136 Comparative Animal Physiology 



two latter substances must be supplied in the diet, whereas the protein and 

 other vitamins can be taken from the host mucosa. -• ^^ 



Larvae of bark beetles, Synchroa and Dendroides, grow on sterile bark but 

 do not pupate unless the rhizomes of a fungus or some nonsterile bark is added. 

 Apparendy the fungus provides some unknown compound essential for pupa- 

 tion. ^^"^ Several beetles have been shown to require the identified B vitamins, 

 except para-aminobenzoic acid and possibly inositol; in addition, Tenehrio 

 requires an unknown called Bt, and Triholiiim needs two different unknown 

 factors found in yeast. •" Roaches fed a synthetic diet equal to the best crude 

 diet for growth nevertheless did not reproduce normally; the necessary factors 

 are unknown (Noland, unpublished). 



The diets of many animals under natural conditions are very restricted. 

 Sometimes dietary restrictions are related to feeding mechanisms (Ch. 6); 

 sometimes they depend on chemical sense and orientation with respect to 

 chemical stimuli. In other animals there appear to be unknown nutritional 

 factors which determine food preference. In no group are there so many 

 examples of restricted diets as among insects, particularly the herbivorous ones, 

 which mav be attracted chemically to single kinds of plant ^'^ and which may 

 eat only certain kinds of leaves. The chemical which attracts an insect to a 

 plant is not necessarily a nutrient. For example, silkworms can be forced to 

 eat some leaves other than mulberry, but they do not grow properly on the 

 substitutes. ^^'^ Female mosquitoes of most species require a blood meal for 

 normal egg development. In several insect groups polymorphism depends on 

 the diet, ^^" as the transformations of Hymenoptera. The identification of the 

 active essential components in the food of specialized insects and the study of 

 the ways these components cause morphological changes have scarcely been 

 started. 



To ascertain the true nutritional needs of an animal it is necessary to 

 measure not only growth rate but also the ability to reproduce or, better 

 still, to survive normally for many generations on an experimental diet. The 

 discovery of new vitamins for common laboratory animals is becoming more 

 difficult and chemical identification is becoming more prompt than in the 

 early period of vitamin research. It is likely, however, that as more kinds of 

 animals are studied as thoroughly as the rat has been, new essential substances 

 will be found for particular species. 



CONCLUSIONS 



It is possible that the first "organisms" were heterotrophic, but certainly 

 animals as we now know them evolved from autotrophic organisms. The 

 history of nutrition shows increasing dependence of animals on autotrophs 

 and a progressive loss of capacity to synthesize. Loss of synthesizing enzymes 

 and increasing dependence on external sources for needed organic compounds 

 is shown particularly well in parasites. For example, the more primitive 

 trypanosomes, parasitic in insects and plants, synthesize their needed hematin, 

 whereas those which parasitize mammals require hematin from outside if they 

 are to grow hi vitro. A comparison of the capacities of free-living and of 

 parasitic helminths to synthesize B vitamins would be of interest. Flagellate 

 protozoans (leucophytes) which can use acetates as carbon source form an 

 interesting scries with respect to ability to use inorganic nitrogen, nitrate, 



