1975] 
Erickson — Papilio polyxenes 
H3 
experimental larvae were fed the same leaves as the maintained cul- 
tures. These randomly collected leaves were split along the midrib, 
one half weighed and offered to the larvae and the other half weighed 
and used to determine the percent dry matter in the leaf material 
(Waldbauer i960, 1964). 
Besides the percent dry matter in the leaf material, the calorific 
and nitrogen content of the leaf material, the larvae, and the feces 
were determined. Calorific values of the larval food plants, feces, 
and larvae were determined by means of a Phillipson non-adiabatic 
microbomb calorimeter (Gentry and Wiegert Inst. Inc., Aiken, S.C.) 
(Phillipson 1964). The lyophilized leaf material, feces, and larvae 
were subjected to 3 replications for the determination of calorific 
values. The organic nitrogen content of the leaf material, the feces 
and the larvae, were determined either by the Kjeldahl method for 
total nitrogen (Williams 1964) or the microKjeldahl method (Mc- 
Kenzie and Wallace 1954). A minimum of 3 replicate samples was 
obtained for the larvae and the feces, as well as each host plant 
species. 
The dry weight of food ingested by the larvae was estimated 
following the techniques of Waldbauer (i960, 1964), and Wald- 
bauer and Fraenkel ( 1961 ) except that plant material was lyophilized 
instead of oven-dried. The dry weight of the food utilized ar assimi- 
lated was assumed to be the dry weight of the food ingested minus 
the dry weight of feces. An additional group of larvae were reared 
along with the experimental larvae, and these were sacrificed to 
determine the dry weights, and thus, the percentage of dry matter of 
the larvae. Indices of food utilization were determined following 
the methods of Waldbauer (i960, 1964, 1968). Many terms have 
been used both by ecologists and by physiologists to describe various 
measures and indices of food utilization and efficiency. Relationships 
between many of these terms are discussed by Kozlovsky (1968) 
and Waldbauer (1968). 
As an index of digestibility, the ratio of the amount of food as- 
similated to the amount of food ingested, referred to as the ‘Assimi- 
lation Efficiency’ (Clark 1946, Odum 1957, Odum 1971), or the 
‘Coefficient of Digestibility’ (Waldbauer 1964, 1968, House 1965), 
was used. In practice, this measure is only an approximation since 
the numerator (as determined by the usual gravimetric procedure) 
does not quite represent the amount of food actually assimilated 
(Waldbauer 1968). This slight error is due to the presence of 
metabolic wastes in addition to the undigested food in the feces 
(Lafon 1951). For this reason Waldbauer (1968) has suggested 
