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.
THE ABDOMEN
The abdomen is composed of ten segments; the first seven bear spiracles or breathing-holes (completely hidden by scales, however) and the last two segments are modified to form external sexual appendages. In the male there is a pair of claspers for holding the female during copulation; in the female there is only a short and simple ovipositor.
THE DIGESTIVE APPARATUS
The alimentary canal, the principal part of the digestive apparatus, is a tube extending through the entire body from the end of the proboscis to the anal opening. Just above the proboscis is a bulb, which is enlarged by the contraction of muscles attached to the hard parts of the head. When the bulb is expanded nectar may be sucked up through the proboscis; then the valve at the end of the proboscis is closed, and the bulb contracted again. By this means the liquid is forced back into the esophagus or gullet, and hence into the crop. Just behind the crop is the stomach, and just behind the stomach is the small intestine. From the small intestine the part of the liquid not absorbed flows into the large intestine, which is divided into a front part or colon, and a rear and lower part called the rectum. From the rectum the fecal matter passes out of the body through the anal opening.
THE CIRCULATORY SYSTEM
The circulatory system consists essentially of a single blood vessel, running the entire length of the body in about the position occupied by the spinal column in the higher animals. This tube is open at the rear, and has valves opening inward all along its sides. In the thorax there is a pulsating enlargement which serves as a simple heart. The blood is a colorless liquid which percolates about through the entire body cavity, not being confined to any particular arteries or veins. It is ultimately collected into the dorsal blood vessel, and the pulsating heart keeps it moving, so that it absorbs food from the stomach and intestines, and distributes it to the various parts of the body.