Bees are insects studied extensively with a wide range of information published annually. The genus Centris Fabricius includes about 230 species of solitary bees spread from Argentina to the southern United States. Species of Centris collect floral oil, a resource used to feed the larvae and to line the brood cells. The subgenus Centris (Paracentris) Cameron, is most commonly represented in semi-desert, Andean regions, xeric areas in North American or semi-arid northeastern Brazil. The production of floral oil is present in 11 plant families that occur mainly in tropical and subtropical regions of the planet. To collect, manipulate and transport the floral lipids the bees have specialized structures in their legs that support this behavior. Nevertheless, some bees of this genus have lost the behavior of collecting oil. This is the case of the Centris (Paracentris) pallida that occur in sympatry with others Centris (Paracentris) species on Sonoran Desert, in USA. The purposes of this project is to study the interaction between bees of the subgenus Centris (Paracentris) with plants of the genus Krameria, a floral-oil producing plant that occur in the Sonora Desert, Arizona, USA. The data found in Brazil for Centris burgdorfi support the idea that Centris (Paracentris) may have preference for Krameria flowers, even when others oil plants are available. Our questions are: (1) Which plants the North-American Centris (Paracentris) use as floral resources to feed their larvae (pollen source as well)? (2) Are olfactory signals of the flowers important for bees to find the host plants? (3) The oil-collecting structures are similar between Centris (Paracentris) species? (4) What is the nutritional value of the diet in the brood cells in species with different strategies (oil-collecting bee vs non-oil-collecting bee)?
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