Clausen (1940) discussed bees in the Stelidae are inquilines in the cells of other bees, principally Megachilidae. Clausen (1940) noted that attack on the early stages of the host in the cell is mainly for the purpose of eliminating a rival for the food supply. In Europe, Stelis nasuta Latr. attacks the cells of Chalicodoma muraria F. (Fabre 1886). Such cells are made of a hard, plaster-like substance, ca. 1 cm. thick, which the female must penetrate before oviposition is possible. The lid is removed with her mandibles, and several eggs are laid next to the host. After ovipositing, the cell is sealed by the stelid. The young larvae of the host and stelid feed in the same area without interference; but the Stelis larva develops more rapidly and the host does not attain sufficient food to complete development. In S. minuta Nyl., the female lays the egg on the food material in the open cell of Osmia leucomeleana Kirby before the latter lays her egg (Verhoeff 1892). The Stelis egg hatches first, and for some time the two larvae feed independently on the beebread, the parasitoid at the bottom and the host at the top. Development of the former is quicker and eventually the latter consumes the host larva.
Extended accounts of the behavior of a member of the family is given by Graenicher (1905b) on Stelis sexmaculata Ashm., which develops in the cells of Andronicus productus Cress. The egg is laid while the host is in either the egg or the larval stage, and the young Stelis larva possesses long falcate mandibles that extent forward similar to those of a chrysopid larva. It is very aggressive and, although considerably smaller than the host (ca. 1/4th as long) it is able to overcome it. The invader feeds on honey and pollen stored in the cell, and finishes development after the host is killed.
Bluhm, B. A., I. R. MacDonald, C. Debenham & K. Iken. 2004. Macro and Megabenthic Communities in the High Arctic Canada Basin: Initial Findings. Polar Biology 28: 218-231