64 PART I. DESCRIPTION AND BACKGROUNDS OF A POLYMORPHISM IN THE PINE LOOPER 1. THE OCCURRENCE OF A YELLOW-GREEN POLYMORPHISM IN PINE LOOPER CATERPILLARS IN THE NETHERLANDS 1.1. SOME GENERAL REMARKS ON THE OBJECTS OF STUDY The pine looper, or bordered white, is the only species of the genus Bupalus belonging to the lepidopterous family of the Geometridae. In Europe this moth has been an object of study since the last century, due to its being a very serious pest insect in some parts of the vast area in which it is found. Its food consists of needles of Scots pine, Pinus sylvestris L. The adult has a wing span of 3.5-4.0 cm. The male is dark brown with white or cream patches on the wings. The female has a greyish brown or an orange brown ground colour. Her markings are more or less similar to those of the male, though less distinct. The eggs are green, about 1 mm long, oval, with a shallow depression at the upper side. The first two larval instars are yellowish green, the first instar having a brown head, and the second instar a yellowish brown head. From the third instar onwards the larvae are green, with several disruptive white or yellowish longitudinal stripes both on the body and the head. An extensive description of the larvae has been given by HERREBOUT, KUYTEN & DE RUITER (1963). The pupae are green during the first few days following pupation, after which they become dark brown. The species is univoltine. The moths emerge from the end of May to the end of June. The eggs are deposited on pine needles, usually on the underside, in longitudinal rows from one to thirty. In the field the clusters appear to be smaller than under laboratory conditions, proba- bly because the wind tends to disturb the female and interrupt egg laying. First instar larvae hatch under field conditions after about three weeks. They do not eat their egg-shells, which are semi-transparent after hatching. The development of the larvae is slow, passing through five or six instars from the beginning of July up to the end of October. The full-grown larvae measuring about 4 cm fall to the ground and pupate in the litter. For more details on the bionomics, the reader is referred to ESCHERICH (1931) and BEVAN (1966). 65 1.2. THE YELLOW MORPH In addition to the common green caterpillars yellow specimens also occur in nature but less frequently. The latter differ from the former morphologically in several respects. Although indistinguishable in the first two instars, the third larval stage of the yellow morph gradually Fig. 1. The reflection spectra of yellow and green pine looper larvae and of pine needles. A general indication of the spectral colours is given below their wave- lenghts. .