The conopidae are also sometimes called the Thick-headed flies. They are closely related to the hoverflies, and are often mimics of wasps. The larvae are parasites of bees and wasps while the adults feed on nectar. As their common name would suggest, the head is usually the widest part of the body. There are around 800 species world wide, and 25 in Great Britain.
Above is an adult Conops quadrifasciatus. In the U. K. adults fly from June - September, and are found on ragwort, umbellifers and composites where females wait to pounce on a bumblebee and lay their egg, but will also attack bumblebees in flight. Once a female has grabbed a bumblebee she quickly injects her egg into the bumblebee's abdomen. The egg hatches inside the bumblebee and feeds off the abdominal contents. The bumblebee eventually dies and the fly larva pupates and hatches as an adult the following spring. Parasitised bumblebees tend to spend more time out of the nest than normal workers, even sleeping outside all night.