b) Aculeata
All social hymenopterans belong to the Aculeata, a clade which has received much attention over the years. Consequently, most of the information on the function of the Dufour’s gland comes from aculeates. Aculeates are characterized by the development of the stinging apparatus, which leads to a separation of the sting from any oviposition related functions (Hunt 2007). In aculeates, the speculations regarding ancestral functions of the Dufour’s gland include sting lubrication, venom constituent or role in oviposition (Dufour 1841, Billen 1987, Howard and Baker 2003, Gnatzy et al. 2004), but definitive demonstrations are lacking. Many studies have looked at the function of the Dufour’s gland in the two well studied groups: the Apoidea and Vespoidea. Among solitary aculeates, the function of Dufour’s gland remains unknown in the Chrysidoidea (cuckoo wasps). Since this group is basal to other aculeates, it would be of interest to investigate the role of the Dufour’s gland in this group. Since the Chrysidoidea is comprised mostly of parasitoids, perhaps the role of the Dufour’s gland here is similar to that in the parasitoid non-aculeates. The Dufour’s gland probably underwent further modifications that were possible with the development of the stinging apparatus of the Aculeata, which removes any constraints related to oviposition from the stinging apparatus (Whitfield 1992), and thereby with the evolution of the sting, the Dufour’s gland can be freed from prior evolutionary constraints. Indeed, structural modifications whereby the Dufour’s gland no longer remains connected to the reproductive tract but opens instead into the sting has been documented (Billen 1987, Gnatzy et al. 2004). The function of the Dufour’s gland in the Aculeata varies widely with variation in mode of living.
In Apoidea, the primary role of the Dufour’s gland in various ground nesting bees appears to be producing the brood cell lining. Dufour’s gland secretions have been found to be involved in forming a protective lining of the brood cells in various ground dwelling bees in Colletidae, Andrenidae, Halictidae, and some Apidae (Anthophorini and Eucerini), and sometimes they also form a hydrophobic lining for their pollen balls (Hefetz et al. 1979, Albans et al. 1980, Cane 1981, Duffield et al. 1981, 1983, Cane and Carlson 1984, Hefetz et al. 1982, 1986, Hefetz 1987). Ground dwelling bees face the challenge of keeping their larval food free from microbial growth, desiccation, and soaking by water. They have evolved to respond to this challenge by coating their brood cell with a hydrophobic lining that forms a sheath around the larval food. Secretions of the Dufour’s gland have been found to be involved in forming this cell lining. Consequently the Dufour’s gland has been found to become hypertrophied with the increase in activity during the nest construction phase (Lello 1971a, 1971b, Kronenberg and Hefetz 1984). Production of brood cell lining is an important adaptation for mass provisioners, ensuring that the food remains fresh and suitable for consumption by the larvae. Unfortunately, it is not known whether a similar function is served by the Dufour’s gland in ground dwelling digger wasps, sand wasps and bee wolves (Sphecidae and Crabronidae), which utilize similar nest substrates. In the Crabronidae there is evidence for the post pharyngeal gland being involved in prey embalming, which may serve the same function as served by the protective hydrophobic cell lining of ground dwelling bees (Strohm et al. 2001). In the melittid bee Hesperapis the Dufour’s glands are tiny and appear to be secondarily derived, as they are correlated with the absence of cell lining or with producing cell lining from exogenously derived substances (Cane 1983). In communally nesting mining bees of genus Andrena (Andrenidae), Dufour’s gland compounds have been found to vary with genetic relatedness, thereby implying a possible role in kin recognition (Ayasse et al. 1990b). Such compounds have also been implicated in marking the nest entrance, helping in locating the nest within the nesting site (Hefetz 1987). In the sweat bee Lasioglossum malachurum (Halictidae), the pattern of Dufour’s gland compounds was found to differ between virgin and mated queens, suggesting a possible involvement inmating and courtship (Ayasse et al. 1993). Mated queens had higher proportion of macrocyclic lactones (used in nest construction), and lower proportion of isopentyl esters (volatiles), as compared to virgin queens, and the composition of Dufour’s gland compounds was found to change within one day of mating (Ayasse et al. 1993). In the same species, Lasioglossum malachurum (Halictidae), the pattern of ester composition has also been found to differ between young virgin queens and old nesting queens, and the pattern has also been correlated with volume of fat body and status of ovarian development, implying possible involvement in sex pheromonal communication and in nest recognition (Ayasse et al. 1990a). Dufour’s gland lactoneshave been found to act as a sex pheromone in Lasioglossum zephyrum (Smith et al. 1985, Barrows 1975a, Barrows 1975b). Dufour’s gland compounds have been implied in nestmate recognition and kin recognition as well in halictine bees (Hefetz et al. 1986, Hefetz 1987, Smith and Wenzel 1988, Soro et al. 2011). In Halictus hesperus (Halictidae) Dufour’s gland secretion has been found to impart structural strength to the opening of the nest (Brooks and Cane 1984). In Eucera palestinae (Apidae: Eucerini), in addition to serving as a cell lining, Dufour’s gland compounds also serve to mark the nest entrance and are used by individuals to locate their nest within dense nest aggregations (Shimron et al. 1985). A similarfunction has been proposed for Colletes cunicularius (Colletidae) and Osmia lignaria (Megachilidae) as well (Guédot et al. 2006). To serve as a semiochemical used in nest recognition, the Dufour’s gland chemicals must show considerable inter-individual variability. Such variability has indeed been reported in some species (Kronenberg and Hefetz 1984, Hefetz 1987). In some Megachile species (leaf cutter bees) (Megachilidae), Dufour’s gland compounds have been found mixed with larval food (bee bread), implying that they may serve as larval nutrition, or possibly to protect the larvae from fungal or bacterial attack (Cane and Carlson 1984, Duffield et al. 1984). Among short-tongued bees (Colletidae, Halictidae, Andrenidae and Melittidae) chemosystematic analysis suggests that Colletidae, Oxaeinae, Nomiinae and Halictinae comprise a monophyletic group (lactone producing bees), sharing macrocyclic lactones that are not found in any other hymenopterans. The other short-tongued bees (Andrenidae, Melittidae and Dufoureinae (currently Systrophinae)) form another monophyletic clade, sharing hydrocarbons (Cane 1983). The function of Dufour’s gland in other solitary Apoidea like the Sphecidae (digger wasps), Crabronidae (sand wasps and bee wolves), and Ampulicidae (cockroach wasps) remains unexplored. Thus in addition to forming the cell lining, nest recognition seems to be another widespread function of Dufour’s gland chemicals in bees. The same chemicals that are already in use for lining the nest surface can also be used by the organisms to identify their nests. Various communicative roles of the Dufour’s gland like kin recognition and nestmate recognition are reported in communally nesting species of Andrenidae and Colletidae, and social Halictidae (Fig. 3).
In the Apidae, the role of the Dufour’s gland in producing brood cell lining is found in ground dwelling bees of the genera Anthophora (Anthophorini) and Eucera (Eucerini) (Stephen and Torchio 1961,Norden et al. 1980,Cane and Carlson 1984,Shimron et al. 1985) and in ground dwelling carpenter bees as well (Xylocopa olivieri, Apidae: Xylocopini) (Kronenberg and Hefetz 1984). Xylocopines generally nest in wood (mostly tree trunks), and ground nesting strategy is an exception for this group. Thus the Dufour’s gland secretions may have secondarily gained the function of producing brood cell lining in xylocopines. The function of the Dufour’s gland in secreting a cell lining in the various ground nesting Apidae can be the result of the retention of a function present in ancestors of this group who diversified to form the different present day phyla. However, cases of convergent evolution owing to similarity in ecological habits cannot be ruled out. Similar to its suggested role in the Megachilidae, the role of the Dufour’s gland in producing larval food has been demonstrated in Anthophora abrupta (Norden et al. 1980). In carpenter bees (Xylocopini), Dufour’s gland has been implicated in foraging site marking, i.e. marking flowers that have been visited for pollen or nectar collection (Fig. 3). This marking is temporary and fades away over a short period of time, over which the flower is able to replenish its nectar. In Xylocopa virginica (Xylocopini) Dufour’s gland secretions are deposited on flowers visited by bees, and serves to detervisits by conspecifics on a short term (Frankie and Vinson 1977, Vinson et al. 1978). In kleptoparasitic Nomadini, male mandibular gland secretion has been found to mimic the Dufour’s gland secretion of their hosts. Males have been found to coat females with their mandibular gland secretion, which renders the chemical profile of the female similar to the cell lining of their host, thereby facilitating kleptoparasitism (Tengö and Bergström 1977).
