Floral Relationships of Bees

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Wind and bees are the world's most important pollinating agents. Bees are either beneficial or actually essential for the pollination, and therefore for the sexual reproduction, of much of the natural vegetation of the world, as well as for many agricultural crops (see Sec. 3). The pollinators are primarily female bees, which collect pollen as the principal protein source in their own food and especially to feed their larvae. Flowers produce not only nectar and sometimes oil but also excess pollen as bait or reward. The pollen that may fertilize ovules is that which bees lose inadvertently on floral stigmata as they go about collecting nectar, pollen, or other material. Male bees of nearly all species, as well as the females of parasitic species, take nectar from flowers but carry only the pollen that happens to stick to them. They thus play a role in pollination, but a less important one than that of the females, which actively collect pollen and (as workers) in eusocial groups are vastly more numerous than males. Parasitic bees are often not very hairy and thus probably play a less significant role in pollination than do males of hairy bees, which are likely to carry abundant pollen. Wcislo and Cane (1996) and Westerkamp (1996) gave excellent recent reviews of floral resource utilization by bees. Barth (1991) provided an excellent account of flowers and their insect pollinators, and referred to older books on the subject.

There must have been a sort of general or diffuse co-evolution, diverse species of plants influencing and being influenced by a diverse fauna ofbees. Many bees take nectar from the same flowers that provide them with pollen. Thus features of a flower that facilitate nectar collecting and the corresponding features of bees that also facilitate nectar collecting are important to the flower because of associated pollen transfer. Short-tongued or minute bees take nectar from shallow flowers like those of Apiaceae. Longer tongues are needed to remove nectar from deeper flowers. Most kinds of bees are generalists in kinds of nectar utilized, although they may exhibit preferences and may be unable to reach nectar in some kinds of flowers. A few bee species, however, have morphological adaptations, such as palpi that fit together to form a sucking tube, that are associated with apparent specialization for gathering nectar from particular kinds of flowers. Examples are described and illustrated by Houston (1983c) and Laroca, Michener, and Hofmeister (1989); see also Figure 19-6.

One group perhaps involved in population- or species-level coevolution with plant hosts consists of the bee Rediviva (Melittidae) and its principal floral host, Diascia (Scrophulariaceae), in South Africa. The front tarsi of females are equipped with fine, dense hairs that sop up oil from inside the floral spurs. The spurs vary in length in various populations or species of Diascia,, and the forelegs of female Rediviva vary in length accordingly (Fig. 6-1); some have forelegs longer than the entire body (Fig. 6-2). Details and alternatives are discussed by Steiner and Whitehead (1990, 1991).

Complex interactions frequently characterize relation ships between even ordinary nectar-collecting and pollen-collecting bees and their floral food sources. Just because a species of bee visits a flower species does not necessarily mean that the bee is a pollinator of that flower. Small bees on large flowers may collect pollen, nectar, or both without going near the stigmata. In this case there is no pollination; the bee is merely a thief. An example is Perdita kiowi Griswold, a whitish bee of the North American high plains that is a specialist harvester of pollen from the long stamens of the large, cream-colored flowers of Mentzelia decapetela that open in the late afternoon. It rarely goes near the pistil; presumably, pollination is ordinarily by moths.

Thievery, such as that described above, merely reduces the amount of pollen available for food or distribution by actual pollinating insects. Some bees, however, damage flowers while at the same time robbing. Various kinds of large bees, especially Bombus and Xylocopa, cut open the sides of tubular flowers and extract nectar without contacting the anthers. Thus not only is the corolla damaged but the amount of nectar reward for legitimate pollinators is greatly reduced. Meliponine bees may chew into closed flowers or anthers, removing nectar or pollen and causing the flower major damage if not its destruction.

