Fig. I.—Diagram showing internal structure. 1, proboscis; 2, bulb; 3, brain; 4, heart; 5, crop; 6, dorsal blood vessel; 7, small intestine; 8, colon; 9, rectum; 10, anal opening; 11, Malpighian tubule; 12, stomach; 13, ventral nerve cord; 14, esophagus; 15, subesophageal ganglion.
THE RESPIRATORY TRACT
Insects have no lungs, and the blood does not carry oxygen about as in the higher animals; air is drawn into the body and brought into direct and immediate contact with the tissues. In the butterfly there are eight pairs of spiracles or breathing-holes—seven pairs in the abdomen and one in the prothorax. These spiracles are connected with large air-sacs reaching from one end of the body to the other, each air-sac being provided with minute branching tubes called tracheae, which carry air directly to the various tissues of the body. The carbon dioxide produced in the respiratory changes passes out through the spiracles, the transfer of gases being produced largely by movements of the abdominal muscles.
THE EXCRETORY ORGANS
The abdomen of the butterfly contains a number of slender Malpighian tubules, in contact with the blood contained in the various cavities. These tubules extract waste matter from the blood, functioning just as kidneys do in the higher animals. The butterfly has no bladder or urethra, however; the Malpighian tubules empty into the small intestine, and the urine passes out of the body with the fecal matter.
THE NERVOUS SYSTEM
The nervous system consists of the brain, the subesophageal ganglion, and the ventral nerve cord, together with branches of these structures. The brain is a large mass of nerve tissue in the head just above the esophagus. The two optic nerves which supply the large compound eyes make up the principal part of the brain, being much larger and more complicated than the cerebrum, which is supposed to be the organ of sensation.
The subesophageal ganglion is a sort of second brain lying just below the esophagus; it gives off nerves which supply the mouth parts and control the mechanism of feeding. The ventral nerve cord runs back from the subesophageal ganglion and traverses almost the entire length of the body, being analogous to the spinal cord of the vertebrates. It bears three ganglia in the thoracic region which give off nerves to the legs, wings, and thoracic muscles. Other ganglia, located in the abdomen, have many branching nerves which are distributed to the abdominal muscles and the viscera.