Although the classification of this family is based almost entirely on wing-nervuration, yet there are some divisions of the Geometridae in which this character is remarkably variable, certain individuals frequently exhibiting considerable abnormality.[[308]] Amphidasis betularia is believed to have changed its variation considerably in the course of the last fifty years. Previous to that time a black variety of the species was unknown, but it has now become common; and it is believed that other species of Geometridae are in process of exhibiting a similar phenomenon.[[309]]
Fam. 37. Noctuidae (Owlet-Moths, Eulen of the Germans).—This very extensive assemblage consists of moths rarely seen in the day-time, of generally sombre colours, with antennae destitute of remarkable developments in the male (except in a small number of forms); proboscis and frenulum both present; a complex sense-organ on each side of the body at the junction of the metathorax and abdomen. The number of species already known can scarcely be less than 8000; owing to their large numbers and the great general resemblance of the forms, their classification is a matter of considerable difficulty. Although the peculiar structure at the base of the thorax was long since pointed out, it has never received any thorough investigation. Few other remarkable structures have yet been discovered: the most interesting is perhaps the peculiarity in the hind wings of the males of certain Ommatophorinae recently pointed out by Sir G. F. Hampson[[310]]: in the genera Patula and Argiva the form of the hind wings is normal in the females, but in the male the anterior one-half of each of these wings is aborted, and the position of the nervures changed; this condition is connected with the development of a glandular patch or fold on the wing, and is remarkable as profoundly affecting a structure which is otherwise so constant that the classification of the family is largely based on it.
Fig. 203—Brephos notha. Larva, newly hatched. Britain.
The larvae are as a rule destitute of the remarkable adornments of hairs and armatures of spines that are so common in many of the families we have previously considered; they are fond of concealing themselves during the day and coming out at night to feed; many of them pass most of their time at, or beneath, the surface of the ground, finding nourishment in roots or the lower parts of the stems of plants; this is notably the case in the genus Agrotis, which is perhaps the most widely distributed of all the genera of moths. Such caterpillars are known as Cut-worms in North America.[[311]] The great resemblance, inter se, of certain of these Cut-worms, much astonished the American naturalist Harris, who found that larvae almost perfectly similar produced very different moths. The majority of Noctuid larvae have the usual number of legs, viz., three pairs of thoracic legs, four pairs of abdominal feet and the terminal claspers. In some divisions of the family there is a departure from this arrangement, and the abdominal feet are reduced to three, or even to two, pairs. One or two larvae are known—e.g. Euclidia mi—in which the claspers have not the usual function, but are free terminal appendages. When the abdominal legs are reduced in number (Plusia, e.g.) the larvae are said to be Half-loopers, or Semi-loopers, as they assume to some extent the peculiar mode of progression of the Geometrid larvae, which are known as Loopers. In the case of certain larvae, e.g. Triphaena, that have the normal number of feet, it has been observed that when first hatched, the one or two anterior pairs of the abdominal set are ill developed, and the larvae do not use them for walking. This is the case with the young larva of our British Brephos notha (Fig. 203). Subsequently, however, this larva undergoes a considerable change, and appears in the form shown in Fig. 204. This interesting larva joins together two or three leaves of aspen and lives between them, an unusual habit for Noctuid larvae. When about to pupate it bores into bark or soft wood to change to a pupa, Fig. 205; the specimen represented closed the hole of entry by placing two separate doors of silk across the burrow, as shown at d. The anal armature of this pupa is terminated by a curious transverse process. The systematic position of this interesting Insect is very uncertain: Meyrick and others associate it with the Geometridae.
Fig. 204—Brephos notha. Adult larva.
Fig. 205 —Brephos notha. A, Pupa, ventral aspect; B, extremity of body, magnified; C, the pupa in wood; d, diaphragms constructed by the larva.
The larva of Leucania unipunctata is the notorious Army-worm that commits great ravages on grass and corn in North America. This species sometimes increases in numbers to a considerable extent without being observed, owing to the retiring habits of the larvae; when, however, the increase of numbers has been so great that food becomes scarce, or for some other cause—for the scarcity of food is supposed not to be the only reason—the larvae become gregarious, and migrate in enormous swarms: whence its popular name. The Cotton-worm, Aletia xylinae is even more notorious on account of its ravages. Riley states[[312]] that in bad years the mischief it commits on the cotton crop causes a loss of £6,000,000, and that for a period of fourteen successive years the annual loss averaged about £3,000,000. This caterpillar strips the cotton plants of all but their branches. It is assisted in its work by another highly destructive Noctuid caterpillar, the Boll-worm, or larva of Heliothis armigera, which bores into the buds and pods. This latter Insect attacks a great variety of plants, and has a very wide distribution, being found even in England, where happily it is always a rare Insect.