In his chapter on the Bombyliidae in Manual of Nearctic Diptera, J.C. Hall describes pupae in this family as having "several pairs of heavily sclerotized horns or prongs on the head." Further, "each abdominal segment is provided dorsally with a row of alternating hooks and long strong bristles." The tip of the abdomen has "one or two pairs of short or long prongs."
Immature Bombyliids are most often associated with bee nests, or with immature Lepidoptera or Coleoptera. A smaller number of Bombyliids are associated with immature Neuroptera, Diptera, or eat the egg masses of Orthoptera.