There are approximately one million insect species in the world, but only 78 are known to digest cellulose. Termites are probably the most known and most efficient cellulose digesters. Although cellulose digestion is rare, it occurs in 20 insect families, indicating that it has evolved several times during evolution.
But why does cellulose digestion even matter? Cellulose digestion is important in forests, because the decaying of wood and other plant materials would be impossible without it. Without cellulose-digesting organisms, dead wood and plants would fill up our forests. Cellulases (which are cellulose-digesting enzymes) have been found in many herbivorous animals, including mammals, birds, reptiles and several invertebrates. Cellulose digestion by white rot fungi is the most precisely understood and three different enzymes operate during the process. However, characterizing the cellulases in insects is challenging. Most cellulase enzymes found from insect guts are not produced by the insects themselves, but by microbial symbionts. Researchers have found certain active enzymes that break down crystalline cellulose. In this way, insects can have active cellulose digestion of their own.
Four types of cellulose digestion occur in insects: exploitation of symbiotic protozoans in the hindgut, utilization of bacteria in the hindgut, usage of fungal cellulases received from food and production of own cellulases. Lower termites and wood roaches use protozoans to digest cellulose, whereas higher termites digest cellulose with the help of bacteria. Beetle larvae and siricid woodwasps get digestive enzymes from their food. But the production of insect cellulases is rare and contested.
The controversy of insect cellulases is based on the low value of cellulose as an energy resource. Insects get most of their carbon from sucrose and other oligosaccharides. Cellulose is difficult to digest, so it does not provide any extra advantages to insects. Nitrogen and water are additionally crucial resources affecting insect growth and reproduction. So what benefits could cellulose digestion provide insects?
Insects with gut symbionts benefit from protozoans and bacteria, as they do not have to produce the cellulases on their own. They receive some of their carbon intake from cellulose, which will increase their energy supplies. The same advantages are apparent in the fungal cellulases that insects receive from their food.
As there is little evidence of insect cellulases, more studies need to be conducted. Gene technology is one alternative for finding watertight evidence. If we could isolate and clone the cellulose genes, we may be able to find the processes behind cellulose digestion. This would put an end to the speculations. Findings of insect cellulases could also bring new information of insect and deadwood relationships.
Knowing whether insects that secrete their own cellulases prefer certain wood types would be interesting. For example, could they select certain tree species instead of other species? Or could individual insect species even be differentiated to individual tree species, as fungi flies are to certain fungi species? Maybe we will have answers to these puzzling questions in the future.