BUTTERFLIES. 733 BUTTERFLIES. series of horizontal, almost parallel, slightly di- vergent nervules. On the position of these the identification of species is most securely based. . . . In the moths, on the other liand. the dis- eoidal cell is less conspicuous." The names of the parts of the wing, and of its veins and nervules, used by entomologists in their descrip- tions of species, are given in the accompanying illustrations. To further increase the power of the pinions, the pair on each side are made to act as one. This adjustment is effected either by an overlapping of the bind wing by tile front wing (buttertiies or some of the larger moths) or the posterior wing possesses a 'frenulum.' composed of one or more bristles, which fits into a 'retina- culum,' a membranous llap or a bunch of scales on the anterior wing (otheii moths) . According to Hampson, "the form of the frenulum is of use in determining sex, as in the males of all the forms that possess it it consists of hairs firmly soldered together so as to form a single bristle, while in nearly all females it consists of three or more bristles, separate and shorter than that of the male." Stales. — The wings of all Lepidoptera, as the word implies, are clothed more or less completely with scales, which are modified hairs — hairs that are very short and much widened; and every gradation may be found, in a species like Itho- uiyia, between the hairs on the body and wings and the scales. They are like small chitinous bags with the sides pressed together, and each one has on its proximal end a short stalk which fits into a cavity of the wing-membrane. They are of various shapes, notched on the posterior margin, striated, etc., and "the males of many species have peculiarly shaped scales arranged in tufts and folds, which are called 'androeonia,' and are useful in microscopically determining species." The scales are in rows, and overlap much as do the scales on a fish or the shingles on the roof of a house. They rub off easily, and entomologists know how to remove them with- out serious injury: but when taken from a living insect they diminish or destroy its ability to fly. They numljer hundreds of thousands, and their use is to strengthen the membranous wings, and when they overlap the wing-membranes at the adges to a considerable extent, as occurs in some cases, they also increase the wing-area. Another use is to bear the colors of the wings, for when the scales are removed the color is gone. This color is due either to pigment contained within the scale or its walls, or to the fine striations on the upper surface which give rise to metallic 'interference colors.' Both albinism and mela- nism occur. The pigments are perhaps in the nature of biliary excretions, such as urates from nitrogenous matter and melanins from carbona- ceous matter. Distinciicms of Sex. — ^The abdomen is com- posed of segments, nine for the female and ten for the male, and contains the viscera, and the lateral spiracles by which air is admitted to the respiratory system. It is shorter in most butter- flies than the hinder wings; and in most moths is tufted along the dorsal line and on the end. The terminal .segment has various appendages, and contains the sexual organs of both sexes. There is often a very striking difference in size, color, and form t)etween the females, especially among the butterllies, where procreation may be the sole duty of the imago during its brief sum- mer existence. In case there are several broods of butterflies in a season, each brood may have its characteristic coloration. Our Ajax butterfly is three-brooded, and before the facts of its life- history were known, each brood had been given a specific name. By artificially varying the tem- perature or moisture, any or all "the sea.sonaI forms may be produced at" will from one and the .same laying of eggs. The males, which are usu- ally more gayly decorated than the females and e.Kceed them in number, are continually in search, about the food-plants, of mates, wlio exert a far- re.iching attracting power. Collectors utilize this instinct: having caught a female they ex- pose it in a cage and soon are likely to" find several males flocking about it. Under certain circumstances eggs may be laid by an unferti- lized female (for which see Repeoductiox and Pabthexogexesis ) . Adherents of the doctrine of sexual selection believe the female exercises a choice among these assembled suitors, selecting for her partner the best, according to the stand- ard of the species, and so maintaining the high quality of the race. A single impregnati(m is sufficient, and the impregnated females soon begin to lay eggs, having accomplished which, they die, in the great majority of cases, the ex- ceptions being those which are double-brooded, or (a -very few) where the adults largely survive the winter. Hibernation and ^Migration. A few butter- llies, such as the mourning-cloak, are able to en- dure in a state of torpidity the winters of the north. . large number winter over as pupie, and others, like the brown and black Isabella cater- pillar, as well-grown caterpillars. Others hatch out only in time to go into winterquarters. Many winter over as eggs, and not a few in two differ- ent stages, the latter having a double chance of surviving. It has been established by at least one set of careful observations that the cabbage butterfly (see Cabbage Insects) of Southern Europe migrates or flies in a general southerly di- rection in the fall and northerly in the spring. In the United States the milkweed butterfly (q.v.) sometimes so migrates in enormous swarms. Such migrations are even more common in the tropics. In his work on Ceylon, Sir .Tames Tennent writes of "the extraordinary sight of flights of these delicate creatures, generally of white or pale yel- low hue, apparently miles in breadth, and of such jirodigious extension as to occupy hours and even days uninterruptedly in their passage." These migrations are at times occasioned by lack of food-plants on which to deposit eggs. In other cases we know they are seasonal. By going south the butterflies find a climate in which they are able to winter. Reproductiox and Metamorphosis. The esgs of all Lepidoptera are laid on or near the food-plant, that is. the plant upon which the young must feed. In number they vary from less than one hundred to several thousand, and are deposited continuouslv and rapidly, as a rule. They may be placed singly, as is coniUKm among butterflies, or, as is more usual among moths, in clusters or masses, adhering to their support and perhaps to each other by a glutinous coat- ing; while some moths prepare a sort of nest of hairs plucked from their bodies upon and within which the eggs rest, or otherwise protect them from observation or the weather, ^specially tho.se destined to last through a northern winter
