1973 ] 
Erickson — Danaus plexippus 
237 
A. curassavica to 117.2 for females reared on A. syriaca. The mean 
per cent of eggs hatching ranged from 87.3% (A. incarnata ) to 
93.2% (A. tuberosa). 
Discussion 
One of the major concerns of modern ecology is the description 
and explanation of the energetic relationships between and within 
various communities. A knowledge of the food utilization efficiency 
of insects is thus of particular importance to ecology since insects 
exert a substantial influence and impact on almost all terrestrial or 
fresh water communities. The ecological significance of such energy 
utilization studies have been extensively reviewed (Englemann 1966, 
Phillipson 1966, and others). 
It seems apparent that adaptive nutritional differences in host 
plants must be sought on a quantitative level and that meaningful 
comparisons of food utilization and nutrition will not emerge until 
quantitative studies are carried out. The determination of absolute 
requirements for dietary constituents depends upon the measurement 
of food or nutrient intake. Differences in food efficiency can be 
demonstrated only be measuring intake and growth. Measurement 
of the food intake and the utilization of this food elucidates to a 
great degree the physiological processes occurring in an insect since 
patterns of utilization may be different although food sources are 
similar in their ability to support growth. For instance, low food 
intake may be offset by a high utilization of ingested or digested food 
and a very high food intake may well lead to a very low efficiency 
in the utilization of ingested or digested matter. 
In this experiment, the assimilation efficiency of the larvae did not 
vary significantly among the various Asclepias host plants except for 
larvae reared on A. syriaca which had an efficiency about 8% higher 
than larvae on the other host plants. This means that the larvae 
reared on the various host plants were digesting and excreting ap- 
proximately equal amounts of food. The efficiency with which 
ingested food and digested food are utilized varied significantly with 
larvae reared on A. curassavica and A. syriaca having the highest 
efficiencies and larvae reared on A. tuberosa the lowest (Table 2). 
The efficiency with which digested food is utilized for growth will 
vary not only with the maintenance and respiration requirements for 
energy but also with the balance of nutrients in the food source. 
Larvae reared on A. tuberosa ingested almost twice as much food 
during the 4th instar as larvae reared on the other three host plants 
