POLLINATION OF THREE SPECIES OF EUPHORBIA SUBGENUS CHAMAESYCE, WITH SPECIAL REFERENCE TO BEES

Euphorbia albomarginata, E. capitellata and E. hyssopifolia together attract over 200 species of insect visitors to their cyathia; bees that are oligolectic on Euphorbia figure prominently among these in both number of species and abundance of individuals. Euphorbia albomarginata, a mat-forming plant, attracts three times as many species as the erect species E. capitellata and E. hyssopifolia. All insects pollinate the plants by "mess and soil" behavior. Bees are more effective pollinators than other in- sects by virtue of their frequent movement on cyathia and their ability to carry pollen. Bees are abundant on the large, conspicuous cyathia of E. albomarginata and E. capitel- lata, but are less common on the small, inconspicuous cyathia of E. hyssopifolia. Plant growth form (mat vs. erect) probably exerts a major influence on the number of insect visitors, but cyathial size determines the abundance of effective pollinators. The evidence suggests that these species of Euphorbia have not evolved in response to their associated bees, despite the important role of the bees in the pollination ecology of the plants. I NTRODUCTION In this paper I describe the insect fauna responsible for pollinating three species of Euphorbia subgenus Chamaesyce (Euphorbiaceae), and evaluate the amount of coevolution that apparently has resulted from the interactions of the plants and the pollinators. Euphorbia albomarginata T. & G. cannot set seed without insect visitors; E. capitellata Engelm. can produce only a small fraction of its normal seed set with- out insects, and E. hyssopifolia L. sets abundant seed in the absence of insects. All three species are self-compatible (Ehrenfeld, 1976). The cyathia of all three species are flat and radially symmetric, their nectar and pollen are readily accessible, and they are visited by a diverse group of insects, of which many are solitary bees that are obligately dependent on these plants for food. I hypothesized that if selection for coadaptation of plants and pollinators were occur- ring, it would involve the bees, which furnish reliable pollination. (Coadaptation is- here taken to mean the result of an evolutionary process in which interdependent selection pressures are exerted by two (or more) groups of organisms on each other, resulting in a reciprocating pattern of evolutionary change in both groups (Baker and Hurd, 1968; Ehrlich and Raven, 1964). ) Coadaptation in this system might be manifested in the two insect-pollinated species of plants through special adapta- tions for promoting bee visits and discouraging other insects, and for increasing the efficacy with which bees pollinate the plants. Selection for coadaptation could also be demonstrated by differences in the plant-insect interactions between the insect- pollinated species and the self-pollinated species. In the course of testing these hypotheses, I have recorded the diversity of insects visiting these plants, and have assessed the effectiveness of the various kinds of insects as pollinators. The plants. -Euphorbia is characterized by a unique floral device, a cyathium, which is an inflorescence composed of many male flowers and one terminal female flower. Each flower consists simply of the sexual parts (in the case of the male flowers, a single stamen), and lacks calyx and corolla. They are tightly clustered within an involucre of five united bracts, each of which bears a nectar gland with an expanded petal-like appendage. Thus, the cyathium resembles a flat, simple