Please refer also to the following link for details on this group:
Halictidae = Link 1
Halictidae. -- The sweat bees are small to moderately sized bees that have a metallic sheen. Their arched basal wing vein distinguishes them. Most nest in ground burrows, either on level ground or in river banks. Their principal tunnel is often vertical with lateral tunnels that branch out to end in a single cell.The genus Sphecodes is known to live at the expense of other insects by commandeering the nests of bees of the genus Halictus and of some Andrenidae. The relationship existing between S. monilicornis Kirby (= subquadratus Smith) and H. malachurus Kirby was discussed by Ferton (1923), who noted that the burrow of the latter is always guarded. The Sphecodes intruder has to kill the defender before taking possession of the nest, and the body of the latter is ejected from the burrow (Clausen 1940/1962).
The lacinia consists of a small lobe separated by a membrane from the rest of the maxilla (Finnamore & Michener 1993). The glossa is of variable length and pointed; a basitibial plate exists in nonparasitic females and many males; the pygidial plate is present in females but frequently hidden under the fifth tergum. The pygidial plate is reduced in parasitic genera. Preepisternal grooves are present (complete, ie., extending ventral to the scrobal groove) in common groups except Nomiinae, including all Canadian species.
The Halictidae and Apidae are the most numerous bees. There are more than 3,525 species known. There are more than 500 species in North America alone. Subfamilies are Halictinae, Nomiinae and Rophitinae. The largest subfamily, the worldwide Halictinae, contains such familiar genera as Halictus and Lasioglossum, and the green Augochlora and related genera, and Agapostemon.
Halictidae nest in soil burrows, but a few Halictinae tunnel in decaying wood also. Most species are solitary, but some Nomiinae and Halictinae are collective and some Halictinae are eusocial with different female castes. Many differences between solitary and fully eusocial exist among the species that have been studied (Finnamore & Michener 1993).
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Engel, M.S. 1995. "Neocorynura electra, a New Fossil Bee Species from Dominican Amber (Hymenoptera:Halictidae)". Journal of the New York Entomological Society 103 (3): 317–323.
Engel, M.S. 2000. "Classification of the bee tribe Augochlorini (Hymenoptera, Halictidae)". Bulletin of the American Museum of Natural History 250: 1-89
Engel,M.S, Archibald,S.B. 2003. An Early Eocene bee (Hymenoptera: Halictidae) from Quilchena, British Columbia. The Canadian Entomologist, Vol. 135, No. 1.
Grimaldi, D. and Engel, M.S. 2005. Evolution of the Insects. Cambridge University Press. ISBN 0-521-82149-5.
Patiny, S et al. 2008. Phylogenetic relationships and host-plant evolution within the basal clade of Halictidae (Hymenoptera, Apoidea). Cladistics 24: 255–269