The bees netsing in the bee hotel attract other guests that are after the bee larvae like this bee fly, Anthrax anthrax [1, 2].
A nicely colored black insect with its grey nuances and white accents.
The fly is a member of the Bee fly family (Bombyliidae), whose members are mostly ectoparasitoïds . In contrast to A. anthrax a typical bee fly has a resemblance to bumble bee with a long snout.
A. anthrax is a common species in the Netherlands. In the garden it appears regularly in smaller numbers of 1 to 3 on the bee hotels.
This species is present from end April until beginning of September with a peak in May and June.
Bombyliidae and thus Anthrax are oviparous which means they lay eggs that hatch outside the mother.
The female is a regular visitor on the bee hotels especially on warm days. They are attracted by the shape and contrast of black discs on different colored backgrounds . becaue the species in the genus Anthrax are ectoparasites on holometabolic insects that use tube shaped nests  in the orders :
- Apoidae (bees)
- Coleoptera (beetles)
- Hymenoptera (wasps)
- Neuroptera (net winged insects)
The female hovers silently like a dark spectre along the bee hotel surface looking for a proper nesting place in which she catapults an egg.
Bee flies are parasitic and the females have developed a complex oviposition, egg-laying, strategy in which their bodies are modified with a san chamber  , an opening in the underside of the abdomen which is filled with ground particles like sand. Using powerfull movements of tergites VIII, IX and X she sweeps up particles from the ground into the sand chamber  to charge it. The eggs are covered with a substance using special glands and are deposited via the oviduct into the charged sand chamber where they are covered with particles .
The females lays the eggs in the open cells of her host during the collection phase while the host is away from the nest foraging for pollen . The A. anthrax female lands next to the target nest to inspect it. After that it will take off, hover with constant distance in front of the opening and in a quick movement catapult one egg, sometimes two, in it  using the flexible back part of the abdomen , see also the filmp from the site Insect Behavior below.
The Bombyliius egg production often is extremely high. Data on this for A. anthrax does not seem available but among the other species in the genus A. limulates produces some 1000 eggs per day  and for A. tigrinus it is estimated that she is capable of producing 2000-3000 eggs .
The development and behavior of the first instar larva, the planidium, the other instars and the pupa is has been thoroughly described for A. limulaus  and to a lesser extent for A. tigrinus . A summary can be found in the next sections to give an impression for the possible development and behavior of A. anthrax.
Planidium (1st instar)
The planidium crawls from the egg and starts searching for the bee larva. It is unclear whether the mobile planidium is able to cross mud walls between the cells .
The larva moves like a catrepillar, is fast and almost never stops. When it reaches the cell it will keep moving around and only when the host moults the first time will it attach itself to the host and feed sparingly. After about 20 days the host will pupate, which will trigger the planidium to moult to the 2nd instar and attach itself with its mandibles to the pupa .
Larvae (2nd – 3rd instars)
The larvae are hypermetamorph, which means that they undergo more than one metamorphosis. The shape of the second and third instars is completely different from the first and resembles a beetle grub. These instars are slow and don’t move much other than repositioning themselves on the host . The second instar grows fast with about 0,5mm a day and will moult after about 7 days. The third instar has a growth rate of 2,5mm per day. About days before pupating the larvae will start to elongate into the pupa shape.
After 4 days on average the third instar puppates. This stage takes 24 days on average to complete . The pupa has a characteristic crest on the head that is used to break through the mud walls that separate the cells in order to reach the exit . During its migration to the exit the pupa will crush all host cocoons in the larval state that occupy the cells it passed through, and contributes this way to a high mortality rate within the host population .
The pupa will stop when the exit is reached and breach the final barrier. After breaching the pupa breaks open and the fly appears. The pupa thorax has long hooked hairs that are used anchor it to the wall and exit. After the fly has emerged the empty shell is often left hanging out the exit.
The photograph below shows two puppae shells within the same bee nest.
Also in this one two flies emerged from the same nest, one of them still had a piece of bee cocoon attached to it.
The adult crawls out the pupa and rest not far from the cell it came out of to harden. When a recently emerged adult is spotted often the empty pupa skin will be visible in one of the cell holes next to it.
About 2 minutes after hatching the wings are completely extended .
Based on own observations the fly will empty its bowels after about 15 – 20 minutes in thick, grey colored drops.
