HOW THE BLACK HORSE FLY: OBTAINS ENERGY
The black horse fly (female) ingests blood from a large mammal or (male) sucks nectar from flowers. This food is then taken into its digestive system. It moves down the esophagus and into the crop where it can be moistened and stored. It then enters the midgut region where it is digested by enzymes that are specific for the foods eaten. Nutrients are absorbed in pouches extending from the midgut called gastric ceca. This digested food then moves through interstitial fluid called hemolymph. Hemolymph enters the heart through ostia and is then pumped into vessels and then sinuses. In these spaces surrounding the organs, hemolymph delivers its energy-yielding nutrients to Joe cell.
OBTAINS OXYGEN AND ELIMINATES CARBON DIOXIDE
All body cells in the black horse fly are virtually exposed to the respiratory system. Tracheae, or air tubes, are branched throughout its body with tracheoles, or very fine branches reaching almost every cell. Air enters the tracheae through spiracles (openings) in the fly’s body. Gas is exchanged by diffusion across the lining of moist epithelium at the ends of the tracheoles. This liquid determines how much oxygen reaches the cells. Since the black horse fly is rather small this diffusion probably brings enough oxygen and removes enough carbon dioxide to support cellular respiration.
In flight, this insect, may consume 10 to 100 times more oxygen than it usually does. Air is, therefore, pumped rapidly though the tracheal system by flight muscles alternately contracting and relaxing.
ELIMINATES ITS NITROGENOUS WASTES
The black horse fly eliminates nitrogenous wastes through Malpighian tubules which are attached to the digestive tract. Nitrogenous wastes and salts accumulate in these tubules from the hemolymph. This material, including water, moves into the rectum where reabsorption of the salts and water takes place across the epithelium. The remaining dry nitrogenous waste in the form of uric acid is excreted through the anus with the feces.
OBTAINS NITROGEN AND PHOSPHORUS
The black horse fly obtains nitrogen and phosphorus in much the same way as it obtains its energy, from its diet. See above.
OBTAINS AND TRANSPORTS WATER
The black horse fly obtains and transports water in much the same way as it obtains its energy, from its diet. Possibly it gets the water it needs from the blood it takes in.
Most of what is mentioned above are anatomical adaptations that make the black horse fly well adapted to its environment:
1. Insects like the black horse fly have tracheae and spiracles which are parts of a respiratory system that is efficient, using very little energy and that is inside the body and protects the fly from losing too much moisture.
2. The Malpighian tubules conserve water as they move waste out of the insect’s body and therefore contribute to the success of these animals on land.
3. The black fly has blade-like mouth parts that allow it to slice into the thick skin of large mammals and enable it to suck their blood.
HOW ENERGY IS OBTAINED BY THE CATTLE EGRET
Chemical energy is obtained by the cattle egret through the oxidation of complex organic molecules. These organic molecules come from its diet. Foods, carbohydrates, fats and proteins, can be broken down and their monomers used to generate ATP through cellular respiration. (See section on obtaining nitrogen.)
HOW THE CATTLE EGRET GETS ITS OXYGEN AND ELIMINATES CARBON DIOXIDE
Air sacs and lungs are found in birds. The cattle egret must inhale and exhale twice for the air to completely pass through its respiratory system. Air passes through the mouth or nostril, down the trachea and enters the posterior air sacs during the first inhalation and bypasses the lungs until exhalation, when air is forced in one direction through the parabronchi. These are tiny parallel tubes that run the length of the lung. The next inhalation ends with the air entering the anterior air sacs. The anterior and posterior sacs fill and empty simultaneously.
The parabronchi are the functional equivalent of alveoli in mammals. And gas exchange occurs in air capillaries extending from them.
Oxygen travels by diffusion from higher partial pressures to lower ones into the interstitial fluid and the bloodstream where it is picked up by hemoglobin. This protein contains iron and has four subunits which hook up with four oxygen molecules and transports them to the left atrium of the heart. This oxygen-rich blood is then pumped into the left ventricle and then pumped out of the heart through the aorta and into the capillary beds of either the head and forelimbs or the abdominal organs and hind limbs. The oxygen is unloaded from the hemoglobin when it reaches a site where there are tissue cells with a lower partial pressure of oxygen. There it moves across the concentration gradient, through the interstitial fluid and into Joe cell.
