Psyche 
[March 
114 
‘Approximate Digestibility’ as a less ambiguous term to describe this 
measure. However, Hiratsuka (1920) and Waldbauer (1964) point 
out that the uric acid content of the phytophagous insects is relatively 
low and that the difference between true and measured assimilation 
efficiencies is negligible. 
The efficiency with which ingested food is converted to biomass is 
calculated by dividing the dry weight of food ingested into the dry 
weight gained by the larva during the instar. This index, referred 
to by the physiologists as the ‘Efficiency of Conversion of Ingested 
Matter’ (Waldbauer 1968) and by ecologists as the ‘Ecological 
Growth Efficiency’ (Gerking 1962, Odum 1971), is an overall 
measure of an animal’s ability to utilize for growth the food in- 
gested. 
The efficiency with which digested food is converted to biomass is 
calculated by dividing the dry weight of food assimilated into the 
dry weight gained by the larva during the instar. This index, re- 
ferred to by Waldbauer (1968) as the ‘Efficiency of Conversion of 
Digested Matter’ and by Gerking (1962) and Odum (1971) as the 
‘Tissue Growth Efficiency’, decreases as the proportion of digested 
food metabolized for energy and maintenance of physiological func- 
tions increases (Waldbauer 1968). 
In his classic work on accessary growth factors, Hopkins (1912) 
pointed out that absolute quantities cannot be used to compare the 
intake of food by animals growing at different rates. Valid com- 
parisons could only be made on the basis of the rate of intake relative 
to the mean weight of the animal during the feeding period. Wald- 
bauer (1964, 1968) working on this basis proposed the ‘Consump- 
tion Index’ calculated in this experiment as : 
dry weight of food ingested 
duration of ^ mean dry weight of the 
feeding period animal during feeding period 
The mean weight of the animal is most accurately calculated from 
the area under its growth curve as determined by integration. A 
weighted average of daily weights will give an almost identical value 
if the growth curve is smooth (Waldbauer 1964). Such three vari- 
able equations are at times difficult to discuss, being included in the 
present work only for later comparison with other insect species (see 
Waldbauer 1968). From the equation, if 2 individual larvae ingest 
the same total amount of food, with the larva on the less nutritiously 
adequate food plant taking a great deal of time and gaining little 
biomass and the other larva on a, more acceptable food plant gaining 
