1973 ] 
Erickson — Danaus plexippus 
235 
be found among the four Asclepias species. The nitrogen content 
of the leaf material did not vary significantly for A. curassavica , 
A. syriaca, and A. incar nata but the leaves of A. tuber osa contained 
a little more than one half the total nitrogen content of the other 
plants examined. 
The proportion of ingested food which was digested and as- 
similated (‘assimilation efficiency’) averaged about 50% for all 
experimental host plants except for A. syriaca which was signifi- 
cantly higher (P<0.05) with a mean of approximately 58%. 
(Table 2). The efficiency with which ingested matter was con- 
verted to biomass ranged from 29.59% for larvae reared on A- 
syriaca to 16.39% for larvae reared on A. tuberosa (P<O.Oi) y 
whereas the efficiency of conversion of digested matter ranged from 
53 - 3 °% for larvae reared on A. curassavica to 34.04% for larvae 
reared on A. tuberosa (P<o.6i). The duration of the 4th stadium 
also varied significantly (P<o.oi) with larvae on A. curassavica 
having the shortest duration (2.15 days) and larvae on A. syriaca 
having the longest duration (2.55 days). The amount of biomass 
gained per day per larva also varied significantly (P<0.05) with 
larvae reared on A. curassavica gaining the most weight (16.42 
mg/day) and larvae on A. incarnata the least (12.70 mg/day). 
Of great interest, however, is that the relative growth rate did not 
vary significantly for the larvae feeding on the four Asclepias species. 
The total allocation of energy by the larvae on the different 
host plants can be found in Table 3. The larvae reared on A. 
tuberosa ingested and assimilated almost twice as many calories as 
larvae reared on A. curassavica or A. syriaca. The total number of 
calories allocated to larval biomass did not vary to any extent among 
the Asclepias species; ingestion of more calories by larvae reared on 
A. incarnata and A. tuberosa was balanced by greater costs for res- 
piration and maintenance. The ratio of the total number of calories- 
allocated to an increase in larval biomass to the total calories 
assimilated ranged from approximately 65% by larvae reared on 
A. curassavica to approximately 32% for larvae reared on A. tuber- 
osa. The respiratory coefficient also ranged from 0.53 (A. curas- 
savica) to 2.16 (A. tuberosa). 
There was no difference in the mean length of the larval stage or 
the pupal stage among larvae reared on the various Asclepias species 
(Table 4). Larval survival was 100% on all host plants except 
A. incarnata where 90% of fhe larvae survived to the adult stage. 
Fecundity and fertility does not vary significantly; the mean number 
of eggs layed per female ranged from 103.7 for females reared on 