After cellular respiration, carbon dioxide is expelled by Joe cell and diffuses across interstitial fluids back into the capillary beds and the blood plasma and into the red blood cells of the bloodstream. There the carbon dioxide is converted first to carbonic acid and then into hydrogen and bicarbonate ions. These ions travel in the blood plasma to the heart via either the anterior or posterior vena cava, into the right atrium where it is pumped into the right ventricle and then into either the right or left pulmonary artery. From there they enter the lung capillary beds where they are converted back into carbon dioxide and diffused out of the capillaries, through the interstitial fluid, and gas exchange occurs through the air capillaries of the lung and air sacs. the carbon dioxide is exhaled from the body through the trachea and out of the mouth or nostrils.
NITROGENOUS WASTES
Ammonia is produced when amino acids are broken down to be used in energy. Joe cell gives up that ammonia and it crosses the interstitial fluid, is taken up into the capillary beds and into the bloodstream. (By what transport mode?) Ammonia molecules are soluble in water and diffuse easily across membranes. (Why do they go?) In the bloodstream, the ammonia travels to the liver, either via the heart (left atrium, left ventricle) into the pulmonary artery, through the lung capillaries, into the pulmonary vein and back to the heart (right atrium, right ventricle) or directly. Ammonia enters the liver with the blood through the hepatic artery. There, the ammonia is converted to urea and leaves through the hepatic vein. Urea is then taken up, by the blood again, and this time is delivered to one of the kidneys.
Birds convert the nitrogenous waste, urea , to uric acid in the kidney. The blood carrying the urea enters the kidney through the renal artery. Blood pressure forces water, urea (and other solutes) into the glomerulus and into the lumen of Bowman’s capsule. The filtrate (of which urea is a part), passes through 3 regions of the nephron, the proximal tubule, the loop of Henle (only birds and mammals have juxtamedullary nephrons), and the distal tubule. Reabsorption of water and some nutrients occurs and uric acid is excreted as a precipitate through an opening at the rear of the bird’s body called the vent.
OBTAINS NITROGEN
Birds obtain nitrogen through their diet of both plant and animal foods. They swallow food that moves down the esophagus and into the crop where they can store it until there is room for it in the stomach, which has two parts. In the first part, digestive enzymes (most likely, HCl and pepsin) are added to the food. In the second part, the gizzard, the food is ground sometimes with gravel that the bird has swallowed. The stomach product, the acid chyme, passes into the small intestine where enteropeptidase (an enzyme which is bound to the intestinal epithelium), begins activating zymogens. These enzymes break up polypeptides into smaller peptides and amino acids. Nucleic acids are also broken down into nucleotides and nucleosides. These are pumped against gradients by the epithelial membranes of the intestinal villi. They cross the interstitial fluid, enter capillaries and are carried into the hepatic portal vessel by the bloodstream.
The amino acids travel via the hepatic portal vessel to the liver and then to the heart and lungs (as mentioned above). They leave the heart through the aorta. Then they may cross through the capillary beds, through the interstitial fluid and enter Joe cell where they can be used to synthesize new proteins
OBTAINS PHOSPHORUS
The cattle egret obtains phosphorus (a mineral) through its diet of insects and fish similar to the way in which it obtains nitrogen. It would not, however, be affected by enzymes in the stomach or small intestine.
OBTAINS AND TRANSPORTS WATER
The cattle egret lives in marshes. In order to obtain water, it would drink water with its mouth in much the same way that it would ingest food. This water would follow a similar path to that of obtaining nitrogen. Esophagus, crop, stomach, gizzard, small intestine, epithelial cells of the villi, across interstitial fluid, through capillaries into the hepatic portal vessel, liver, right side of the heart, lungs, left side of the heart, aorta, capillary bed, interstitial fluid, Joe cell. Because water follows salt, minerals in the fluids help distribute the fluids inside and outside of cells.
Some adaptations that make the cattle egret well adapted to its environment are:
1. Birds have juxtamedullary nephrons in their kidneys which allow for conservation of water.
2. Air sacs allow the bird to be more buoyant and cool the animal’s body during flight - when much heat is being generated.
3. Birds have fewer organs than most vertebrates which makes their bodies lighter in weight for flight.
BIBLIOGRAPHY
Campbell, Neil A., Biology. The Benjamin/Cummings Publishing Company, Inc., Menlo Park, California, 1996.
Klein, Animal Physiology. 199?.
gopher://psupena.psu.edu:70/0$d%20292011318 Food for Thought Jeanne Goldberg, 1993.
Whitney, Eleanor Noss and Sharon Rady Rolfes, Understanding Nutrition. West Publishing Company, Minneapolis, Minnesota, 1996.
The World Book Encyclopedia. World Book, Inc., Chicago, Illinois, 1988.