Compare the thorax with that of the beetle and that of the locust. The _prothorax_ is a freely movable narrow ring or collar. The _mesothorax_ and _metathorax_ are fused to form a large convex mass, of which fully five-sixths is mesothorax and only one-sixth metathorax. Try to distinguish the boundaries of the two segments. Note the three pairs of legs; the differences in size among them, and the differences between them and the legs of the locust and water-beetle. In one of the legs determine the _coxa_, _trochanter_, _femur_, _tibia_ and _tarsal segments_. Note the differences between the wings of the butterfly and those of the locust and beetle. Note that the wings are membranous, but are covered with many fine _scales_ (fig. 42), as is, indeed, the whole body. Rub off some of these scales on a glass slide and examine; note shape, little stem or pedicel of insertion, and longitudinal striations. Examine under microscope a bit of wing from which some of the scales have been rubbed. How are the scales attached to the wing membranes? How are the scales arranged? Note that the wing is colorless where the scales have been removed. All the colors and patterns of the wings of butterflies are produced by the scales. Make drawings of scales; of parts of denuded wings, and of bit of wing covered with scales. Remove all or nearly all the scales from a wing and note the arrangement of the _veins_ (_venation_). Compare with venation in wings of locust. Make drawing showing venation in the butterfly's wings. [Illustration: FIG. 42.--Bit of wing of Monarch butterfly, _Anosia plexippus_, magnified to show the scales; some scales removed to show the insertion-pits and their regular arrangement. (From specimen.)] The venation of insects' wings is much used in insect classification, and the various veins have been given names. The names of the veins in the butterfly's wings are given in fig. 43. When the veins in the wings of all the various groups of insects are studied, it is evident that the principal ones are the same in all insects, so that the costa, sub-costa, radius, media, cubitus and anal veins of the butterfly's wings can be compared with the corresponding veins in the wings of a beetle or wasp or fly. Noting the differences in the number and character of branching of these principal veins, and the number and disposition of the cross-veins which connect the longitudinal veins, the various kinds of insects can be to a large extent properly grouped or classified. A detailed account of the wing-veins of insects is given in Comstock and Kellogg's "Elements of Insect Anatomy," chap. VII.