The effectiveness of a bee as a pollinator depends on many factors, unfortunately not always studied by people investigating pollination. A bee that has come from other flowers on the same plant or the same clone is unlikely to cross-pollinate. A bee that combs pollen off most of its body and appendages for transport in the scopa (pollen-transporting brushes or areas) is probably less likely to pollinate the next flower than a bee that leaves the pollen where it lodges on its body as it seeks more. A bee that moistens pollen with nectar or oil for transport is presumably less likely to pollinate than a bee that carries pollen dry and loose. And the location where pollen is deposited on the body of the bee can be critical for later b // '4

Figure 6-1. Front legs of females of Rediviva, showing elongation for oil collecting. a, R. rufocincta (Cockerell); b, R. colorata Mich-ener; c, R. peringueyi(Friese); d, R. longimanus Michener; e, R. emdeorum Vogel and Michener. (Scale line = 1 mm.) From Vogel and Michener, 1985.

Figure 6-1. Front legs of females of Rediviva, showing elongation for oil collecting. a, R. rufocincta (Cockerell); b, R. colorata Mich-ener; c, R. peringueyi(Friese); d, R. longimanus Michener; e, R. emdeorum Vogel and Michener. (Scale line = 1 mm.) From Vogel and Michener, 1985.

Figure 6-2. Rediviva emdeorum Vogel and Michener, female, showing the long front legs, which function to withdraw oil from spurs of Diascia flowers. From Vogel and Michener, 1985.

pickup by a floral stigma. Such factors depend not only on the floral structure but also on the movement patterns of bees, which may differ among different individual bees because they are partly learned, and will be different for different kinds ofbees because they are partly species-specific. Students of pollination biology need to pay attention to these and many related matters. Too frequently, the assumption is made that because a particular bee species visits a flower species, that bee is a pollinator of that flower. Another unfortunate assumption is that bees of a common size (usually small) can be lumped as a single functional pollinating unit.

Nearly all eusocial bees and many solitary bees are floral generalists, whereas some solitary bees are floral specialists. Social bees are usually active for long seasons, so that, for them, floral specialization is impractical, because few flower species are in bloom for so long a period. Bom-bus consobrinus Dahlbom of Northern Europe, however, is a specialist on Aconitum and eusocial after an initial subsocial phase, like all nonparasitic Bombus (Mjelde, 1983). Eusocial bees often do show distinct "preferences," such that at a given time and place, the bee species visiting one flower species may be different from those visiting another. Such preferences are especially obvious in the American tropics, where numerous species of Meliponini are commonly active in the same vicinity, some of them segregated onto particular flowers. Unlike social bees, many solitary bees have short seasons of adult flight activity, and can therefore be specialists even if their favorite plant is in bloom for only a few weeks each year.

One would expect plants to evolve in ways that would promote floral specialization by bees, because a specialist is more likely to carry pollen to another plant of the same species than is a generalist that may next visit an entirely different kind of flower. This may not be as important a consideration as one might think, however, because of bee behavior that is called floral constancy: On any one trip, or during a longer period of time, individual bees tend to visit flowers of the same species. Whereas floral specialization by bees is presumably a result of inherent neural or morphological constraints, constancy is learned by each individual bee and may change with new opportunities, or may differ among individuals of the same species at the same time and place. Foraging generalist bees probably exhibit constancy because they can forage more efficiently (i.e., realize more gain per unit time) on one familiar floral type than on a diversity of types, each of which must be manipulated differently. Such aspects of bee behavior may be as important for pollination biology as is the bee's level of oligolecty or polylecty (see the definitions below).

Nectar and oil. Sugars in nectar are the principal source of carbohydrates in bees' diets. Nectar is eaten by adults as an energy source and mixed with pollen to make larval food. Nectar also contains some amino acids, and thus may also contribute toward a bee's nitrogen metabolism. Nectar for regurgitation into brood cells or for storage is carried to the nest in the crop.

Ingestion of nectar, of course, is by way of the proboscis. The gross structure of certain bee proboscides is shown in Figures 19-1 to 19-5. The actual mouth opening is on the anterior surface of the proboscis, near its base (Fig. 19-1c). The details of how nectar moves up the proboscis to the mouth are not fully understood and must vary in different kinds of bees. They involve a sheath consisting of the maxillary galeae, supplemented in long-tongued bees by the labial palpi, which surround the glossa. The flow by capillarity and labial, especially glossal, movement takes nectar toward the base of the proboscis. Some details for Andrena are provided by Harder (1983) and for Apis by Snodgrass (1956). The glossa is elaborately hairy and a significant part of the process may involve variations in the volume of nectar held among these hairs as they are alternately erected and depressed with protraction and retraction of the glossa. As shown in Figures 61-3, 86-1, and 116-2, the hairs, although sometimes simple, may be flattened and lanceolate, capitate, or branched in various ways.

