Nests and Food Storage

The nests of bees are the places where their young are reared. They are always to some degree made by the mother, or, in social bees, by the workers. Nests and especially cells, their provisions, and larval behavior are full of meaningful details of importance not only for bee survival but also for our understanding of adaptations and of phylogeny. Malyshev's many papers were among the most important and detailed early studies of these matters, culminating in his summary work (Malyshev, 1935). Some of the best and most critical recent accounts are in Rozen's papers. References to many papers that include information on nest architecture are in the accounts of taxa in later parts of this book. General accounts were by Michener (1961a), Iwata (1976), Radchenko and Pesenko (1994a, b), and Radchenko (1995), and detailed summary accounts of certain taxa are those by Wille and Michener (1973) for the Meliponini and by Sakagami and Mich-ener (1962) for the Halictinae.

Bee nests ordinarily contain or consist of brood cells (Fig. 7-1). A cell serves to protect the delicate immature

Fossilized Bee Nest

Figure 7-1. Cells of an anthidiine bee, Dianthidium concinnum (Cresson), made of pebbles and resin, constructed on an elm twig in Kansas. Emergence openings are shown on righthand photograph. From Fischer, 1951.

stages, and in most cases the food, of the growing larva. It is the space in which a single immature bee grows, although in most species of Bombus a cluster of eggs is placed together in a small wax cell, and the cell is enlarged as the resulting larvae grow. Except in Bombus, cells are big enough initially to contain one mature bee each.

Most bee nests consist of more than cells, being burrows in the soil, in wood, or in pith. Typically, and probably ancestrally (because the pattern is common in sphe-coid wasps), the nests are in the soil and the main burrow gives rise to lateral burrows, each of which ends in a single cell (Michener, 1964b; Radchenko and Pesenko, 1994a, b; Radchenko, 1995). As to nests of ancestral bees, note that Budrys (2001) believed that nests of ancestral

Figure 7-2. Diagrams of nests of a eucerine bee, Peponapis fer-vens (Smith), excavated in soil in Brazil. At left is a mature nest with the lateral burrows filled with earth and unrecognizable, their exact positions not determined. The other nests are relatively new, each with one newly constructed lateral burrow and an unprovisioned cell. (Scale line = 6 cm.) From Michener and Lange, 1958c.

Figure 7-1. Cells of an anthidiine bee, Dianthidium concinnum (Cresson), made of pebbles and resin, constructed on an elm twig in Kansas. Emergence openings are shown on righthand photograph. From Fischer, 1951.

Figure 7-2. Diagrams of nests of a eucerine bee, Peponapis fer-vens (Smith), excavated in soil in Brazil. At left is a mature nest with the lateral burrows filled with earth and unrecognizable, their exact positions not determined. The other nests are relatively new, each with one newly constructed lateral burrow and an unprovisioned cell. (Scale line = 6 cm.) From Michener and Lange, 1958c.

Figure 7-3. Diagram of three nests of a colonial halictine bee, Halictus ligatus Say, excavated in soil in Trinidad, showing sessile cells. Cells shown by dots were abandoned, earth-filled; the contents of other cells are indicated as follows: e, empty; E, egg; SL, small larva; ML, medium-sized larva; PP, large larva, usually prepupa; the sex symbols identify pupae of the sexes indicated. At upper left is a sectional view of a cell showing shape, earth closure (dotted), and feces of larva (black). (Scale lines = 5 mm for the cell; 10 cm for the nests.) From Michener and Bennett, 1977.

sphecoid wasps were in wood cavities. If this is true, ancestral bees may have occupied similar holes instead of burrowing in the ground. However, since sphecoid wasps are paraphyletic relative to bees and related lineages are ground-nesting, it is more likely that ground-nesting is ancestral for bees, as also concluded by Engel (2001b).

The cells are lined or unlined; the burrows themselves are unlined. Typically, each lateral is filled as a new lateral is excavated, saving the bees the trouble of pushing the excavated earth to the surface. Laterals may be well separated up and down the nest, as shown by the mature nest in Figure 7-2, or may all radiate from one level. Among the many other modifications of such architecture are horizontal cells, instead of vertical cells as in Figure 7-2; two or more cells per lateral; shortening of the laterals until the cells are sessile, arising directly from the main burrow (Fig. 7-3); and rotation until the main burrow is horizontal, entering a vertical bank instead of flat ground.

In some bees the cells, like the burrows, are unlined, mere excavations into the soil, usually broader than the burrows leading to them. Such bees include many Melit-

tidae, most Xylocopinae, Fideliinae, and the genus Perdita in the Panurginae. Ifthis cell type, resembling that of most sphecoid wasps, is ancestral for bees, it supports the Perkins-McGinley hypothesis of the proto-bee as a form with a pointed glossa like a melittid; see Section 20. The alternative, that the proto-bee had a broad glossa like that of a colletid, would favor the proto-bee's lining each cell with a secreted film, applied with the broad glossa as do colletids.

