CHAPTER II
THE BUTTERFLY’S BODY
The body of a butterfly, like that of any other insect, is divided by constrictions into three parts: the head, the thorax, and the abdomen. The head carries the eyes, antennae, and mouth parts; the thorax bears the legs and wings; and the abdomen the sexual appendages.
THE HEAD, EYES, AND MOUTH PARTS
The head is globular, usually a little flattened from front to rear. Two large compound eyes are located at the sides of the head, and the face or front consists largely of a plate called the clypeus. Above the clypeus and between the eyes are the antennae or feelers, which are believed to be the organs of hearing, smell, and touch. Below the clypeus is the labrum or upper lip, and the rudimentary mandibles; just below these are the two maxillae, which unite to form a tube called the proboscis, used in sucking nectar out of flowers. When not in use the proboscis is coiled up like a watch-spring between the two three-jointed labial palpi. The labium or lower lip is very small in butterflies.
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 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 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 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.
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.
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 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 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.
The ovaries in the female butterfly are sometimes so large as to crowd the other organs in the abdominal cavity. They communicate by means of tubes called oviducts with the copulatory apparatus at the end of the abdomen. The testes of the male butterfly are usually combined into a single organ; they discharge the seminal fluid into the vas deferens, whence it is conducted to a sort of pouch near the penultimate segment of the abdomen. In copulation the ends of the male and female abdomens are locked together by certain clasping appendages, and the seminal fluid of the male is forced into the body of the female, where it meets and fertilizes the eggs as they descend from the ovaries.
