1973 ] 
Erickson — Danaus plexippus 
233 
of calories lost through respiration and maintenance per calorie 
allocated to biomass. 
Of general interest to ecologists is the ‘Principle of Allocation’ 
described by Cody (1966). Organisms have a limited amount of 
energy to spend and will be selected to partition this energy in dif- 
ferent ways depending upon changing physiological or environmental 
conditions. Any activity of an organism, or more precisely, the 
energy expenditure for that activity, can be viewed only in relation 
to all other demands for energy. To descry any increased ‘cost’ of 
detoxifying or incorporating cardenolides by D. plexippus larvae 
when reared on Asclepias host plants with known toxicity spectrum, 
a larval energy budget based on dry weights was constructed. Calo- 
rific values of the larval food plants, feces and larvae were deter- 
mined by means of a Phillipson non-adiabatic microbomb calorimeter 
(Gentry and Wiegert Inst. Inc., Aiken, C.S.) (Phillipson 1964). 
The lyophilized plant material was subjected to five replications 
whereas lyophilized larvae and feces were subjected to three replica- 
tions for the determination of calorific values. 
The nitrogen content of the leaf material was determined by the 
Kjeldahl method for total nitrogen (Williams 1964). A minimum 
of three replicate samples were obtained for each of the plant species. 
For the purposes of food utilization and efficiency determinations, 
the experiment was concluded when the larvae molted into the 
ultimate instar. The larvae were then reared through to the adult 
stage on the same experimental plants that they fed upon in the 
utilization experiments. All adult females were hand paired and 
placed individually in a 1 meter square screen cage with one A. 
curassavica and one A. syriaca plant of approximately the same age 
and condition. Records were kept each day of the number of eggs 
layed per female and the percentage of these eggs which were fertile. 
The data are generally presented as a mean and standard error 
for the larvae in any particular treatment group. The various 
experimental parameters and indices were subjected to one-way 
analysis of variance to determine differences in efficiencies or develop- 
mental rates among the various larval food plants. 
Results 
Various plant parameters differed greatly among the four Asclepias 
species offered to the monarch larvae (Table 1). The leaves of 
A. tuberosa were significantly higher in dry matter content than the 
other three species. No significant difference in caloric content could 
