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Bees play an important and necessary role in the pollination of agricultural crops grown for fruit and seed and of forage and browse plants important in range management for production of both livestock and game animals. Nearly 20,000 different kinds of bees are now known, some, like the imported honey bee, the bumble bees, and the stingless bees of the tropics, are social and live together in colonies subject to manipulation and movement from place to place. However, by far the majority of bees are solitary, living in burrows in the ground, in wood, or in plant stems, where they store pollen and nectar as food for their young. Many of these take pollen only from certain kinds of plants and are very efficient pollinators of these. Others, like the honey bee, visit a wide variety of pollen plants, both native and introduced. Although management procedures have been developed for certain crops in certain areas which permit the effective use of honey bees for increased production through better pollination, these are not universally applicable to our agriculture as a whole. For certain crops, both cultivated and wild, supplemental pollination is necessary. The development of management procedures which will permit increased utilization of native solitary bees and perhaps lead to the importation of foreign species native to the areas in which our agricultural crops originated, requires a detailed knowledge of the ecology, or environmental relationships, of these bees. This article is intended to review as briefly as possible, the current status of our knowledge of the subject and to indicate published papers in which various aspects may be pursued further. Prominence has been given to publications of the last fifteen years and to those which are most comprehensive and include references to earlier literature. Factors which appear to have had an influence in the economy, evolution, and survival of solitary bees have been especially emphasized.

The interaction between plants and extremely diverse habitats and environmental conditions of California has resulted in rapid evolution in many plant groups. A small number of studies suggest that evolution in many of these groups has been accompanied and perhaps paralleled by evolution in particular groups of bees (Linsley and MacSwain, 1957, 1959; Linsley et al., 1963a, 1963b; 1964; Thorp, 1969). The interaction between these bees and their pollen plants is oligolectic. An oligolectic relationship is defined by Linsley and MacSwain (1957, 1958) as one ". . . in which the individual members of the population, throughout its range and in the presence of other pollen sources, consistently and regularly collect pollen from but a single plant species or from a group of similar or related plant species, turning to other sources, if at all, only when there is a local shortage or absence of that pollen." Oligolectic relationships may be nonselective or co-evolved. Included in the first class are Andrena linsleyi Timberlake, which collects pollen of Oenothera deltoides Torr. and Frem. in the morning after sphinx-moths have pollinated the flowers (Linsley et al., 1963; Gregory, 1964), and Andrena nemophilae Ribble which is discussed below. Such a bee must adapt to the evolutionary vicissitudes of the flower and its pollinator(s). The second class includes those bees which are pollinators of the host plant. Thus they may influence the evolution of the plant as well as being influenced by the plant. This group of bees and their pollen plants are of particular interest to the student of pollination biology and evolution. Although a number of studies have dealt with oligolectic bees, the basis of the relationship remains an enigma. The flight period of most oligolectic bees is short and the adult bee's only previous contact with a particular pollen source is the pollen and nectar it consumed as a larva. Three explanations have been suggested to explain oligolecty. First, it is genetically controlled (Thorp, 1969). Second, a preimaginal conditioning from the pollen and nectar that the larva consumes (Linsley, 1958). Third, some combination of the above, i.e., genetical control modified by conditioning (Thorp, 1969). In this paper my objectives are (1) to present a broad picture of the plant Nemophila menziesii H. and A. s.l. and its pollinators and (2) to re-examine particular questions concerning evolution within oligolectic bees. Specifically, I discuss the distribution, floral behavior and breeding system of the plant, Nemophila as a bee plant, the oligolectic Nemophila bees, the role of polylectic bees in pollination, and fly pollination. In the final section I discuss host plant recognition and evolution in oligolectic bees

