the next year, possibly earlier than hibernating caterpillars. On the other hand, perhaps owing to adverse weather conditions, feeding after hibernation may be continued well on into the autumn, when the caterpillars pupate,
but emergence of the moth is postponed until the following year, the second after hatching from the egg.
In the case of callunæ, at least as regards its normal habit in Scotland and southwards to the moorland districts of Yorkshire and Lancashire, the young caterpillar hibernates the first winter, feeds through the following summer, and passes the second winter as a chrysalis, the moth emerging in the following May or June.
Generally speaking, then, it may be stated that quercus has a twelve-month life cycle, whilst that of callunæ extends almost or quite to twenty-four months, of which at least twelve months are passed as a caterpillar. However, as has been noted, quercus sometimes passes one winter as a caterpillar, and another as a chrysalis, thus assuming the callunæ habit; whilst callunæ occasionally attains the perfect state during the summer following that in which the caterpillar left the egg.
The food plants comprise bramble, dogwood, hawthorn, heather (Calluna), and various low plants; it is even content with ivy.
Newman, in the Entomologist for 1845, gives a life history of the Northern Eggar (callunæ), and from this the following details are extracted. The male flies rapidly over the heather by day at the latter end of May or beginning of June; its flight is jerking or zigzag, and its object is evidently to find the female, who rarely moves until impregnation has taken place. Subsequently the female flies over the heather, dropping her eggs at random as she flies, and the eggs, having no glutinous covering, do not adhere to any object which they may accidentally touch in falling. On emergence from the egg the young caterpillar is dark ash-coloured, the divisions between the rings of the body being indicated by two minute orange streaks, each of which is accompanied by a small black spot. After the first moult the ground colour becomes more smoky, the divisions velvety black, and on each ring a triangular orange spot appears; these markings become more conspicuous later on, and by the end of October, when it hibernates, they are very distinct. It rests in a straight position, and, if disturbed, falls off its food plant, and rolls in a ring with its head slightly on one side.