As in most of biology there are exceptions to the generalities. Thus some plants in diverse families (Cu-curbitaceae, Iridaceae, Krameriaceae, Malpighiaceae, Orchidaceae, Primulaceae, Scrophulariaceae, Solanace-ae) secrete, instead of nectar, floral oils, which certain specialist bees collect and carry to the nest externally, i.e., in the scopal hairs, to mix with pollen and sometimes nectar to make larval food. The oils are believed to replace sugars in nectar as the larval energy source, but at least in Centris vittata Lepeletier both nectar and oils are included in larval food (Pereira and Garofalo, 1996). A review was by Buchmann (1987). Bees of the genus Macropis (Melittidae) collect floral oil from Lysimachia (Primulaceae) and use it in part to line (presumably to

Figure 6-3. Ventral views of basitarsi of females. a, Centris (Ptiloto-pus) sp.; b, C. (Paracentris) near tricolor Friese; c, C. (Heterocen-tris) trigonoides Lepeletier. Combs of setae are for oil collecting. From Neff and Simpson, 1981.

waterproof) their brood cells (Cane et al., 1983). Adult bees rarely if ever ingest the oils and thus, like other bees, are dependent on nectar for their own energy sources. Since oil flowers do not produce nectar, oil-collecting bees must get needed sugars from other flowers. Oil-collecting bees have a striking array of pads, brushes, or combs of flattened setae (Figs. 6-3, 110-3a) with which to absorb or scoop the oil and to transport it back to the nest, sometimes mixed with pollen. The morphological details are discussed and illustrated by Vogel (1966 to 1990), Neff and Simpson (1981), and Cocucci, Sersic, and Roig-Alsina (2000). Bees with such structures are found in the Centridini, Ctenoplectrini, Tapinotaspi-dini, and Tetrapediini in the Apidae and in the Melitti-nae in the Melittidae. Obviously, oil utilization has arisen independently, probably in each of these groups, just as the production of oil for reward has evolved independently in different families of plants (Vogel, 1988). The holarctic and Old World oil-collecting bees are specific to particular genera of plants, i.e., Ctenoplectra to Momordica and Thladiantha (Cucurbitaceae), Macropis to Lysimachia (Primulaceae), and Rediviva species mostly to Diascia (Scrophulariaceae). In the neotropics, however, oil-using bee genera and often species are not always specific to particular oil-producing genera or even families of plants.

Pollen. For most bees, pollen is the principal protein source; it is collected and carried to the nest as food for larvae and is also eaten by adults, especially females producing eggs. After dissecting for other purposes a thousand or more females of social halictine bees (mostly Lasio-glossum, subgenus Dialictus), my impression is that large quantities of pollen in the crop were frequent in young adults, whose ovaries might enlarge, and in egg layers with large ovaries, but were virtually absent in old bees with slender ovaries, i.e., workers. Even among workers of highly social bees, whose ovaries will not enlarge greatly, it is the young ones that eat the most pollen, perhaps promoting development of their exocrine glands (Cruz-Landim and Serrao, 1994).

Pollen may initially stick to the bee's legs and body because it is spiny or sticky, or because of electrostatic charges. Some bees carry it back to the nest dry. Others (many Panurginae, Stenotritidae, Melittidae, and the corbiculate Apinae) moisten it with nectar to form a firm

Figure 6-3. Ventral views of basitarsi of females. a, Centris (Ptiloto-pus) sp.; b, C. (Paracentris) near tricolor Friese; c, C. (Heterocen-tris) trigonoides Lepeletier. Combs of setae are for oil collecting. From Neff and Simpson, 1981.

mass that can be carried with relatively few hairs to hold it in place. Oil-collecting bees moisten it with floral oils and possibly also nectar, thus sticking it to the oil-carrying scopal hairs. Finally, although pollen in bees' crops is partly used for their own nutrition, some is carried to the nests and regurgitated. All of the pollen used by bees of the subfamilies Hylaeinae and Euryglossinae to provision cells is carried in the crop, for these bees lack scopae for carrying pollen externally.

