Bombylius major exhibits a unique flight behavior known as "yawing"[1] and plays a role in general pollination, without preference of flower types.

[3] However, the fly uses this mimicry of bumblebees to its own advantage, allowing close access to host solitary bee and wasp nests in order to deposit its eggs.

[2] Bombylius major can be found from April to June throughout temperate Europe, North America and some parts of Asia, concentrated in the northern hemisphere.

It has dark patches on the anterior half of the wings and long hairy legs that dangle while in flight.

[6] Their boldly patterned wings have a distinct dividing border through the horizontal middle between the dark and clear portions.

[2] While its wings continue to beat, its front legs grip the flower and its long rigid beak is inserted to collect the nectar.

However, there are several distinguishing features: B. major has only one pair of wings (bees have two), extremely thin legs, and the head is very small, with a long rigid proboscis.

[10] The larvae are hypermetamorphic parasitoids which then feed on the food stored, as well as the young solitary bees or wasps.

[10] The parasitic eggs of B. major are produced in large numbers, however few will make it to the host insect burrow entrance.

[9] White larvae gradually turn into a yellowish brown pupa, with distinct mouthparts, wings, antennae, and legs.

[13] Along with one other species, Bombylius pygmaeus, B. major was observed to selectively (and almost exclusively) visit bluets at several North American sites despite the abundant presence of many other flowers.

[14] In Russia, female B. major act in a narrow oligophagous manner, favoring a limited number of food sources, including pollen grains of Siberian squill (Scilla siberica), lesser celandine (Ficaria verna), and willow (Salix caprea) over dandelion (Taraxacum officinale), coltsfoot (Tussilago farfara), and primrose (Primula vulgaris).

However, it is still unknown what can cause this behavior to be triggered and what purpose it serves, but a proposed explanation includes mating habits.

Some flower species, such as Delphinium tricorne, are even specifically adapted to the fly in terms of color, shape, and form.