Some time after this the fly may start to walk over the surface in characteristic slow manner. After about 2 hours the fly is complete hardened .
Adult Bombyliidae drink nectar, and the females eat pollen as well to support egg production . The sub-family Anthricinae that includes the Anthrax genus is seen less frequently on flowers [4, 10] and has been seen to specialize on specific flowers .
The pupa of this and most other Bobmyliidae species is recognizable by the characteristic hairs and spineson the head, thorax and abdomen. The crest used to break through cell walls consists of a number of thick spines that are developed on the pupa labrum, the lip.
The pupa also has a large number of hooked long hairs on its thorax that are used to anchor itself in the exit . Without hooking these hairs the pupa is incapable to hatch .
The adult fly measures 7 – 13 mm in size .
The female does not have rotated tergites.
Photos 1 – 3 are taken in sequence and show that the genital section is outward extendable. Photo 4 is of a different specimen that recently hatched and was drying. It shows clearly that the whole genital section of tergites VIII – X is fully extended. Possibly the section will retract inward as the animal hardens.
The sand chamber is a structure formed by the elongated sides of tergite VIII and the invagination and vertical repositioning of both tergite and sternite VIII . Tergite VIII forms the upperside and sides of the sand chamber and has a brush of long hairs on the posterior rim .
A. anthrax is a parasite on the following species in the garden:
- metselbij Osmia cornuta 
- metselbij Osmia bicornis 
References1 Nederlands Soortenregister
2 Smit, J.T., 2016. Soortzoeker wolzwevers van de Benelux. Naturalis Biodiversity Center & EIS Kenniscentrum Insecten en andere ongewervelden, Leiden.
3 KRUNIC, M & Stanisavljević, Ljubiša & PINZAUTI, M. (2005). The accompanying fauna of Osmia cornuta and Osmia rufa and effective measures of protection. Bull. Insectol. 58. 141-152.
4 MARSTON, Norman L. Revision of New World species of Anthrax (Diptera: Bombyliidae), other than the Anthrax albofasciatus group. Smithsonian Contributions to Zoology, 1970.
5 David K. Yeates, David Greathead, The evolutionary pattern of host use in the Bombyliidae (Diptera): a diverse family of parasitoid flies, Biological Journal of the Linnean Society, Volume 60, Issue 2, February 1997, Pages 149–185, https://doi.org/10.1111/j.1095-8312.1997.tb01490.x
6 Karsten Seidelmann, Open-cell parasitism shapes maternal investment patterns in the Red Mason bee Osmia rufa, Behavioral Ecology, Volume 17, Issue 5, September/October 2006, Pages 839–848, https://doi.org/10.1093/beheco/arl017
7 Yeates, David. (1994). The cladistics and classification of the Bombyliidae (Diptera: Asiloidea). Bulletin of the American Museum of Natural History. 219. 1-191.
8 Gerling, Dan & Hermann, H. (1976). The oviposition and life cycle of Anthrax tigrinus, [Dipt.: Bombyliidae] a parasite of carpenter bees [Hym.: Xylocopidae]. BioControl. 21. 227-233. 10.1007/BF02371755.
9 WIJNGAARD, Wopke. Control of hovering flight during oviposition by two species of Bombyliidae. In: Proceedings of the Netherlands Entomological Society Meeting. 2012. p. 9-20.
10 MARSTON, Norman Lee. A monograph of the nearctic species of the Albofasciatus group of the genus Anthrax scopoli (Diptera: Bombyliidae). 1962.
11 EL-MOURSY, Aly, et al. Foraging behaviour of anthracine flies (Diptera: Bombyliidae) in southern Sinai, Egypt. Egyptian Journal of Biology, 1999, 1.1: 87-95.
12 Peeters, T.M.J., C. van Achterberg, W.R.B. Heitmans, W.F. Klein, V. Lefeber, A.J. van Loon, A.A. Mabelis, H. Nieuwen-huijsen, M. Reemer, J. de Rond, J. Smit, H.H.W. Velthuis, 2004. De wespen en mieren van Nederland (Hymenoptera: Aculeata). – Nederlandse Fauna 6. Nationaal Natuurhistorisch Museum Naturalis, Leiden, knnv Uitgeverij, Utrecht & European Invertebrate Survey – Nederland, Leiden.