THE B VITAMIN REQUIREMENTS OF ANIMALS AND PLANTS 309 



ments of the parasite from the nutritional status of the host, since it is 

 extremely improbable that the diminished amounts of thiamine and 

 riboflavin per se improved the nutrition of the parasite. Unfortunately, 

 much of our knowledge of the nutrition of parasitic organisms stems 

 from data of this nature. 



If the information so far presented seems sparse, it is even more strik- 

 ing that data are virtually nonexistent regarding the B vitamin require- 

 ments of the Echinodermata and Annelida. This is particularly strange 

 in view of the relative ease in the handling of such animals as the earth- 

 worms and leeches. Similarly, but more obviously because of experi- 

 mental difficulties, nothing at all is known of the B vitamin requirements 

 of Mollusca, although some indirect data 1G suggest that the requirement 

 is a complex one. 



The Arthropoda. We have seen that the requirement for no single 

 B vitamin has been definitely established for any metazoan invertebrate 

 below the Arthropoda, and a similar lack of information exists with 

 regard to the economically important Crustacea. The Insecta, then, repre- 

 sent the only class of metazoan invertebrates concerning which we have 

 reliable information as to B vitamin requirements, and for some insects 

 relatively complete data are available. For this reason alone, the discus- 

 sion of this class is more extensive than that of the other phyla. 



Aside from the need for information regarding the nutritional require- 

 ments of insects, there are many factors to encourage their study. In 

 general, they are readily bred in a small space and with little time and 

 effort, and require a minimum of food and equipment for their study. 

 Various species are herbivorous, carnivorous, and omnivorous, and hence 

 vary widely in their natural nutrition. Factors such as weight, food and 

 liquid intake, and environment may be readily controlled, and for these 

 reasons insects are generally ideal experimental animals. Insects have 

 the further advantage in nutritional experimentation in that they possess 

 symbiotic microorganisms which may function to meet their requirements, 

 in a manner similar to that of the vertebrates. Thus of five beetles studied 

 by Fraenkel and Blewett, 17 Tribolium confusum and Ptinus Tectus 

 required thiamine, riboflavin, nicotinic acid, pyridoxine, pantothenic acid, 

 biotin and choline, while Lasioderma serricorne, Sitodrepa panicea, and 

 Silvanus surinamesis did not require all of these because of the presence 

 of intraocellular symbionts. It is possible, on the other hand, to raise many 

 insects in a germ-free condition, a feat involving great difficulties with 

 higher animals. Under such sterile conditions the nutritional require- 

 ments are generally found to be more fastidious, reflecting extra needs, 

 for metamorphosis, etc. 