Thorp (1979) provided an excellent review of adaptations of bees for collecting and carrying pollen. These adaptations are both structural and behavioral. The details of hair structure associated with pollen gathering, manipulation, and transport have received considerable attention (Braue, 1913; Roberts and Vallespir, 1978; Thorp, 1979; Müller, 1996d; and papers by Pasteels and Pasteels cited in Pasteels, Pasteels, and Vos, 1983). Some aspects of these structures are characters of taxa described in the parts of this book on systematics. Figure 6-4 shows the scopa on the hind tibia and basitarsus of a eucerine bee, and Figures 102-2 and 120-11 show scopae reduced to form pollen baskets or corbiculae on the hind tibiae of corbiculate Apidae. Modified grooming movements are used by female bees for pollen handling. Pollen is commonly removed from anthers by the front tarsi or is dusted onto the body of the bee by its movement among floral parts. The forelegs may be pulled through the mouthparts if the bee eats the pollen, or they are pulled through the flexed middle legs whose opposable mid-femoral and midtibial brushes remove the pollen. The pollen is then transferred to the hind legs, where it may be either held in the leg scopa for transport or, in mega-chilines among others, passed on to the metasomal scopa. Pollen dusted onto the bee's body is groomed off by the legs and transferred backward to the scopa. Details of these movements, and their many variations among taxa of bees, are described by Jander (1976) and Thorp (1979). Some of the best-known variations are in the remarkable ways in which pollen is loaded into the tibial corbicula by corbiculate Apidae, i.e., Apini, Bombini,

Outer Surface Tibia Bee
Figure 6-4. Hind leg of a female of Svastra obliqua (Say), a eucerine (L-T) bee, showing the scopa for transporting dry pollen on the tibia and basitar-sus. The bare area on the lower outer surface of the femur constitutes the femoral corbicula in many S-T bees. Drawing by D. J. Brothers.

Euglossini, and Meliponini (Michener, Winston, and Jander, 1978).

Bees such as Apis mellifera Linnaeus are extreme gen-eralists, and many others take pollen from various unrelated kinds of flowers. Such bees are called polylectic. Bee species or genera that specialize on a particular pollen taxon are called oligolectic. Some will collect pollen from a number of plant species of the same or related or even superficially similar families. These can be called broadly oligolectic. Others collect pollen from a few closely related species and are called narrowly oligolectic. The boundaries are indefinite, for there seems to be a continuum from the most broadly polylectic to the most narrowly oligolectic. Some authors have quantified this terminology. For example, Müller (1996b) suggested the following: oligolectic, at least 95 percent of the pollen grains from the scopa belong to one family, subfamily, or tribe; polylectic with strong preference for one plant family, 70 to 94 percent of the pollen grains, etc.; polylectic, 69 percent or less of the pollen grains, etc. Variations in the abundance ofvarious plants, however, are likely to render such a system ineffective. Although these terms relate to pollen collecting, nectar or oil specialists are mostly also pollen specialists and thus oligolectic.

Frequently in any one area, or throughout its range, a bee species is restricted in its pollen collecting to a particular species ofplant that has no close relatives in the vicinity. It is the usual view, based on considerable experience, that if a related plant species were present, the bees would utilize its pollen also, and that in other regions where related plants do exist, they will be visited by the same species of bee. For these reasons, the term monolectic is almost unused. Use of the term is appropriate, however, if it is clear that close relatives of the plant host are absent or not flowering in the area and season under study. For example, about 22 species ofbees collect pollen only from Larrea divaricata in the southwestern United States (Hurd and Linsley, 1975). They are monolectic; there are no closely related plants in North America. Nonetheless, we usually call these bees oligolectic, thereby predicting that if other species of Larrea were present, they also would be utilized. The word "monolectic" is especially appropriate for a species of bee that collects pollen only from one species of flower, even in the presence of closely related flowers. Anthemurgus passiflorae (Robertson), a specialist on flowers of Passiflora lutea in the eastern United States, may be such a bee, for it does not visit other species of Passiflora so far as is known; but the size and color of flowers of the other regional species are entirely different from those of P. lutea..

