Bomber Flies: these flies launch their own eggs into the nests of wasps and bees, hovering in mid-air as they do so; the females also have a unique perivaginal pouch that is known as a "sand chamber" because it is filled with sand
Above: Anastoechus sp. (top) and Anastoechus nitidulus (bottom)
The flies of this family (Bombyliidae) are generally referred to as "bee flies," because they are known to mimic bees and bumblebees, but many of them are also known as "bomber flies," thanks to their unusual ovipository technique.
Above: a bomber fly of the genus Anastoechus
As the female bomber fly is preparing to lay her eggs, she will usually look for a soil-hidden nest that already contains the eggs/larvae of another insect. Most bomber flies lay their eggs in the nests of wasps and bees, but there are a few species that are known to target beetles, grasshoppers, locusts, other flies, and/or caterpillars, instead. Once the bomber fly has found a suitable nest, she will hover over the entrance to the burrow and rapidly flick her abdomen up and down until she has launched a single egg into the nest. She must then go searching for more nests in which to lay her other eggs.
After hatching, the parasitoid larvae of the bomber fly feeds on the eggs/larvae of the host species (i.e. the nest's original occupants). As this article explains:
When a bee fly egg hatches, a tiny larva called a planidium emerges. The planidium is quite unlike most fly larvae (also known as maggots); they are adapted for moving quickly in search of a host. Once the planidium finds a host, it will attach itself to the host’s exterior, like a leech or sea lamprey, and begin to feed. Yes folks, this is the stuff of nightmares.
Female bomber flies also have another peculiar trait: their abdomen contains a specialized perivaginal pouch known as a "sand chamber." During the ovipository process, the fly fills this pouch with fine grains of sand, and her eggs are then coated in the sand as they pass through the perivaginal pouch. The pouch is also lined with elongated setae ("hairs") and enlarged spikes that prevent the grains of sand from spilling back out.
The overall purpose of the "sand chamber" is still unclear, but many researchers believe that coating the eggs in a layer of sand may help to improve the viability of the eggs by preventing them from being damaged as they are launched to the ground, and that it also provides them with some camouflage after they have landed. Some researchers have argued that it prevents the eggs from becoming dehydrated, because the barrier helps to improve fluid retention, while others believe that it improves the mother's aim by increasing the weight of each egg. There is some evidence to suggest that it could also prevent the eggs from being detected (and then destroyed, abandoned, or ejected) by the host species, because the sand helps to mask/mute the egg's biochemical signals.
Above: Anastoechus barbatus
Adult bomber flies tend to feed on pollen and nectar. They are excellent pollinators, and their parasitoid larvae may also help to keep wasp and bee populations in check.
This article notes that bee flies (particularly Anastoechus nitidulus) likely served as the inspiration for the Pokémon known as cutiefly:
Biological Journal of the Linnean Society: The Evolutionary Pattern of Host Use in the Bombyliidae (Diptera) Family
YouTube: Video of a Bomber Fly (Bombylius major) Selecting a Nest and then Launching her Egg into the Burrow
Smithsonian Institution Press: Bee Flies of the World (pages 6 and 27)
Michigan State University: Bee Flies
Animal Diversity Web: Genus Bombylius
Animal Diversity Web: Anthrax albofasciatus
Journal of Geek Studies: The Entomological Diversity of Pokémon
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