THE THORAX, WINGS AND LEGS
The thorax is composed of three segments, the front part or prothorax, the middle part or mesothorax, and the hind part or metathorax. The prothorax bears the front legs; the mesothorax the second pair of legs and the fore wings; the metathorax carries the third pair of legs and the hind wings. The under side of the thorax is called the pectus or breast. The large muscles which operate the legs and wings are contained in the thorax. One pair of spiracles or breathing-holes is found in the prothorax; the other seven pairs are located in the abdominal segments.
The butterfly has four wings, which are the largest and most conspicuous part of the insect. The wings consist of membranes stretched over horny tubes called veins; in the newly emerged insect the veins contain both blood and air, but the veins of the adult contain air only. The colors of the wing are due to minute scales which cover the membranes in an overlapping fashion like shingles on a roof. The scales vary considerably in size and form as well as in color, and the males of some species bear specialized scales known as androconia, which produce odors attractive to the females. The third of the wing nearest the body is the base; the middle part of the wing is the median or discal area; the outer portion is called the limbal area. The front edge is the costal margin; the outer edge is the external margin; the posterior edge is known as the inner margin. The angle of the outer and inner margins of the front wings is called the outer angle, and the corresponding angle of the hind wings is the inner or anal angle. The outmost tip of the front wing is called the apex.
The fore wing has three simple veins: the costal, the radial, and the submedian. There are also two branching veins, the median and the subcostal. The median vein has three branches or nervules, while the subcostal usually has four or five. The hind wing has five simple veins: the costal, subcostal, upper radial, lower radial, submedian, and internal. The costal vein in the hind wing usually has a short ascending spur called the precostal vein but it is classed as a simple vein none the less. The median vein has three nervules, as in the fore wing. In both fore and hind wings, between the subcostal and median veins, there is an area called the cell, which is often closed or partially closed on the outer side by three discocellular veins, designated as upper, middle, and lower.
Each of the six legs is divided into five parts. The section nearest the body is the coxa, which is attached to the ring-like trachanter. Next beyond the trachanter is the femur, then the tibia, and finally the tarsus or foot bearing the tarsal claws, which are used in clinging to various objects when the butterfly is at rest. In some species the fore legs are small and quite useless, a fact which is used in classification.