Many oligolectic bee taxa (e.g., subgenera or genera) consist of related species specializing on the same or related plants. Examples are Systropha (Rophitinae), all species of which, so far as I know, use Convolvulus pollen more or less exclusively; Macropis (Melittinae), all species of which use pollen of Lysimachia; and the Proteriades group of Hoplitis (Osmiini), most members of which use Cryptantha pollen more or less exclusively, although visits to other plants for nectar result in taking some pollen.

Although many oligolectic bees appear to be dependent on their particular flowers, and do not occur outside of the ranges of those flowers, the plants are generally not dependent for pollination on their oligoleges. Plants often occur and reproduce outside the ranges of their oligoleges; pollination by polylectic bees or other insects is adequate for the plants' needs. Examples are given by Michener (1979a). As noted above, one can rarely recognize the coevolution of particular species of plants and bees; rather, the bees appear to have adapted to plant floral structure and chemistry, while the plant has commonly not adapted to any one oligolectic bee species or genus. In fact, readily accessible pollen characterizes some plants, such as willows (Salix), that host numerous oligolectic species of bees. Often the bee's adaptation appears to be only behavioral, but there are many cases of probable morphological adaptation of a bee to a particular kind of flower. A common example is the sparse and often coarsely branched scopal hairs of bees such as Tetralonia malvae (Rossi) (Eucerini) and most Diadasia (Emphorini) that use coarse pollen like that of Malvaceae and Cactaceae. Hooked hairs on the mouthparts or front tarsi of females, which pull pollen away from anthers located deep in a small corolla, are other examples that occur in various unrelated bees. North American examples are Andrena osmioides Cockerell (Andreninae) and the above-mentioned Proteriadesgroup of Hoplitis (Osmiini) on Cryptantha (Boraginaceae) and Calliopsis (Verbenapis) (Panurginae) on Verbena (Verbenaceae). European examples include Colletes nasutusSmith (Colletinae), Andrena nasuta Giraud (Andreninae), and Cubitalia parvicornis (Mocsary) (Eucerini), all oligolectic on Boraginaceae (Müller, 1995). Some narrowly polylectic bees that frequently collect pollen from Boraginaceae have similar hooked hairs, as shown by the same author. A scopa consisting of simple sparse bristles is characteristic of bees that specialize on pollen of Onagraceae, plants whose pollen grains are webbed together by viscin threads. Examples are Svastra (Anthedonia) (Eucerini) and Lasioglos-sum (Sphecodogastra) (Halictini); for others, see Thorp (1979).

A morphological feature that has arisen independently in various groups of bees appears to be adaptive for col lecting pollen from Lamiaceae and Scrophulariaceae, particularly from Salvia and its relatives. The facial vesti-ture consists of erect, rather short hairs having stiff, thickened bases tapering to slender tails that are usually hooked, bent to one side, or wavy. Such hairs are usually on the clypeus but are on the frons in Rophites s. str. In the best-developed cases, the face is flatter than in related species lacking such hairs. Müller (1996a) reviewed such bees in Europe and found them to be mostly oligolectic on Lamiaceae or narrowly polylectic on that family, Fabaceae, and Scrophulariaceae. He observed that such bees rub the anthers with their faces and remove the pollen from their faces with the front basitarsi; obviously, they then transfer the pollen to the scopa. Certain species in each of the following genera have such facial modifications: Caupolicana (Colletidae); Andrena (Andren-idae); Rophites (Halictidae); Anthidium, Trachusa, Osmia, and Megachile (Megachilidae); Anthophora, Amegilla, Habropoda, and Tetraloniella (Apidae).

