In this post, I wrote about a dead fly I had found in a container of land snail shells collected last May and which turned out to be Pherbellia albovaria (Diptera: Sciomyzidae). The larvae of sciomyzids are snail predators. Bratt et al. (1969) gave several descriptions of the feeding habits of the larvae of P. albovaria. Here is one:
A larva that hatched on August 10 killed and ate 5 snails (1 [Discus] patulus, 1 [Mesodon] thyroidus, and 3 [Zonitoides] arboreus) during the 44 days it required to complete development to the pupal stage. Each of these snails was killed slowly, and the larva continued to feed in the rotting tissues for several days after the host died. The fifth snail was invaded on September 13 but did not die until September 16, and the larva continued feeding in it until puparium formation on September 22.The last instar larva pupates inside the shell of its last victim and after its development is completed, emerges as an adult fly to continue the cycle.
In the original post, I had a picture of a M. thyroidus shell that I thought contained the puparium of the dead fly. But I was mistaken; what I saw inside that shell was only some soil. Subsequently, I found the fly's puparium inside an Anguispira fergusoni shell.
The arrows mark the ends of the puparium. After I took this picture, I opened a hole in the shell and visually confirmed that the object inside was indeed a puparium.
Last Thursday afternoon I was at the Smithsonian's National Museum of Natural History and gave the fly and the shell with the puparium to the entomology collection. I am now preparing a short note for publication.
Here is another picture of the snail killer identified as Pherbellia albovaria by Lloyd V. Knutson. From the front of its head to the tips of its wings it was ~7.5 mm; in comparison, the diameter of the A. fergusoni shell was 9 mm.
Bratt, Knutson, Foote & Berg. (1969). Biology of Pherbellia (Diptera: Sciomyzidae). Cornell University Agricultural Experiment Station, Memoir 404. [I am grateful to Wayne Mathis of the NMNH for giving me a copy of this book.]