The Alimentary Canal, please note the hindgut and Malpighian tubules. (From The Insects: Structure and Function, By Reginald Frederick Chapman)
This week Trish gave an enlightening presentation about the hindgut, its function and the importance and variability of the Malpighian tubules. The hindgut is the terminal segment of the insect’s alimentary canal and is comprised of the Malpighian tubules, the pylorus region, the ileum and the rectum. Reabsorbing water and salts from the excrement of the insect before it is excreted is the primary function of the hindgut. The most anterior section of the hindgut, the Malpighian tubules, look like vermicelli noodles that are attached at one end to the hindgut, the pylorus region, and are free-floating in the hemocoel at the other end. They are responsible for absorbing the nitrogenous waste from the hemolymph, which is then transported into the hindgut to be excreted. The number and presence of Malpighian tubules can be extremely variable throughout Arthropoda. Schistocerca has 250 (!!) and at the other end of the spectrum Coccidae have two. Then there is Collembola, which doesn’t have any!
Colin presented a paper about Reticulitermes flavipes, the common wood-eating termite in the southeastern United States. Wood-eating termites have carbon-rich diet but are deficient in nitrogen. The termites synthesize uric acid but lack the enzyme, uricase, to brake it down into the usable form of nitrogen that they need. By using multiple tracer experiments the researchers were able to follow the path of uric acid through the termite’s system and they found that the uric acid is absorbed by the Malpighian tubules and transported to the sac in the ileum filled with symbiotic but bacteria. The bacteria brake down the uric acid into two parts, nitrogen and ammonia. The ammonia is excreted and the nitrogen is reabsorbed.
Keith presented a paper that explained how Chironomid larvae use their hindgut and Malpighian tubules to protect them from high levels of cadmium in their environment. Cadmium is a heavy metal that is highly toxic to living organisms because it is similar to calcium and will block calcium receptors causing bone softening in vertebrates, but many aquatic insects are insensitive to cadmium. The researchers in this paper found that Chironomids are capable of withstanding 25,000 to 500,000 times the LC50 values recommended by the United States Environmental Protection Agency. How do they do it?! Aquatic insects that are able to withstand such a toxic brew utilize at least one of three strategies. They may form a physical barrier (limits the uptake of the metal), excrete the metal through the fecal matter or urine, and metal detoxification. In detoxification the metal is sequestered by proteins and/or incorporated into inorganic crystalline concretions which makes the metal incapable of exerting its toxic effects. The researchers in this paper found that sequestration through the midgut and Malpighian tubules was very important method for Chironomid larvae. The cadmium would be pulled from the hemolymph by the Malpighian tubules and transported to the hindgut and eliminated. The most fascinating thing about this system is that if there is too much cadmium in the larva it will store it in the Malpighian tubules and not pass it into the hindgut. When the insect pupates, to make room for the adult tubules, the larval tubules are eliminated and the cadmium goes with it. How cool it that?
These two papers are a tie for the best of the week, cadmium sequestration vs. waste recylcing, it is just too difficult to pick one!!
Other papers of note:
1. How do mosquito larvae survive when the salinity concentration in their body is so much higher than that of their environment? Here is a nice photo she drew for us to help explain.
2. Malpighian tubules have stand alone pacemakers?! In Drosophila they do!
3. We know water is reabsorbed in the hindgut, but how does it happen. READ IT HERE!
