172 Comparative Animal Physiology 



animals utilize different compounds. In the digestive tract of Teredo wood 

 loses 80 per cent of its cellulose and 15 to 50 per cent of its hemicellulose.''"' ■*^ 

 The nutrition of wood-boring insects is summarized by Wigglesworth,^^" 

 and Mansour and Mansour-Bek, ^^' and data for beetles are given by Man- 

 sour and Mansour-Bek, ^^- Ripper, '■'" and Parkin, ^■^- and data for term- 

 ites by Cleveland •''^' ^" and Hungate. "'' Wood insects can be grouped into 

 several categories: (1) In some— Lyctidae, or powder post beetles, and Bos- 

 trychidae— only the stored food, starch and sugar in wood is used, and the 

 cellulose is unchanged in the digestive tract; these insects contain no micro- 

 organisms which can act on cellulose. (2) The bark beetles (Scolytidae) 

 consume the cell contents of the wood and can also digest hemicelluloses. 

 Other specific compounds are digested by other insects, as xylan by the Ceram- 

 bycid larva Phymatodes. m.^i- (3) Numerous Cerambycidae and Anobiidae 

 do digest cellulose. These insects, which either lack organisms that can digest 

 cellulose or else have them at early stages and then lose such organisms, have 

 true cellulase in their own digestive secretions and can attack heartwood, 

 which is low in sugar. (4) Several groups of insects harbor in their digestive 

 tracts bacteria and flagellate Protozoa which can digest cellulose. Larvae of 

 lamellicorn beetles have in the proctodaeal chamber of the digestive tube 

 bacteria which can split cellulose, and flagellates which cannot do so. In three 

 families of termites and in the wood-feeding cockroach Cryptocercns the 

 hind-gut is laden with bacteria and flagellates. '^^^ •^'■^' ■*" A cellulase can be 

 extracted from the hind-gut contents; the insects can live well on pure cellu- 

 lose. The protozoan fauna can be removed if the host animals are incubated, 

 starved, subjected to high oxygen tension or high temperature, and after 

 defaunation (but while still possessing bacteria) the termites or roaches die 

 in a few days or weeks on a diet of cellulose. Such defaunated termites can 

 survive if fed glucose or cellulose predigested by fungus, or if the Protozoa are 

 restored. The flagellates produce glucose from cellulose in mass culture and 

 live on purified cellulose as the exclusive carbohydrate. ^*''^ Particles of wood 

 are engulfed and digested inside the protozoan. The flagellates also produce 

 lower fatty acids, particularly acetic acid; their metabolism is anaerobic. '^'^ 

 In one family of termites bacteria are the organisms acting on cellulose. There 

 has been some question whether the termites derive their nourishment from 

 the bodies of the flagellates, ^^^ from the glucose formed, "*" or from the lower 

 fatty acids. ^'^ Current evidence favors the fatty acids as being the most 

 important. Termites survive but do not mature and reproduce on cellulose 

 without nitrogen; wood contains some nitrogen, and in decaying logs fungi 

 transport soil nitrogen to the wood from which it is taken by the termites. '^ 

 Proteolytic enzymes of termites have not been investigated; however, numerous 

 wood-boring beetles are known to have proteases. ^•^- 



In ruminant animals symbiotic microorganisms also digest cellulose. The 

 rumen usually contains a mixture of bacteria, yeast, and ciliated Protozoa. 

 Cultures of the Protozoa showed that Entodinium does not digest cellulose, 

 whereas several Diplodinia do digest it. '" However, the action of these 

 Protozoa is small, compared with that of the bacteria, and in numerous experi- 

 ments, ruminants (lambs and goats) have been freed of the Protozoa by 

 starvation and injections of copper sulfate into the paunch. Such defaunated 

 animals shows no nutritional deficiency. ^^^ '-^ Digestion of cellulose is car 