Unlike the taxa listed above, most bees excavate cells in a substrate (usually soil), line them with a smooth earthen layer, often made of fine clay from elsewhere in the burrow, tamp the cell surface smooth with the pygidial plate, and apply to this surface a secreted film of cellophane-like or waxlike material (Fig. 7-5). The "waxlike" material is a mixture that may not include wax; J. Rozen (in litt.) prefers simply to call it a shining secretion. For additional information, see Section 111. Two views of such cells are shown in Figure 7-4; see also Plate 16. Both the earthen layer and the secreted lining are derived features relative to those of sphecoid wasps; when sphecoid wasps make

Figure 7-4. Cells of Augochloropsis sparsilis (Vachal) excavated into soil, showing the characteristic cell shape (saggital section at left, frontal section at right) as well as a pollen mass and egg. (The scale at the right is in millimeters.) From Michener and Lange, 1959.

similar structures, such as the lined cells made by some Pemphredoninae, the lining is not homologous to that constructed by bees. The earthen layer and secreted lining may also be derived features relative to those of the proto-bee. Such cells can be isolated or grouped in clusters made by excavating them close together. Some halic-tids, however, construct clusters of similar cells in cavities that the bees have excavated in the soil (Fig. 7-5); the cells are made from the homologues of the earthen cell linings. The view of Radchenko and Pesenko (1994a) that such construction of cells does not occur is incorrect.

Cells made by subdividing a burrow with transverse partitions, as is done by many Megachilinae, Hy-laeinae, Ceratinini, and others, are usually not identical in shape, i.e., they are heteromorphic. Cells excavated or constructed in the soil or other substrate, such as rotting wood, are usually alike, i.e., homomorphic, for any one species. In the homomorphic cells of some common taxa like Andrenidae and Halictidae, one surface (the lower surface if the cells are horizontal) is flatter than the other surfaces, each cell thus being bilaterally symmetrical about a sagittal plane (Figs. 7-3, 7-4).

Megachiline bees usually make cells [sometimes only by means of partitions in an unlined burrow (Fig. 7-6; Pl. 16), but usually with whole cell walls] using foreign materials carried to the nest. Such materials can be cut pieces of leaves, chewed leaf pulp, plant hairs (sometimes supplemented with sticky material from stem or foliar tri-chomes, Müller, 1996c), resin, pebbles (Fig. 7-1), mud, etc. A secreted lining seems to be absent. In a few Megachilidae [Heriades spiniscutis (Cameron) (Fig. 7-6), Michener, 1968b; Osmia (Metallinella), Radchenko, 1978; Megachile (Sayapis) policaris Say, Krombein, 1967; Fidelia, Rozen, 1977c; Lithurgini, Malyshev, 1930b], partitions between cells are sometimes or always omitted, so that larvae are reared in a common space with separate or contiguous food masses. Some megachilid bee nests are so constructed that they consist only ofone or several cells made of resin, resin and pebbles, leaf pulp, or mud on the surfaces of rocks, walls, stems, or leaves. Examples are An-thidiellum s. str., whose nests usually consist of a single resinous cell exposed on a leaf, stem, or rock surface (Pl. 8), and most species of Dianthidium s. str. and Megachile

Nest Diagram Colletidae

Figure 7-5. Nests of Augochlorella striata (Provancher) excavated into soil. At left, a cell cluster exposed by digging. At center, a nest poured full of plaster of Paris, then exposed by digging. At right, the same cell cluster opened to show three cells (the oldest in the center) in saggital section, the very thin earthen cell walls, and the earthen pillars supporting the cell cluster in the space here filled with plaster. (The scales are in millimeters; that at the right center relates only to the righthand photograph.) Photos by E. Ordway (left) and C. Rettenmeyer.

Figure 7-5. Nests of Augochlorella striata (Provancher) excavated into soil. At left, a cell cluster exposed by digging. At center, a nest poured full of plaster of Paris, then exposed by digging. At right, the same cell cluster opened to show three cells (the oldest in the center) in saggital section, the very thin earthen cell walls, and the earthen pillars supporting the cell cluster in the space here filled with plaster. (The scales are in millimeters; that at the right center relates only to the righthand photograph.) Photos by E. Ordway (left) and C. Rettenmeyer.

Figure 7-6. Parts of three nests of Heriades spiniscutis (Cameron) in dead, dry stems. The nest at the right had thin partitions made of pith fragments between the cells; the partitions are marked by horizontal lines. The other nests lack partitions. All the nests contain eggs or very young larvae in the upper ends of masses of provisions. To show the eggs, loose pollen was blown away from the nest at the left before photographing. From Michener, 1968b.