Among the corbiculate apids (Apini, Bombini, Meliponini and Euglossini), communicative functions for the Dufour’s gland secretions have been reported in various taxa. In bumble bees (Bombini), the Dufour’s gland has been found to increase in size and activity with the reproductive activity and age of queens, and queen worker differences in gland size have also been reported, implying a possible role as an egg marking pheromone (Abdalla et al. 1999). Queen signaling and nestmate recognition through Dufour’s gland has also been implied (Tengö et al. 1991, Oldham et al. 1994, Ayasse et al. 1999). In Bombus terrestris it has been found that workers may advertise their sterility through esters secreted in their Dufour’s gland, and thereby avoid being policed by queens or other egg laying workers. These esters disappear when an individual starts to develop ovaries (Amsalem et al. 2009, Amsalem and Hefetz 2010). Cuckoo bumblebees (subgenus psithyrus) can be attracted by the Dufour’s gland odors of their hosts (other Bombus) to parasitize them (Fisher et al. 1993). In stingless bees (Meliponini) it has been found that reproductively active queens may have larger Dufour’s glands than virgin queens and the gland may be absent in workers; however the function remains unknown and has been speculated as involvement in reproduction or in queen signaling, and it has been suggested that the Dufour’s gland secretions may act as a queen pheromone, as virgin queens have hydrocarbons in their Dufour’s gland, while mated physogastric queens have esters in addition to hydrocarbons (Abdalla and Cruz-Landim 2004, Abdalla et al. 2004). In honey bees (Apini), there are multiple evidences for Dufour’s gland esters functioning in pheromonal queen signaling. Such esters are initially absent in workers, but start being secreted in egg laying workers (Katzav-Gozansky et al. 2002, Oldroyd et al. 2002, Katzav-Gozansky et al. 2007, Malka et al. 2008). It has been proposed that multiple mating of queens triggers the production of esters from the Dufour’s gland (Richard et al. 2011), while in the case ofworkers it is probably triggered by the absence of queen pheromone. The egg laying workers of Apis mellifera capensis, who act as social parasites of Apis mellifera scutellata, produce Dufour’s gland secretions that have been found to mimic the queen gland secretion better than that of egg laying Apis mellifera scutellata workers, thereby increasing the efficiency of their social parasitism (Sole et al. 2002). The role of Dufour’s gland in Euglossini remains unknown. Thus in honey bees (Apini), bumble bees (Bombini), and stingless bees (Meliponini) the Dufour’s gland serves communicative functions like fertility signaling (queen pheromone) and nestmate recognition (Fig. 3).
The Vespoidea contains two major groups of social Hymenoptera: the Formicidae and Vespidae. Apart from these two families, the function of Dufour’s gland remains unexplored in other Vespoidea (Tiphiidae, Sapygidae, Mutillidae, Pompilidae, Rhopalosomatidae, Scoliidae, and Sierolomorphidae). In ants (Formicidae), Dufour’s gland secretions have often been found to act as a trail pheromone in Formicinae and Ponerinae (Law et al. 1965, Hölldobler and Wilson 1970, Williams et al. 1981, Bestmann et al. 1995, Blatrix et al. 2002, Jeanson et al. 2003). They have a recruitment effect, serve in territorial marking, or in setting foraging direction (Cammaerts et al. 1977, Greene and Gordon 2007). Often the Dufour’s gland acts in unison with other glands, like the poison gland, in communicating recruitment signals for recruitment of workers to food or nesting sites. Secretions of the Dufour’s gland are often applied on the substrate by extruding the sting and dragging it (Hölldobler and Wilson 1970, Morgan 2009). However, other glandular sources of trail pheromone originating in the metasoma (hindgut, pygidial gland, post-pygidial gland) or legs (tibial glands, footprint glands on the hind pre-tarsi) are also known (Morgan 2009). The Dufour’s gland secretion can have colony specificity and species specificity, thereby ensuring fidelity of the trail (Traniello 1980, Haak et al. 1996). The Dufour’s gland has also been implicated as a storage site for chemicals involved in the alarm-defense system of ants, whereby the same chemicals can produce an alarm reaction among nestmates, while simultaneously acting as repellents to intruders (Wilson and Regnier 1971, Whitehouse and Jaffe 1996). In slave making ants of the genus Polygerus Dufour’s gland chemicals have been found to be used in manipulating the behavior of the host species. Dufour’s gland compounds facilitate the new queen of the slave making ant to invade a host colony, and reduce any aggression shown by the host workers towards the usurper (Topoff et al. 1988, Mori et al. 2000). Such compounds may also act as a repellent and thereby facilitate usurpation of a host colony (Ruano et al. 2005). Another function of Dufour’s gland in slave making ants is to act as a source of propaganda substances that elicit panic among defending host workers, thereby increasing the efficiency of slave capturing raids (Brandt et al. 2006). Dufour’s gland compounds have also been found to act as sex pheromones, and can also be involved in the calling behavior exhibited by females, whereby virgin gynes place themselves in strategic positions outside their nests and “call” for mates by releasing sex pheromones (Hölldobler and Wust 1973, Hölldobler and Wilson 1990, Ayasse et al. 2001).
