Moth,(Rudolf5) 



UNE JULY AUG. SEPT. OCT. NOV. DEC. JAN. FEB. 



1186 FAT METABOLISM [pt. iii 



An interesting study of the fat metabolism of the eggs of the lackey 

 moth or tent caterpillar, Malacosoma americana, was made by Rudolfs. 

 Fig. 369, constructed from his data, shows the progressive decUne in 

 total fatty acids during the development of this insect. Evidently a 

 very large proportion is used 

 up, probably by combustion, in 

 agreement with the findings on ^ 

 the silkworm and the grasshop- : 

 per. As the chorionic coverings 

 constitute 52 per cent, of the 

 whole egg-mass, the fat-content 

 of the G.gg itself on laying would 

 be about 9-0 per cent, dry 

 weight. As the water-content Y\^. 369. 



decreased only slightly during 



development (see Fig. 369), the fat/water ratio decreased a good deal; 

 thus at laying there would be 9 mgm. of fatty acids per 100 mgm. of 

 water, but at hatching only about 2 mgm. 



1 1 -8. Combustion and Synthesis of Fatty Acids in Relation 

 to Metabolic Water 



It is exceedingly interesting to note the similarity between the 

 insect c^g and the egg of the chick. All these terrestrial eggs 

 burn relatively large amounts of fat, and, though this is perhaps 

 associated with the ease with which fatty acids can be stored in a 

 small space, yet it may also be related to the fact that fat, when it 

 burns, leaves behind all its original weight in the form of water. We 

 have already seen that one of the most difficult problems confronting 

 the earliest terrestrial animals must have been the proper supply of 

 water for their embryos, and it is therefore likely that the use of fat 

 as the principal source of energy is associated with the importance 

 of the water balance. Aquatic embryos which have no trouble in 

 this direction do not, as far as we know, burn large quantities of fat. 

 Babcock and others have calculated that 100 gm. of fat on com- 

 bustion yield 107-1 gm. of water, 100 gm. of carbohydrate, 55*5 gm. 

 of water, and 100 gm. of protein only 41-3 gm. of water. From 

 Table 126 of Section 7 it is clear that the chick burns much more 

 fat than the frog (approximately 80 per cent, in the former case and 

 20 per cent, in the latter case, of the total material combusted), and 