Figure 7-6. Parts of three nests of Heriades spiniscutis (Cameron) in dead, dry stems. The nest at the right had thin partitions made of pith fragments between the cells; the partitions are marked by horizontal lines. The other nests lack partitions. All the nests contain eggs or very young larvae in the upper ends of masses of provisions. To show the eggs, loose pollen was blown away from the nest at the left before photographing. From Michener, 1968b.

(Chalicodoma), whose nests consist of clusters of cells similarly exposed (Fig. 7-1). Those of Dianthidium are made of pebbles in a matrix of resin, whereas those of Chalicodoma are made of mud or sand impregnated with a secretion (probably of the labial glands, since these glands are enlarged) that renders the nest hydrophobic and able to withstand rain (Kronenberg and Hefetz, 1984b).

The simplest but architecturally derived bee nests are those of the allodapines. Such a nest is a cavity in a hollow stem, a burrow in a stem made by some other insect, or an unbranched burrow made by the female bee in a pithy stem. If necessary, such a tubular hollow is cleaned out by the bee, the bottom rounded out by tamped particles. A collar of pith or wood particles cemented together, probably by salivary materials, is constructed in such a manner that it narrows the entrance, permitting more efficient guarding. When there is a threat, the entrance is plugged by the somewhat flattened metasoma of the female. Immature stages are reared together, usually fed progressively, in the nest burrow. For illustrations, see Michener (1971a, 1990d) and Figures 90-4 and 90-5. Such nests, though simple, are not the ancestral type of bee nest; no doubt they are derived from nests like those of Ceratina, which are burrows in stems, subdivided into mass-provisioned cells by partitions of pith particles (Fig. 90-5a). The allodapine subgenus Compsomelissa (Halter-

apis), unlike most of its relatives, makes mass-provisioned nests somewhat like those of Ceratina but with the partitions omitted.

In contrast, in the corbiculate tribes Apini, Bombini, and Meliponini, cells are built of wax secreted by the metasomal wax glands, and except in Apini, mixed with other materials such as resin or pollen. The cells are in clusters or in combs (i.e., regular layers), usually in a cavity in a tree or in the ground, or in a cavity in a larger nest. Rarely, as in the groups of Apis dorsata Fabricius and A. florea Fabricius, the combs of cells are exposed, but protected by layers of bees. Details of cells of corbiculate Apidae and the nests in which they are found are explained by Michener (1961a, 1974a), Wille and Michener (1973), and numerous other works.

The most elaborate bee nests are those of the Meli-ponini (Figs. 7-7, 7-8, and 120-3), in which the clusters or combs of wax brood cells are surrounded by one or multiple layers of resin or wax involucrum. These layers, and masses of food-storage pots, are usually surrounded by batumen consisting of one or multiple layers of wax mixed with either resin or mud, sometimes forming an enormous, exposed nest, more often a nest hidden in a hollow tree or in the ground. For clarification of terminology, see Figure 7-8. The mixture of wax and resin is called cerumen. The multiple layers of cerumen around

Fossilized Trees With Bee Nest
Figure 7-7. The subterranean nest of a colony of stingless bees (Meliponini), Partamona testacea (Klug). The horizontal combs of brood cells are supported by slender vertical pillars; the brood chamber is surrounded by multiple layers of cerumen, constituting

the involucrum. The batumen is reduced to the thin lining of the cavity in the soil. (Enlarged drawings of the storage pots are at the upper left, of brood comb at lower right.) Drawing by C. M. F. de Ca-margo, from Michener, 1974a.

Food Stored Stem Diagram

Figure 7-8. Diagram of a meliponine nest in a hollow tree, with parts labeled. The batumen typically surrounds the entire nest; here it consists of two plates that limit the nest area and a thin lining of batumen (not illustrated) that lines the entire cavity between the plates. From Wille and Michener, 1973.

Figure 7-8. Diagram of a meliponine nest in a hollow tree, with parts labeled. The batumen typically surrounds the entire nest; here it consists of two plates that limit the nest area and a thin lining of batumen (not illustrated) that lines the entire cavity between the plates. From Wille and Michener, 1973.

the brood chamber are called the involucrum, and the plates or layers enclosing the whole nest are called batumen. Many works describe and illustrate such nests; some are Michener (1961a), Wille and Michener (1973), and especially the beautifully illustrated works ofJ. M. F. de Camargo, e.g., Camargo (1970) and Kerr et al. (1967).

