The bees nesting in the bee hotel attract other guests that are after the bee larvae like this bee fly, Anthrax anthrax [Soortenregister],[Smit 2016].
A nicely colored black insect with its grey nuances and white accents.
BOMBYLIIDAE
The fly is a member of the Bee fly family (Bombyliidae), whose members are mostly ectoparasitoïds [Yeates & Greathead 1997]. In contrast to A. anthrax a typical bee fly has a resemblance to bumble bee with a long snout.
1. DISTRIBUTION
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.
2. BEHAVIOUR
2.1. ACTIVITY
This species is present from end April until beginning of September with a peak in May and June.
2.2. DEVELOPMENT
Bombyliidae and thus Anthrax are oviparous which means they lay eggs that hatch outside the mother.
Beehotel
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 [Stanisavljevic 2005] since the species in the genus Anthrax are ectoparasites on holometabolic insects that use tube shaped nests [Marston 1970] in the orders [Yeates & Greathead 1997]:
- 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.
Oviposition
Bee flies are parasitic and the females have developed a complex oviposition, egg-laying, strategy in which their bodies are modified with a sand chamber [Yeates 1994], an opening in the underside of the abdomen which is filled with ground particles like sand. Using powerful movements of tergites VIII, IX and X she sweeps up particles from the ground into the sand chamber [Yeates 1994] 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 [Yeates 1994].
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 [Seidelmann 2006]. 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 [Wijngaard 2012] using the flexible back part of the abdomen [Stanisavljevic 2005].
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 [Yeates & Greathead 1997] and for A. tigrinus it is estimated that she is capable of producing 2000-3000 eggs [Gerling & Hermann 1976].
The development and behaviour of the first instar larva, the planidium, the other instars and the pupa is has been thoroughly described for A. limulaus [Marston 1962] and to a lesser extent for A. tigrinus [Gerling & Hermann 1976]. A summary can be found in the next sections to give an impression for the possible development and behaviour 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 [Stanisavljevic 2005].
The larva moves like a caterpillar, 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 [Marston 1962].
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 [Marston 1962]. 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.
Pupa
After 4 days on average the third instar pupates. This stage takes 24 days on average to complete [Marston 1962]. 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 [Stanisavljevic 2005]. 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 [Stanisavljevic 2005].
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.
Adult
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 [Marston 1962].
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 [Marston 1962].
2.3. FOOD
Adult Bombyliidae drink nectar, and the females eat pollen as well to support egg production [Yeates 1994]. The sub-family Anthricinae that includes the Anthrax genus is seen less frequently on flowers [Marston 1970], [Marston 1962] and has been seen to specialize on specific flowers [El-Moursy et al. 1999].
3. IDENTIFICATION
3.1. PUPA
The pupa of this and most other Bobmyliidae species is recognizable by the characteristic hairs and spines on 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 [Gerling & Hermann 1976]. Without hooking these hairs the pupa is incapable to hatch [Marston 1962].
3.2. ADULT
The adult fly measures 7 – 13 mm in size [Smit 2016].
Male
Females
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.
Sand chamber
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 [Yeates 1994]. Tergite VIII forms the upperside and sides of the sand chamber and has a brush of long hairs on the posterior rim [Yeates 1994].
4. PARASITE
A. anthrax is a parasite on the following species in the garden:
- Osmia cornuta [Stanisavljevic 2005]
- Osmia bicornis [Stanisavljevic 2005]
Literature
El-Moursy et al. 1999 El-Moursy, A., Gilbert, F., Zalat, S., & El-Hawagry, M., 1999. Foraging behaviour of anthracine flies (Diptera: Bombyliidae) in southern Sinai, Egypt. Egyptian Journal of Biology, 1, 87-95.Gerling & Hermann 1976 Gerling, D., & Hermann, H., 1976. The oviposition and life cycle of Anthrax tigrinus,[Dipt.: Bombyliidae] a parasite of carpenter bees [Hym.: Xylocopidae]. Entomophaga, 21(3), 227-233.
Marston 1962 Marston, N. L., 1962. A monograph of the nearctic species of the Albofasciatus group of the genus Anthrax scopoli (Diptera: Bombyliidae).
Marston 1970 Marston, N. L., 1970. Revision of New World species of Anthrax (Diptera: Bombyliidae), other than the Anthrax albofasciatus group. Smithsonian Contributions to Zoology, 1970.
Seidelmann 2006 Seidelmann, K., 2006. Open-cell parasitism shapes maternal investment patterns in the Red Mason bee Osmia rufa. Behavioral Ecology, 17(5), 839-848.
Smit 2016 Smit, J.T., 2016. Soortzoeker wolzwevers van de Benelux. Naturalis Biodiversity Center & EIS Kenniscentrum Insecten en andere ongewervelden, Leiden.
Soortenregister Nederlands Soortenregister
Stanisavljevic 2005 Stanisavljevic, L., 2005. The accompanying fauna of Osmia cornuta and Osmia rufa and effective measures of protection. Bulletin of Insectology, 58(2), 141-152.
Wijngaard 2012 Wijngaard, W., 2012. Control of hovering flight during oviposition by two species of Bombyliidae. In Proceedings of the Netherlands Entomological Society Meeting (Vol. 23, pp. 9-20).
Yeates 1994 Yeates, D. K., 1994. The cladistics and classification of the Bombyliidae (Diptera: Asiloidea). Bulletin of the American Museum of Natural History. 219. 1-191.
Yeates & Greathead 1997 Yeates, D. K., & Greathead, D., 1997. The evolutionary pattern of host use in the Bombyliidae (Diptera): a diverse family of parasitoid flies. Biological Journal of the Linnean Society, 60(2), 149-185.
Citation
Krischan, O.R., 2025. Anthrax anthrax. Kerfdier, www.kerfdier.nl. Accessed on 27 April 2025.