The bee genus Diadasia Patton is a relatively large group of solitary or gregarious ground nesting species. It occurs only in the Western Hemisphere and is abundantly represented in southwestern United States and Mexico and in the arid and semi-arid portions of South America, but is rare in the moist tropics (Michener, 1954). Biological characteristics of Diadasia have been discussed elsewhere (Linsley, MacSwain and Smith, 1952, 1956; Linsley and MacSwain, 1957). It is the object of the present paper to emphasize the role that food habits and flower relationships may have had in evolution and speciation in the group. As with most organisms, food habits have played an important role also in the evolution and speciation of bees. The choice of larval food, as is usually the case with plant feeding insects, is made by the adult. The bee larva, to survive, must accept the food since it is unable to make an alternative choice. The larval food is pollen, usually mixed with some nectar, gathered from flowers by the female bee. However, bees visit flowers for a variety of reasons, other than pollen collecting. Flowers provide adult food (especially nectar) for both sexes of all species, the site of mating for many, and a male sleeping place for others. Of all these functions, the collection of pollen by females invokes the most consistently specific response to flower species, although, in some cases, the mating search by males may be equally so. Bees in which the females are highly specific in their choice of pollen sources are termed oligolectic, those which are not specific, polylectic. Flower constancy in the collection of pollen is a characteristic of bees in general. Two principal forms of constancy are involved. The first is characteristic of individual bees which, on one or several successive trips, may gather pure or nearly pure loads of pollen from a single kind of plant, although the species as a whole may be polylectic and not sharply limited in the kind and number of pollen sources utilized. This type of constancy has long been known to floral biologists, students of insect behavior, and research workers in certain production aspects of agriculture. It is characteristic of social bees (e.g. Apis, Bombus, Melipona), semisocial bees (e.g. Halictus), and many solitary bees (e.g., some Anthophora, Andrena, Megachile). The second kind of constancy was partially recognized by Loew (1886) and was more clearly defined by Robertson (1925).1 It is adaptive in nature and is characteristic of species, genera, and even higher groups of bees. This form of flower constancy is

The papers presented in this symposium focus on the reciprocity between plants and their associated fauna. Some studies approach plant-pollinator interactions at generic or familial levels and treat distributions over broad expanses of evolutionary time (e.g. Beattie, Gentry, and Ramirez). Others examine selected habitats and community interactions on ecological time scales (e.g. Ayensu, Heithaus, Macior, and Strong). Generally, one goal is to document specific boundaries which allow definition of floral resources relative to specific faunal consumers. I want to relate some of these new findings to recent research on consumer "switching" between alternative food resources. Early studies of pollination did not consider the gathering of pollen and nectar as generalized modes of foraging, but Robertson (1899) did recognize the similarity when he stated, "The relations of the host-bees to the flowers from which they get pollen are quite analogous to the relations of parasites to their hosts, of phytophagous insects to their food plants, or of predaceous insects to the insects upon which they feed ...." Pollination ecologists currently use terminologies and classifications for describing pollinator behavior which are different from those applied to predatory behavior, but there is a continued recognition of the general similarity between these distinct types of foraging. For instance, a pollinator which visits a single plant species for pollen and nectar is termed "monotropic," and Faegri and Pijl (1971) comment, similar to Robertson, that "its relation to the blossom is more or less on the lines of a host-parasite relationship." Michener (1954) has also noted that bees which collect pollen from a few kinds of flowers (termed "oligolecty") represent a "form of host specificity" (for a review of these terms seeLiusley, 1959). Yet, there has been little attempted transfer of methodologies from related studies on selective predation to the techniques used for studying selective pollination. In the following discussion, I propose an extension of a measure that I have recently developed for determining "rate of resource substitutability" among selective consumers (Covich, 1972a) to studies of pollinator energetics. Before discussing these points, I want to compare plant-pollinator and preypredator interactions. By definition, a pollinator is a dispersal agent for highly specialized pollen grains which transmit genetic information and produce sexual recombination and heterozygosis. Being direct links in the reproductive pathways of many flowering plants, pollen transporters have a strong influence on floral evolution and are tightly coupled to development of complex morphological and biochemical adaptations. Predators also have a direct effect on their prey populations, but predation (in the strict sense) can have a less intense impact if non-reproductive individuals (immature, weakened, or senescent com