Comparative morphology of spermathecae in solitary and primitively eusocial bees (Hymenoptera; Apoidea)

Spermathecae of solitary and primitively eusocial bees from five major families were compared according to their chamber size, epithelial cell height (wall thickness), number and shape of glands, number of sperm pumps, and diameter of the duct, using whole mounts and serial sections. A thick spermathecal wall with a small chamber in our exemplar colletid, megachilid, and andrenid bees, a large chamber with a thick wall in social Apidae, and a thin wall with a large chamber in social halictids indicate that a big chamber may be associated with increased colony size. A thin epithelium in the receptacle of halictid bees suggests relegation of spermathecal secretion to the spermathecal gland. A relatively thick spermathecal wall in the Apinae indicates the potential importance of both the epithelial cell layer of the receptacle and the enlarged spermathecal gland in maintaining large numbers of stored sperm. There was one sperm pump in all taxa surveyed except the representative Halictidae, in which this structure was paired. Elongated spermathecal glands may typify the spermathecae of eusocial bees. RCsumC : La spermathkque a CtC CtudiCe par examen de priparations entikres et de coupes sCriCes chez des guCpes solitaires et des guCpes eusociales primitives appartenant h cinq grandes familles : la taille de la chambre, la hauteur des cellules CpithCliales (Cpaisseur de la paroi), le nombre et la forme des glandes, le nombre de pompes spermatiques et le diamktre du canal ont CtC comparCs chez ces insectes. Une paroi Cpaisse avec petite chambre chez les collCtides, mkgachilides et andrinides, une grande chambre avec paroi Cpaisse chez les apides sociales et une paroi mince avec grande chambre chez les guCpes halictides sociales semblent indiquer qu'une chambre de grande taille est associie aux colonies de grande taille. L'CpithClium mince du riceptacle des halictides semble indiquer que les sCcrCtions de la spermathkque proviennent exclusivement de la glande de la spermathkque. La paroi relativement Cpaisse de la spermathkque des Apinae indique que la couche de cellules CpithCliales du receptacle et la glande amplifiCe de la spermathkque concourent probablement toutes deux h la mise en risewe de grands nombres de spermatozoi'des. I1 n'y a qu'une seule pompe spermatique chez tous les taxons examinks, h l'exception du reprisentant des Halictidae qui compte une paire de pompes. Les glandes allongCes de la spermathkque chez les guCpes eusociales sont probablement un caractkre typique. [Traduit par la RCdaction] Introduction spermatozoa and the duration of their storage by females. Following copulation, females of most insects store spermatozoa in the spermathecae, where they remain until they are used for fertilising eggs. Extensive studies of the spermatheca and its secretions in nonsocial species have explored the mechanisms that keep stored spermatozoa viable during the reproductive life of the female (Davey and Webster 1967; Clements and Potter 1967; Tombes and Roppel 197 1, 1972; Villavaso 1975; Ahmed and Gillott 1982). Spermathecae provide both a suitable environment for the spermatozoa and a source of metabolites for their maintenance, the latter derived from secretions of glandular cells that either line the chamber or are separately located (Davey 1985; Gillott 1988; Kaulenas 1992). In the Hymenoptera, there is particularly wide variation in both the number of eggs laid and the length of time over which they are produced, giving rise to widely differing requirements in terms of numbers of ~ e e s , in particular, display a wide array of social behaviours and range from solitary to eusocial forms, with multiple independent origins of a reproductive division of labour (Wilson 197 1 ; Michener 1974). This unique situation lends itself to the study of reproductive mechanisms that accompany the evolution of social behaviour in this group. Females of the solitary mining bee, Andrena erythronii, and the alfalfa leaf-cutter bee, Megachile rotundata, lay an average of approximately 8 and 20 diploid eggs, respectively, in their lifetime (Michener and Rettenmeyer 1956; Gerber and Klostermeyer 1970). Foundresses of the primitively eusocial Lasioglossum laevissimum produce a maximum of 52 worker-brood individuals (in a multiple-foundress nest) and 66 reproductives (Packer 1992), giving a total of approximately 75 diploid eggs requiring sperm for fertilisation. These are comparatively low reproductive outputs requiring fewer stored spermatozoa than, for example, in the perennial colonies of the primitively eusocial halictine bee Lasioglossum marginatum, where a single queen may need to fertilise over Received June 28, 1995. Accepted November 2 1, 1995. 2000 eggs (Plateaux-Quenu 1960). Spermathecae of Apis N. Pabalan, K.G. Davey, and L. Packer. Department of mellifera queens store sufficient spermatozoa to last their Biology, York University, 4700 Keele Street, North York, lifetime, during which hundreds of thousands of fertilised ON M3J 1P3, Canada. eggs are laid (Bresslau 1906; Snodgrass 1956; Poole 1970; Can. J. Zool. 74: 802-808 (1996). 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