A type of pollen presentation that has received considerable attention is in tubular anthers that perhaps protect pollen from damage by rain. Instead of dehiscing in usual ways, such anthers, found in diverse families, are porici-dal, i.e., tubular with one or two holes in the distal ends through which pollen must escape. Such plants usually produce no nectar, but depend on pollen as a reward for bees. Many kinds of bees, both oligolectic and polylectic, obtain pollen from such flowers by vibrating (sonicating) them, the anther aperture usually directed toward the bee. Pollen shoots out and some of it clings to the bee, after which it can be handled in the usual way. The vibrating, caused by the wing muscles, results in bursts of audible sound, hence "buzz-pollination." A review is by Buchmann (in Jones and Little, 1983). Müller (1996a) records buzzing during pollen collecting from Lamiaceae by bees with bristles on the frons (Rophites) or clypeus. Such flowers do not have tubular anthers, but perhaps vibrations help to release the pollen from the anthers. An interesting aspect of sonication by bees is that not all kinds of bees do it. Conspicuous among bees that do not is Apis mellifera Linnaeus. Moreover, minute bees usually do not do it.

Vibrating behavior is widespread among bees and wasps, and is usually used by individuals finding it difficult to push through a small space or to loosen a pebble in nest construction. This is probably the ancestral function of such vibrating. Minute bees probably do not have the mass and energy to liberate pollen or pebbles in this way. Apis may have lost the tendency to sonicate anthers because it nests in the open or in large cavities and builds with malleable wax rather than hard soil, pebbles, etc., and therefore rarely needs sonication in nest construction. Melipona does sonicate flowers; this may seem to negate the argument based on Apis. The intricacies of its nests and the small nest entrances may have promoted retention of the behavior.

An unsolved question is whether oligolecty or poly-lecty is the ancestral condition for bees. No doubt evolution can go in both directions, but it seems reasonable to suppose that oligolecty is a specialized condition and therefore derived. Probable evidence for this supposition comes from unusual oligolectic species of generally poly-

lectic groups. An example is Lasioglossum (Hemihalictus) lusrans (Cockerell), an oligolege on Pyrrhopappus (Aster-aceae), in the midst of the huge and generally polylectic tribe Halictini. There is no reason to believe that L. lus-trans is exhibiting a plesiomorphic condition; on the basis of morphology, it seems to be a derived species, although this is an impression, not based on a phylogenetic analysis.

Conversely, in diverse groups of bees all species are oligolectic, but on plants of different and often unrelated families. Examples are the tribe Perditini in the Pa-nurginae and the tribe Emphorini in the Apinae. In such cases it seems clear that oligoleges have given rise to other oligoleges dependent on different host flowers. We have no evidence concerning how they became oligolectic in the first place. There are, however, various archaic bee taxa, i.e., basal branches of clades, that consist entirely or largely of oligolectic species. Examples are the Fideliinae, Lithurgini, Rophitinae, and Melittidae. Their phyletic positions suggest that more derived taxa containing many polylectic species may have arisen from taxa consisting of oligolectic species. In their study of the phylogeny of bees at the family level, Danforth et al. (2006) reached the same conclusion. One can support this idea with the notion that a specialist need be adapted to only a limited environment, e.g., the chemicals in its pollen food, or the floral structure of its host plant, to which it must adjust. A generalist, on the contrary, must be able to deal with environmental diversity, e.g., different chemicals in pollens and diverse floral structures, in different plants. Much evolution, therefore may have been from the simpler requirements of a specialist to the complex requirements of a generalist. The obvious advantage would be access to the much increased resources available to the generalist. As species-level phylogenies are worked out in genera like Andrena, Colletes, Leioproctus, and Megachile that contain both polylectic and oligolectic species, better understanding of this topic will develop. Müller (1996b) made such a study for western palearctic An-thidiini. He found evidence for transitions from oligolecty to polylecty and for transitions of oligoleges from one floral host to another, but he found no transitions from polylecty to oligolecty.

An interesting observation is that some species of plants have many oligolectic visitors while others have none. For example, in North America there are many oligoleges on Helianthus (Hurd, LaBerge, and Linsley, 1980). Some of them occasionally take pollen from other large Asteraceae, but most are almost exclusively dependent for pollen on Helianthus, to judge by my observations and collecting records. But no oligolege is known for the similar flowers of another large Asteraceae, Sil-phium, even though Helianthus and Silphium often flower in the same vicinity. I have no explanation for this rather common phenomenon. A combined botanical and entomological study would probably be worthwhile.

