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. [Illustration: 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