Cellophane-like cell lining of Colletidae. (a) Linear array of nest cells of Colletes sp. (Arizona, USA) in a single polyester tubular membrane (picture by James H. Cane). (b) Brood cell of Colletes validus Cresson filled with provision and an egg (picture by James H. Cane). (c) Prepupae of Hylaeus mesillae (Cockerell) viewed through the nest cell lining (picture by William Nye, USDA-ARS Bee Biology and Systematics Lab; courtesy of James H. Cane). 

... the ground (Houston and Thorp, 1984; Houston, 1975, 1984, 1987). The second group includes species reported to nest inside stems, soft wood, pre- existing cavities, as well as in soil. Included in this group are the species of Callomelitta (Rayment, 1935), Hylaeinae, Euryglossinae, and Xeromelissinae. These bees are, on av- erage, smaller, and less hairy. The species of Euryglossinae and Hylaeinae lack an external pollen-carrying scopa because females trans- port pollen internally. Most species of Colletinae are ground nesters, but at least one South American species nests in pithy stems instead: Colletes rubicola Benoist (Benoist, 1942). Many but not all species of Colletes have a quite peculiar nest architecture compared to other ground- nesting bees of their and other families – the lateral burrows of some Colletes species are subdivided into series of cells (Michener, 2000: 130, and references therein; for an account of the diversity of nest architectures of seven South American species of Colletes , see Claude-Joseph, 1926: 125–139). In contrast, lateral burrows of typical soil-nesting bee end in a single cell (Michener, 1964, 2000). The linear cell arrangement observed in Colletes nests (Fig. 3a) resembles that of stem- nesting bees (Fig. 3c). Moreover, Colletes have reduced basitibial and pygidial plates, two generally well-developed morphological structures in soil-nesting bees (Batra, 1980: 525; Radchenko and Pesenko, 1996). These morphological characters plus the nest architecture (in addition to the known case of stem- nesting behavior) suggest the possibility that extant species of Colletes descended from a stem-nesting ancestor. Regrettably, nest information is not available for Hemicotelles, Mourecotelles , and Xanthocotelles , the three basal-most lineages of Colletinae (Almeida, 2007). Nests of Xeromelissinae are generally built in holes in hollow stems or beetle burrows in wood or stems, but some re- cently described species are ground nesters (Michener and Rozen, 1999; Packer, 2004). All species of Chilicola whose nesting behavior has been studied were found to nest in woody substrates, i.e. stems, abandoned beetle burrows, etc. (Packer, 2004: 818– 819, and references therein). Claude-Joseph (1926) described nests of Chilicola inermis (Friese) in hollow bamboo stems and of Chilicola friesei (Ducke) occupying abandoned stem nests of Manuelia (Apidae). Eickwort (1967) studied nests of Chilicola ashmeadi (Crawford) and Michener (2002) described nests of Chilicola espeleticola Michener and Chilicola styliventris (Friese). Stem-nests of Chilimelissa generally follow the same plan as that of Chilicola (Michener, 1995). Michener (1995: 33) pointed out that, in general, xeromelissine nests do not di ff er conspicuously from those of Hylaeus . This is quite significant given a well-supported sister-group relation- ship between Xeromelissinae and Hylaeinae (Fig. 2). Ground-nesting species were more re- cently documented for this subfamily: Geodiscelis megacephala Michener and Rozen, Chilimelissa australis Toro and Moldenke (Michener and Rozen, 1999; Packer, 2004), and probably also G. longicephala Packer (as suggested by morphological adaptations of bees of this species for sand nesting – Packer, 2005). Houston’s (1969) observations of various nests of Euryglossinae species illustrate the diversity of nesting habits for these bees. Some groups were found to nest in the ground ( Eu- ryglossa sp., Euryglossula chalcosoma (Cockerell), and Brachyhesma perlutea (Cockerell)), whereas others nest in wood ( Euryglossina hypochroma Cockerell, Euryglossina pulchra Exley, Pachyprosopis haematostoma Cockerell, and Pachyprosopis indicans Cockerell). Euhesma fasciatella (Cockerell) and Eury- glossa ephippiata Smith also nest in the soil (Rayment, 1935, 1948, respectively). Hylaeinae seem to include mostly stem- nesting species (Michener, 2000), but some species use abandoned nests of other insects, pre-existing cavities, volcanic rock, and earth banks (Rayment, 1935; Sakagami and Zucchi, 1978; Michener, 2000; Daly and Magnacca, 2003). Examples of stem-nesting species include Amphylaeus (Spessa et al., 2000) and Meroglossa (Michener, 1960). Hylaeus ( Nesoprosopis ) includes both soil- and stem-nesters (Daly and Magnacca, 2003). Michener (1964) compared nesting substrates used by di ff erent groups of bees and concluded that soil nesting is probably a plesiomorphic character state for bee nesting. It seems that soil nesting is the ancestral con- dition for the clade formed by Colletidae and Stenotritidae, given the soil-nesting behavior of Stenotritidae, Diphaglossinae, and Paracolletinae, and the phylogenetic relationships within this group (Fig. 2). Callomelitta perpicta Cockerell