In the hover wasps (Vespidae: Stenogastrinae) the Dufour’s gland substances act as a substrate on which eggs are laid and food is placed for subsequent consumption by larvae and adults (Sledge et al. 2000, Fortunato and Turillazzi 2012). The secretion of a jelly-like substance by the Dufour’s gland that functions in larval nutrition has been proposed as an important step towards the evolution of sociality in this lineage. It enables anchoring eggs and larvae, and the storage of liquid food in the nest, subsequently facilitating the evolution of behavioral mechanisms that facilitate social interaction (Turillazzi 1989, Cervo et al. 2002). Apart from these functions, Dufour’s gland secretions also serve as ant repellents. They are used to construct sticky barriers which serve as ant guards around the nest (Sledge et al. 2000, Fortunato and Turillazzi 2012). These aid in preventing predation of immature brood by ants. Dufour’s gland compounds may also function in nestmate recognition (Cervo et al. 2002). Additionally, the Dufour’s gland secretions serve as food for both larvae and adults and can be stored for future consumption. This seems to be an important development that may have facilitated the evolution of sociality in this group (Turillazzi 1989, Cervo et al. 2002, Fortunato and Turillazzi 2012). The Dufour’s gland secretions function as ant repellants (Sledge et al. 2000), and this should be another important factor facilitating progressive provisioning of brood. Progressive provisioning of brood is a character shared by most eusocial Hymenoptera, and is important for the evolution of sociality (Hunt 2007). Evidence for Dufour’s gland compounds acting in nestmate recognition in stenogastrines reinforces the idea of chemicals having other primary functions being secondarily involved in chemical communication. Since the gland secretions have to be applied inside cells, and also outside and around the nest, to repel ants, this should facilitate nest or nestmate recognition by nest odor. Thus the Dufour’s gland secretions can be intertwined with the coevolution of brood rearing, avoiding brood predation, and maintaining colony fidelity, and appear to have an important role in the evolution of sociality in hover wasps.
Among other vespids, in the Vespinae the Dufour’s gland has been postulated to facilitate social parasitism by increasing acceptance of parasites among host individuals, or by acting as an alarm pheromone, eventually serving as a means of usurping and controlling the host colony (Jeanne 1977, Reed 1982, Reed and Akre 1982). The Dufour’s gland secretion of Vespa orientalis has been reported to be slightly lethal to honey bees, thereby implicating a role in venom secretion (Barr-Nea et al. 1976). This may be another example of an adaptation of Dufour’s gland secretions for a function not reported in other hymenopteran taxa. In Polistinae, Dufour’s gland compounds have been implicated in dominance interactions, egg marking, nestmate recognition, and queen signaling (Downing and Jeanne 1983, Downing 1991, Dani et al. 1996a, 1996b, Mitra et al. 2011, Mitra and Gadagkar 2011, Mitra and Gadagkar 2012a, 2012b). In the polistine wasp Ropalidia marginata the Dufour’s gland has been shown to be involved in producing the queen pheromone, using which the queen conveys her presence to workers and thereby maintains reproductive monopoly (Mitra et al. 2011). It has also been found that the Dufour’s gland hydrocarbon composition varies as a function of ovarian development, thereby suggesting that such chemicals can act as an honest signal of the queen’s fertility (Mitra and Gadagkar 2011, Mitra and Gadagkar 2012a, 2012b). Interestingly it has been found that the cuticle and the haemolymph of Ropalidia marginata also contain the same set of hydrocarbons as found in the Dufour’s gland, thereby implying that these hydrocarbons can be synthesized in the oenocytes of fat bodies, from where they enter circulation in the haemoplymph and are finally sequestered and stored in the Dufour’s gland or secreted on the cuticle (Mitra and Gadagkar: in press). The function of the Dufour’s gland remains unexplored in other Vespidae like potter wasps (Eumeninae) or pollen wasps (Masarinae). Examples of chemical communication through Dufour’s gland in vespids, like dominance signaling, queen signaling, nestmate recognition or social parasitism (Fig. 3), again suggest adaptations in a case specific manner. The ability to perceive chemicals secreted from the Dufour’s gland should be necessary to facilitate the evolution of adaptive communicative functions. Interestingly, in a polistine wasp it was found that dominance signaling through Dufour’s gland secretions may act via egg marking (Downing and Jeanne 1983, Downing 1991), perhaps exemplifying the ancestral function of egg coating giving rise to novel communicative roles. The role of the Dufour’s gland in communication in vespids may have coevolved with the evolution of various behavioral and chemical communication systems exhibited by vespids, or it may also have evolved coincidentally with the evolution of vespid sociality.