Genus: Coelioxys Latreille, 1809
Subgenera: Allocoelioxys, Austrocleptria, Boreocoelioxys, Callosarissa, Coelioxys, Cyrtocoelioxys, Dasycoelioxys, Eingana, Glyptocoelioxys, Leuraspidia, Liothyrapis, Melissoctonia, Neocoelioxys, Paracoelioxys, Platycoelioxys, Rhinocoelioxys, Rozeniana, Synocoelioxys, Torridapis, Xerocoelioxys
Common name: cuckoo bees
Coelioxys are black to dark brown bees that range in body length from 5–22 mm. Intraspecific variation can be high, with color and structural characters usually of value in identification between individuals in addition to large variations in adult body size (Michener 2007). Sometimes the abdomen has reddish areas with pale apical hair bands on the terga (Michener 2007). The thorax is black and usually possesses axilla that are angled or protruding as a spine. Overall, there is a reduction in hair across the body. Females are easily recognized by their pointed abdomen and lack of scopae (Droege 2015). T6 of the male is usually ornamented with an arrangement of knobs, teeth, or spikes (Michener 2007).
Coelioxys consists of over 500 species in 20 subgenera worldwide; around 45 species in 9 subgenera in the United States and 10 species in Canada (Michener 2007; Wilson and Carril 2016). A recent revision expanded the number of world subgenera from 15 to 20 (Rocha-Filho and Packer 2016).
(modified from Michener 2007)
Coelioxys may be confused with species in the genera Radoszkowskiana due to tapering of the abdomen after the second segment in females, lack of scopa, and presence of preoccipital carina on the side of the head (Michener 2007). Coelioxys can be differentiated from Radoszkowskiana based on the combination of distinguishing characteristics listed above. In addition, as larvae Coelioxys can be differentiated from Radoszkowskiana by the curvature of the first instar’s mandibles: Radoszkowskiana curve orally, while Coelioxys bend caudally (Rozen and Kamel 2008).
Coelioxys may be also confused with Dioxys due to similar appearance, with females having reduced scopal hairs and a pointed abdomen with black and white banding. However, Coelioxys lacks the medial spine on the metanotum that is present in Dioxys (Michener 2007).
Coelioxys coturnix, a native of the Mediterranean, southwestern Europe, North Africa, and India was first noticed in the United States in 2000. It was unintentionally introduced to the U.S., and has been found in Maryland, Washington, D.C., southern New England, and southern Pennsylvania. Its presumed host in the U.S. is Megachile rotundata (Droege 2015).
Female Coelioxys do not gather pollen from flowers since the larvae develop parasitically on their host’s pollen provisions (Michener 2007). It is presumed that wide varieties of flowers are used for nectar.
Coelioxys are one of the various groups of solitary brood parasites collectively referred to as “cuckoo bees” or cleptoparasites. Most known hosts are Megachile (Droege 2015), but a few are known to parasitize orchid bees and cavity-nesting species of Centris. After a host nest is found, a small, curved, specialized egg is laid inside a host cell that is being provisioned by the host female, or is injected through a slit made into the cell after it has been sealed off (Baker 1971). Eggs may be placed near or on host eggs, oviposited through the pollen provisions, or stuck between pieces of leaf that make up the cell wall (Scott et al. 2000). Unlike most bee larvae, they are active and develop a sclerotized head bearing extended sickle-shaped mandibles that may be further modified with secondary spines or prongs (Baker 1971; Rozen and Kamel 2008). The mandibles are used to kill the host egg or larva. The larva has a “hospicidal” body form temporarily before molting into a more ordinary grub-like form (Rozen and Kamel 2008). If more than one egg is laid by the parasitic female, the hatched larvae will destroy sibling eggs as well as the host’s egg (Baker 1971; Scott et al. 2000). Subsequently, it feeds on the pollen stores of its host (Scott et al. 2000).
Coelioxys is distributed worldwide, with high species diversity in South America, and relatively little diversity in Australia. In North America, they are found transcontinentally, from the Arctic to Florida, in a wide variety of habitats (Michener 2007; Wilson and Carril 2016).