I have written about the dead insects that show up in sealed containers of empty land snail shells. They are the luckless individuals that were metamorphosing in one of the shells at the time of the collection.
Yesterday I found a dead insect in a container that had 3 shells I had collected during a field trip in May.
I think it is a fly, although I may be wrong.
Scale in millimeters.
Which of the 3 shells, 2 Anguispira fergusoni and 1 Mesodon thyroidus, had the fly in it? The smaller A. fergusoni was probably too small for the fly and the larger one appeared empty when I examined it against a bright light. On the other hand, there is a dark object inside the Mesodon shell that may be the puparium. If I can get the fly identified, I will break open the shell to confirm the location of the fly’s site of metamorphosis.
Update added 27 June 2008: Lloyd V. Knutson and Wayne Mathis have identified the fly as Pherbellia albovaria (Diptera: Sciomyzidae). Sciomyzids are snail predators. Details in this post.
Update added 13 July 2008: The fly's puparium turned out to be in another snail shell. Details in this post.