Terms that relate to brood cell usage are mass provisioning vs. progressive feeding. Most bees provision each cell with enough food to suffice for all of larval growth; then after an egg is placed in a cell, the cell is closed. Usually, it is not opened again by the mother. These bees are mass provisioners. Some bees, however, feed the growing larvae at intervals. These are the progressive feeders. The only progressive-feeding bees are Apis (honey bees), most Allodapini (which, in fact, do not make cells; see above), and Bombus (bumble bees, in which the extent and nature of progressive feeding varies with the season, caste, and species). In certain Halictini (see the review in Michener, 1974a) and many Ceratina (Ceratinini) (see the review in Michener, 1990d), although mass provisioning occurs, cells are opened, feces are removed, and the cells are reclosed. This activity has led to apparently incorrect reports of progressive feeding (see Michener, 1974a).

In mass-provisioning species, cells serve to hold the provisions. The walls are often lined with a secreted layer that is impervious to water, so that if the provisions are liquid, they do not seep away. The cell walls must also be important in protecting the larva and pupa both from desiccation and from drowning, and in preventing the hygroscopic provisions (when not already liquid) from liquefying because of excess water. These rather obvious functions of cells have been dealt with by various authors such as Radchenko and Pesenko (1994) and Stephen, Bo-hart, and Torchio (1969). Less well-known functions in clude bactericidal and fungicidal activity that reduces invasion of cells by microorganisms and consequent spoilage of the provisions (Cane, Gerdin, and Wife, 1983). (Immature bees, at least the prepupae of Nomia melanderi Cockerell, also have bacteriostatic materials in or on the integument; Bienvenu, Atchison, and Cross, 1968). Water relations inside cells deserve further study. In some Halictinae the mature larva weighs over 60 percent more than the combined weight of the pollen mass and egg, because of water absorption by the larva from the atmosphere (May, 1972). Another suggestive observation is that in some ground-nesting bees like Perdita (Pa-nurginae) that do not secrete a cell lining, there is instead a secreted covering protecting the food mass.

The secreted cell lining is probably mostly derived from products of Dufour's gland, which opens at the base of the sting (Cane, 1981, 1983b). It tends to be large in bees that construct cells in the ground, and it is commonly small in bees that construct cells elsewhere and that do not line their cells (see Pesotskaya, 1929, and papers by Lello cited in Lello, 1976). Salivary-gland products probably cause polymerization and solidification of the Dufour's gland product, which is initially a liquid (Albans et al., 1980). In Anthophora it seems that the Du-four's gland secretion consists largely of liquid triglycerides that are transformed into solid diglycerides (Batra and Norden, 1996). In this case the salivary secretion is evidently added to the food mass, and solidification of the liquid cell lining occurs on contact with the food mass. Dufour's gland products may also be found in the food mass in the cells of some bees but, contrary to old literature, in which it was called the alkaline gland, Dufour's gland does not contribute to the sting venom.

The food stored for larval consumption in brood cells of mass-provisioning bees takes various forms. Sometimes, as in Hylaeus and Colletes (both Colletidae), it is liquid, consisting primarily of nectar with some pollen admixture. In many others, e.g., Anthophora, Megachile, and Trigona, it is more viscous, as a result of containing more pollen, but nonetheless fills and takes the form of the part of the cell in which it is placed. In still other bees, such as Leioproctus (Colletidae), Halictidae, Andrenidae, Melit-tidae, and Xylocopini, the food mass is firm and carefully shaped, commonly a spheroidal or a flattened sphere, but sometimes, as in Exomalopsis (Apinae), Dasypoda, and Macropis (Melittidae), with basal projections that support the rest of the food mass. Such firm food masses are considered the ancestral condition by Radchenko and Pesenko (1994a, b). The spherical form and especially the supporting projections, as well as the loaf-shaped provisions of Ceratina and Xylocopa, minimize areas of contact with the cell surface, and possibly thus minimize moist sites where destructive molds may gain a foothold. A series of illustrations of various cell and provision types was presented by Stephen, Bohart, and Torchio (1969). In addition to nectar and pollen, the larval provisions sometimes, or perhaps regularly, contain glandular secretions. Salivary glands clearly contribute to the larval food, probably especially in the corbiculate Apidae but also in Anthophora (Batra and Norden, 1996). In addition to providing nutrition, the salivary secretions no doubt contribute to the preservation of larval food as well as of food stored in pots by the necrophagous species of Trigona (Meliponini) and probably others.

Food storage (both pollen and nectar or honey) for adult consumption or transferral to larvae occurs in the nests of relatively few bees. In Apis such storage is in cells much like worker brood cells; in Meliponini and Bom-

bini it is in pots, quite different from brood cells, those of Bombus usually made from abandoned cocoons. In the Allodapini, pollen is stored on the walls of the nest burrows; nectar or honey is stored in large drops on the bodies ofthe larvae, where it will not be absorbed into the pith that usually forms the walls of the nest burrows.

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