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Parasitic fly - Stylops melittae
Family - Stylopidaed
Also known as - Twisted wing parasite

Stylops melittae is a species of parasitic fly that parasitize mostly females of various mining bee species of the family Andrena.  The female ♀ produces a large number of first generation larvae which are themselves mainly female, to be distributed on flowers so that when a mining bee of a species that Stylops melittae prey on visits that flower for pollen it also picks up a parasitic load, transporting it back to the bee's nest.  These parasitic larvae then enter a larval stage of the new host species, moulting into a secondary larvae to feed and establish themselves in the abdomen.  Some time later after several further moults their anterior portion breaks through between two abdominal segments of the now adult bees skin, which in case of a male Stylops melittae they then pupate into flies, to go on and mate, produce more first generation larvae and re-infect more bees.  A bee infected with a parasite is called a "stylopised" bee.  The host species develops somewhat normally but with reduced function, females who seem to be more often infected are are supposedly sterile and exhibit male behaviour, however stylopised female bees have been photographed mating which questions their sterility.  The less common male stylopised bees tend to be more "female".

A genus of some 600 species only around 17 live in the U.K., extremely small and inconspicuous and commonly called twistedwing parasistes, the only time a Stylops is usually seen is when a stylopised bee is found.  Stylops melittae live by filtering particals out of the blood of the host, with females developing and remaing in the host as a larval form with only their heads protruding, they never fully develop into a fly but remain to await a male.  Males do develop into a full flying insect and leave the host to find a female and mate, after which they die.  The right hand photo below is of an older Chocolate Mining Bee heavily pollen covered which has been parasitised by a Stylops melittae larvae, in the photo a small yellow protusion will be seen on the bee's abdomen, this is the Stylops anterior portion.  The centre photo has a clearer view of another parasitized Bee.

Parasitic fly - Stylops melittae, click for a larger image, photo licensed for reuse
Photo ©2011 Aiwok
Parasitic fly - Stylops melittae, click for a larger image, photo licensed for reuse
Photo ©2011 Aiwok
Parasitic fly - Stylops melittae, click for a larger image
Photo ©2006
J. Lance, FBCP
Click any photo for a larger image

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Britain. Because they are extremely hard to find, it is possible there are many more species, probably even in Britain. In Europe most species are parasites to wasps and bees. A few species are known to attack bugs. Most species are so you'll never see them at all. Only one species, Stylops melittae, is sometimes seen, when the adult females stick their head out of the host, invariably a bee. A bee infected with a parasite is called a stylopised bee. Such a bee sometimes behaves oddly and it is claimed he or she is infertile. The picture in the middle proves that a stylopised bee is still capable of mating. You can see the head of the parasite sticking out between two segments of the female bee just in front of the males front leg. We are probably talking about Andrena tibialis, a mining bee. Mining bees are regularly found to be .