Termites have a symbiotic relationship with both flagellates and bacteria. The reason for this is that although they eat cellulose, they can not digestion cellulose
The flagellates are able to ingest the small particles of wood that have inturn been ingested by the termite. The flagellates produce cellulase and enzyme that works on cellulose. There are some questions as to whether the termites use the breakdown products released by the flagellates or whether they actually consume the flagellates themselves. If they did the latter, it would be somewhat self-defeating. One possible alternative is that they only eat the "aged" flagellates.
The termites pass their intestinal fauna on to their young, as the young are born with "clean" guts. This can be accomplished in at least two different ways. The young can pick up the symbionts by ingesting fecal materials produced by the adults. The other way is that the adults can regurgitate food materials to the young (such as is done by workers with respect to potential queens in a beehive.
Female mosquitos need a blood material before they can lay their eggs. Their mouthparts are designed to function as a hypodermic needle that passes through the skin of a host and into sub-epidermal capillary bed. In order to keep the blood flowing, they secrete via their salivary material an anti-coagulant. The blood passes into the digestive tract where breakdown occurs. The peculiar habits of mosquitos make them suitable as vectors for a variety of diseases
Perhaps one of the most important diseases is that of malaria. This sporozoan parasite reaches its host via injection of the parasite by a mosquito obtaining a blood meal. The transmissive stage is located in the salivary glands and gets carried into the host via the salivary fluid. One in the hosts digestive tract, the parasite moves through the intestinal wall and into the blood stream. The malarial parasite then has a number of options open to it. One option is to enter erythrocytes and go through a multiplication cycle. This cycle results in the fevers associated with malaria, as the parasites all are released from the red blood cells along with the waste product synchronously. The response of the host is to become feverous. Another alternative is for the parasite to go to the liver where a somewhat resistant stage is formed. The resistant stages in the liver explain why there can be reoccurrences of malarian many years after the original infection. The third alternative is to go into a transmissive stage for reconsumption by the mosquito host. The next time a mosquito bites, the infective stages pass into the gut of the mosquito and then pass through the digestive tract wall and finally wend their way to the salivary glands where the cycle can be started all over
An aside--in Central Africa where malaria use to be fairly prevalent, the inhabitants developed a natural defense against malaria. This defense is called sickle cell anemia. People with normal hemoglobin seem to be more susceptible to malaria. If the hemoglobin is sickled, the person is less susceptible. In this instance selection favors the heterozygote, as the homozygote dominant has non-sickled hemoglobin and gets malaria and the homozygote recessive has only sickled hemoglobin and is anemic and less fit. When Africans moved to climes without malaria, sickling of the hemoglobin was no longer favored and there has been some gene flows back to values favoring normal hemoglobin.
Tse tse flies have mouth parts somewhat like mosquitos and therefore also function as vectors in the transmission of disease. In this case, the disease is African Sleeping Sickness. Cattle and other ungulates function as the normal hosts, but human can be infected. If so, the infected person is very lethargic, enough so in some cases that they are incapable of reproducing.
Grasshoppers have chewing mouth parts. As their mandibles mascerate the food materials, they add saliva to the mixture. The saliva serves a variety of functions, one of which is to bind the food material together for the passage down the esophagus. A side effect of the saliva is that it apparently has an affect on the regrowth of the material the animals are feeding on. Studies have been done to ascertain whether it is the consumption of the food materials that causes them to regrow, or whether it is some substance present in the saliva. Although the former is partially true, the studies show that there is also a material in the salivary fluids that enhances plant growth.
Sea cucumbers are worm-like echinoderms that live on the ocean bottom. Their radial symmetry has been modified by the worm-like bodies to be a bilateral/radial symmetry. One species, Holothuria atra, from the Hawaiian Islands consumes "sand", actually a mixture of broken shells, coral, etc. They use their five tentacles to push the sand into their mouths. One major question concerns how much nutrient material is in sand. The sand itself has none, but it is often covered with a microflora and fauna. The question is then does the cucumber depend upon that material for its metabolic needs. The amount of organic matter in the sand can be analyzed both before and after consumption. Some studies have shown that there is the same amount of organic matter in the fecal materials as in the materials ingested. Out of this comes another possible explanation, that being that due to the slow passage of the material through the cucumber, the digestive tract is functioning as an incubation chamber and the sea cucumber is using the production of new creatures in its gut for its metabolic purposes.