973 ] 
Erickson — Danaus plexippus 
239 
(see Table 3). As the ingestion rate increases, there is a negative 
correlation with the efficiency with which ingested or digested matter 
are utilized (Waldbauer 1968). This reduction in overall gross and 
net efficiency has been shown for Prodenia larvae (Soo Hoo and 
Fraenkel 1966) and by House (1965). The larvae reared on A. 
tuberosa may be viewed as being extremely ‘wasteful’ of the potential 
caloric content of their food, most likely passing great quantities of 
food through the digestive tract to secure a supply of some limiting 
nutrient or substance. The leaf material does not vary to any sig- 
nificant degree among the Asclepias species tested in terms of caloric 
content (Table 1), whereas the leaves of A. tuberosa contain a little 
more than one half the nitrogen content of the other three species. 
It has been suggested that the water content of food material will 
greatly affect the utilization of this material. It is generally the case 
with the swallowtail butterfly, Papilio polyxenes , that the efficiency 
of utilization of food decreases as the dry matter content of the food 
plant increases (Erickson and Feeny, in preparation). In this ex- 
periment, there is a significant difference in the dry matter content 
of the leaves, with A. tuberosa having the highest dry matter con- 
tent at approximately 21% and A. incar nata and A. syriaca the 
lowest dry matter content at about 17% (Table 1). It has been 
shown in this laboratory, that by varying the water content of leaf 
material, the utilization of food by the cecropia moth, Hyalophora 
cecropia is greatly affected (J. M. Scriber, in preparation). The 
effect is shown not in an increased intake rate, as occurs in monarch 
larvae, but in lengthened larval development times, in which the 
total food intake is increased but the rate of this intake remains 
relatively constant. It thus appears that the dry matter content of 
Asclepias leaves may have some influence on the utilization efficiencies 
but not to the degree found in this experiment. 
In keeping with Cody’s (1966) ‘Principle of Allocation’, I at- 
tempted to descry any increased ‘cost’ for D. plexippus larvae to 
detoxify or to incorporate cardenolides. It is well known that A. 
curassavica is highly toxic and contains at least 15 cardenolidies 
(Kupchan et al. 1964, Duffey 1970), three of which have been 
isolated from distasteful D. plexippus larvae (Parsons 1965, Reich- 
stein 1967) incorporated there as a defense against vertebrate preda- 
tors (Brower and Brower 1964, Brower et al. 1967, Brower 1969). 
Asclepias syriaca has been found to be only slightly toxic to verte- 
brates and to contain at least seven cardiac glycosides (Reynard and 
Morton 1942, Duffey 1970, and others). Asclepias incarnata and 
A. tuberosa are known to contain cardenolides but in much lower 
