50 INSECT PHYSIOLOGY 



system which will effectively break the sulphur linkages by means of 

 which the adjacent polypeptide chains in keratin are bound together. 

 Once reduced in this way the chains are hydrolysed by means of a 

 protease of tryptic type which is itself resistant to the very low oxi- 

 dation-reduction potential in the gut. The bee moth (Galleria) can 

 digest all the components of beeswax (except paraffin), but certain 

 bacteria in the gut are largely responsible for this. 



So many insects feed on the foliage and woody tissues of plants 

 that particular interest attaches to the question of cellulose digestion. 

 In the vast majority of species, the cellulose is quite unaffected by 

 passage through the intestinal canal: those plant cells that are not 

 ruptured by the jaws of the caterpillar pass through the gut with their 

 walls intact, although their contents may be digested ; the excrement 

 of many wood-boring beetles contains all the lignin and cellulose 

 that have been ingested with the food ; the very starch grains may be 

 protected from digestion by their pectin covering. On the other hand, 

 many insects can digest cellulose; sometimes by means of cellulase 

 they secrete themselves, sometimes with the help of symbiotic micro- 

 organisms; and hemicellulose (lichenin, &c.) is digested by most 

 phytophagous insects. 



Symbionts in digestion 



Many insects have a rich fauna of bacteria or protozoa in the gut, 

 but in none is this so prominent as in the hind-gut of wood-boring 

 termites. Here there is an amazing population of flagellates, ciliates, 

 and spirochaetes, which perform for their host the invaluable func- 

 tion of digesting cellulose. Termites with this fauna intact can thrive 

 indefinitely upon pure cellulose; but if the insect is deprived of these 

 organisms (by exposing it to a high tension of oxygen, for example) 

 its powers of digesting cellulose are completely lost, and unless re- 

 infected it soon dies. Cellulase can be extracted from the flagellates 

 in the gut of these termites and of certain cockroaches (Crypto- 

 cercus) ; whereas this enzyme is absent from defaunated insects. 



Certain Lamellicorn beetle larvae {Cetonia, Oryctes, Osmoderma) 

 that feed on pine-needles and such like, ingest with their food those 

 micro-organisms which ordinarily ferment cellulose in nature. These 

 flourish in the dilated hind intestine (the so-called 'fermentation 



