1973 ] 
Erickson — Danaus plexippus 
231 
general viability of monarch adults which have stored higher con- 
centrations of cardiac glycosides. To test this hypothesis the fertility 
and fecundity of adult monarchs was determined. 
Methods and Materials 
Groups of intact larvae of D. plexippus 1 were taken from the 
second generation of a culture founded from wild insects taken near 
Ithaca, New York, and were reared in the laboratory on one of the 
following species of Asclepias 1 host plants: A. curassavica , A. syriaca, 
A. incarnata, or A. tuberosa. Newly molted 4th-instar larvae were 
placed individually in glass petri dishes (Pyrex, 100mm X 15mm) 
lined on the bottom with a piece of Whatman No. 1 filter paper. 
Mature and uninjured leaves of the native species were gathered in 
the field each day from plants growing in open sunlit areas, and 
leaves of A. curassavica were collected from plants grown in the 
greenhouse. All leaves were sealed in plastic bags and used within 
2 hours. These randomly collected leaves were split along the mid- 
rib, one half weighed and offered to the larvae and the other half 
used to determine the percent dry matter in the leaf material 
(Waldbauer i960, 1964). Leaves were replaced and feces collected 
every 24 hours. 
All the larvae were placed in a controlled temperature room, 
except for the period of time each day during which new food was 
offered to the larvae and the feces collected. The room was kept 
relatively constant with day-night temperatures of 22° and 18 0 , 
respectively, and with a relative humidity of approximately 55%. 
The photoperiod was regulated at a 16-8 hour light-dark cycle. 
The dry weight of food ingested was estimated following the 
techniques of Waldbauer (i960, 1964), except that plant material 
was lyophilized instead of oven-dried. The dry weight of the food 
utilized or assimilated was assumed to be the dry weight of the food 
ingested minus the dry weight of feces. An additional group of 
larvae was reared along with the experimental larvae, and these 
were sacrificed to determine the dry weights, and thus, the per- 
centage dry matter of the larvae. Indices of food utilization were 
determined following the methods of Waldbauer (i960, 1964, 
1968). Many terms have been used both by ecologists and by 
physiologists to describe various measures and indices of food 
Specimens of the insects used in this research have been deposited in the 
Cornell University Insect Collection, Lot 1023, Sublot 14. Specimens of the 
plants have been deposited in the Bailey Hortorium, Cornell University. 