The frequency of oligolecty among bees also varies regionally. Michener (1954b) observed that oligoleges form a smaller percentage of the bee fauna in the moist tropics than in temperate regions, and that the maximum percentage of oligolectic species seems to be in xeric warm-temperate areas, at least in the Western Hemisphere. Good data are difficult to obtain, partly because of problems with the definition of the terms, but I believe that this regional pattern in percentage of oligolectic species is real, and occurs more or less worldwide. This pattern could be accentuated in the Western Hemisphere by the abundance of the largely oligolectic Panurginae in the xeric regions of both North and South America, but Pesenko (in Banaszak, 1995) wrote that in the former U.S.S.R. nearly half of the nonparasitic bee species of steppes and deserts are oligolectic, the percentage apparently being much less in more humid regions and in boreal regions. The same pattern is subjectively recognizable in Africa in spite of the scarcity of Panurginae there.

Even within broad areas, such as the Sonoran deserts of the United States and Mexico, the prevalence of oligolecty varies among districts. Minckley, Cane, and Kervin (2000) showed that bees oligolectic on Larrea are concentrated in and presumably most frequently arise in areas of least predictable flowering. Larrea flowers after rains of 12 mm or more; the driest deserts, where the plants may fail to flower for years, appear to be most prone to produce oligoleges. Relatives of most of them are oligoleges on other flowers.

Other substances collected by bees. Aside from materials for nest construction collected by many megachiline bees, many bees assiduously collect certain other substances. These include water, employed for temperature control in colonies, as in Apis, and for softening hard soil while excavating, as in Ptilothrix (Emphorini). Sweat bees (Halictinae) and some Meliponini take perspiration, probably for its water and salts, and can be quite bothersome to people in the process. These same groups of bees sometimes take salts from other sources, e.g., soil moistened by urine. Roubik (1989) lists various other bees that appear to be attracted to, and to take, inorganic salts.

Male euglossine bees collect aromatic fragrances from orchid flowers as well as from flowers of certain Araceae and a few other plant families. (Even larger quantities of the same and similar chemicals may come from rotting logs, fungi, and perhaps other objects in tropical forests, as noted by Whitten, Long, and Stern, 1993.) The functions of these compounds in euglossine bee biology are not clear (see Sec. 118, on Euglossini), but they are the bait or reward that attracts the bees to the flowers. Male euglossine bees are the sole pollinators of many species of neotropical orchids (see Dressler, 1968), but that function does not depend on pollen collected by the bees or dusted onto the bees' bodies. Orchid pollen is in fact useless for bees, because it is produced in saclike pollinia. The often complex orchid floral structures stick pollinia to bees' bodies at sites that later will come in contact with the stigmatic surfaces of other orchid flowers as the bees seek more of the fragrant compounds.

Just as some bees collect oil in place of the usual nectar for larval food, some species of Trigona (Meliponini) have another source, meat, to fill part or all of their protein needs. Some species not only collect pollen, but frequently visit carcasses of dead animals, where they collect bits of tissue, perhaps for nest construction but probably in some cases also for larval food. Three neotropical species of the same genus are obligately necrophagous; they do not collect pollen but take tissue from animal carcasses instead (Roubik, 1982; Baumgartner and Roubik, 1989). These bees, which can rather quickly skeletonize the carcass of a small animal, do not even collect nectar from flowers but use fruits and extrafloral nectaries as sugar sources (Noll et al., 1997). At least one of the carrion-feeding Trigona species is sometimes predaceous on soft protein sources—living wasp larvae in nests abandoned by the adult wasps, and eggs of toads (Bufo sp.) stranded by lowering water levels (Mateus and Noll, 2004; D. Roubik, personal comm., 2004).

Worker honey bees sometimes collect such strange materials as coal dust, brick dust, and flour. Presumably, such substances have no function in the hive; probably they are discarded.

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  • anya
    What do bees use the oil from the diascia for?
    2 years ago

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