43^ THE INVERTEBRATA 



eat their usual diet. Since the trichonymphids are known to digest 

 wood inside their own bodies, it is probably only indirectly that the 

 termites benefit from the wood, the flagellates being their immediate 

 source of food. Termites will live on a diet of cellulose (e.g. cotton 

 wool) but not when the last traces of nitrogenous material have been 

 removed. 



The majority of so-called saprophagous insects are really phyto- 

 phagous, in that they feed on yeasts, and micro-organisms effecting 

 the decomposition of the decaying matter. The house-fly is probably 

 such a case. Blow-fly larvae feeding on decaying meat do, however, 

 employ proteolytic enzymes, and to this extent are truly saprophagous, 

 as is also the dung beetle Geotrupes. The flesh-fly Luctlia, though 

 saprophagous in this way, still requires the microflora of the decaying 

 food to complete a diet suitable for full development, these organisms 

 supplying the vitamines necessary for growth. 



The great range of environments occupied by insects as a whole is 

 largely an expression of their diverse feeding habits, and few materials 

 have escaped their attentions. In addition to the foods mentioned 

 above, may be noted keratin, which undergoes fermentative digestion 

 in the larval gut of the clothes-moth Tmea biselliella. Silk can be 

 utilized as the sole diet of the museum beetle Anthrenus museoruniy the 

 amino-acids in this case supplanting both fats and carbohydrates. 



The saliva of various insects shows great variety according to their 

 habits; thus the larva of the tiger-beetle (Ctcindela), the flesh-eating 

 larvae of flies, e.g. Sarcophaga, and the aquatic larva of Corethra, pour 

 their saliva, which contains a proteolytic enzyme, on their food and 

 suck up the products of digestion {external digestion). Bees, with their 

 reliance on pollen and honey as food, have four different kinds of 

 salivary glands. These probably serve different purposes such as to 

 invert sugars, to ensure preservation of food by adding formic acid, 

 and to predigest pollen in the manufacture of " bee bread" on which 

 the young are fed. The proportion of carbohydrate to fat and protein 

 in the food after the early stages of feeding, determines whether a 

 larval bee shall become a queen (fertile female) or a worker (sterile 

 female). The former is fed throughout on a richer protein diet pre- 

 pared from pharyngeal glands while the latter has its diet changed to 

 pollen and nectar containing a higher carbohydrate content. In 

 wood-boring larvae the secretion of a mandibular gland softens the 

 wood and thus assists mastication, while, in caterpiUars, silk pro- 

 duction is the main function of labial glands. 



The principal excretory organs are the Malpighian tubules, opening 

 into the anterior end of the hind gut, and therefore are just as much 

 ectodermal structures as the nephridia of annelids. The proof of their 

 function is the presence of crystals, which can be identified micro- 



