1975] 
Erickson — Papilio polyxenes 
1 19 
figure converts to approximately 80 wet wt. mg which for a normal 
4th instar caterpillar comprises about 20% of its maximum net 
weight. In terms of the consumption index, or the rate, corrected for 
larval weight, at which food enters the gut, both groups of larvae 
were ingesting at approximately the same rate (P < .1), although 
larvae reared on wild umbellifer species did display a slightly higher 
consumption (Table 3). In terms of larval weight gained per day, 
the larvae reared on the cultivated umbellifer species gained about 
1 1.6 dry wt. mgs per day whereas larvae reared on the wild species 
averaged on the whole about 8.7 dry wt. mgs per day (P < 0.05). 
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 and energy utilization 
of insects is thus of particular importance to ecology since insects 
exert a substantial influence and impact on almost all terrestrial com- 
munities. Energy utilization studies and their ecological significance 
have been extensively reviewed (Englemann 1966, Phillipson 1966, 
and others) . 
Adaptive nutritional differences in host plants must be sought on a 
quantitative level and meaningful comparisons of food utilization and 
nutrition will not emerge until quantitative studies are carried out 
(Erickson 1972). The determination of absolute requirements for 
dietary constituents depends upon the measurement of food or nutri- 
ent intake. Differences in food utilization efficiency can be demon- 
strated only by 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 utiliza- 
tion of ingested or digested food. 
In this experiment, the larvae of the black swallowtail, P. polyx- 
enes, display a marked differential growth rate when reared on 
cultivated versus weedy umbellifer species. Larvae reared on cul- 
tivated umbellifer species gained approximately 25% more weight 
during the 4th instar than larvae reared on the wild or weedy um- 
bellifer species (Table 3). Since the larvae were generally from the 
same genetic background and were reared under similar conditions, 