nests in de- caying wood, as reported by Rayment (1935: 97); unfortunately he neither illustrated nor described in detail the architecture of this bee’s nests. The phylogenetic placement of Callomelitta is not completely understood (Fig. 3), but this genus is part of clade comprised of Colletinae, Euryglossinae, Hylaeinae, Scrapterinae, and Xeromelissinae. It is possible that the common ancestor of this clade was a stem-nesting bee, and multiple reversals to soil nesting (and to rotten wood) would then have taken place during the diversification of the group. The application a waterproof lining on the brood cells wall is a synapomorphy of bees and is probably associated with the pollen- feeding habits of bees (Michener, 1964, 2000). The origin of this behavior and the secretion of the lining by bee’s glands are associated with one another, as no species of apoid wasps was reported to do it (Hefetz et al., 1979; Espelie et al., 1992). The glandular lipoidal secretion is sometimes replaced by oils collected in flowers, as in the case of species of Centris (Apidae) (Vinson et al., 1997) or Macropis (Melittidae) (Cane et al., 1983a), and a few bees do not line their cells at all (e.g. Hesperapis and Dasypoda : Melittidae) (Rozen, 1987; Michener, 2000). The importance of cell lining certainly has to do with reduction of water exchange between the brood cells and the environment (Michener, 1964, 2000; May, 1972; Cane, 1983), and it may also be resistant to fungal hyphae (Albans et al., 1980). Prior to lining the brood cells, some bees apply a mandibular gland secretion that is very e ff ec- tive both for fungistasis as well as for bacte- riostasis (Cane et al., 1983b). Populations of certain species of Colletes , e.g. C. halophilus Verhoe ff , are often flooded while overwintering but the cells are completely waterproof because of the lining along the cell walls and a flap that seals the opening (Albans et al., 1980). In some other species of colletid and stenotritid bees, the main problem may be desiccation, and a waterproof lining should work just as well. Colletidae, in particular Colletes spp., became well known for the apparently unique lining of their nest cell walls. As early as 1742, Réaumur perceived the uncommon nature of this cell lining and described the multi-layered silky membrane of colletid brood cells (Batra, 1980). Colletid bees (and most other ground- nesting bees) generally apply a waterproof lining to their brood cells but not to the nest tunnels. Nonetheless, the hylaeine Meroglossa torrida (Smith) can line the entire cavity of a twig with cellophane-like material, and this lining can even extend outside the nest en- trance (Michener, 1960). Cell linings of Colletes are insoluble in either aqueous or organic solvents, and are not degraded by either acidic or basic hydroly- sis (Hefetz et al., 1979). Jakobi (1964) tried 11 di ff erent solvents on nest cell linings of diverse bees: Colletes was unique in that lining was not soluble in chloroform, but was soluble in pyridin (unlike halictids and apids, among others). Colletid bees employ their brush-like glossa as the main tool for applying the secretion (e.g. Janvier, 1933; Malyshev, 1968; Batra, 1980). By the end of the 1970’s, behavioral and morphological evidence suggested that the cell-lining secretion was produced by the female Dufour’s gland, associated with the sting apparatus (e.g. Batra, 1964, 1966, 1970; Lello, 1971). The Dufour’s gland is a blind sac that empties its secretions into the base of the sting (Fig. 4; e.g. Lello 1971; Batra, 1980; Cane, 1981; Du ffi eld et al., 1984). This gland can be very large in bees that actively secrete and it may represent up to 10% of the live weight of a Colletes bee (Du ffi eld et al., 1984) and it may occupy 20–50% of the abdominal cavity of a Colletes (Batra, 1980). Dufour’s gland functions in Hymenoptera are diverse, and include defense and alarm pheromones, trail pheromone, host marking and discrimina- tion, and sexual attraction (Hefetz et al., 1978). Cane (1983: 658) provides a helpful histori- cal account of research dealing with Dufour’s gland and its secretions. Lello conducted comparative studies of the Dufour’s gland in many groups of bees (Lello, 1971 [for Colletidae], other papers by Lello cited in Lello, 1976; see also Du ffi eld et al., 1984: 403–414 for an anatomical and functional review of this gland in bees). Hefetz et al. (1979) compared their chemical nature of the contents of the Dufour’s glands and nest cell lining of three species of Colletes . The main chemicals found in both the gland secretion and the lining were macrocyclic ω -lactones, hydrocarbons, and aldehy- des. Macrocyclic ω -lactones are the precursors of lipid polyester of the nest cell lining, the latter being composed of ω -hydroxy acid units (Hefetz et al., 1979; Cane, 1981). Although the detection of macrocyclic lactones inside the Dufour’s gland of Colletes had already been accomplished by Bergström (1974), the comparative chemical analysis of nest cell lining and gland secretion of the same bee species, side-by-side, was only attained later (Hefetz et al., 1979; Albans et al., 1980). The membrane made of natural high ...