I found this firefly larva 2 years ago at the University of Michigan's Biological Station in Michigan's Upper Peninsula. When I first saw it in the woods, it was eating a snail. Even after I picked it up and carried it to my cabin in a vial, it kept on eating the snail. Subsequently, I brought it to Maryland, where it survived in my lab for several months feeding on snails and slugs. It went through one instar stage and I was hoping that it would eventually turn into an adult, but it died before becoming one.
The species of snails it ate in the lab were Cochlicopa sp., Oxyloma retusa (pictures above), Ventridens ligera and a juvenile specimen of the slug Deroceras reticulatum. It never seemed to have any difficulty overcoming the snails; it would simply force its way into the snail's aperture and attack it right away. Deroceras reticulatum, on the other hand, gave it a hard time. The first time it attacked the slug, the latter secreted such a large amount of slime that the larva got immobilized for more than 10 minutes before it could eat its way out of the slime. In the meantime, the slug crawled away. Under natural conditions that would have been the end of it. But they were in a small container and I was anxious to see if the larva would succeed. So, I kept bringing them together. Eventually, after several more attacks, the larva managed to overcome the slug.
LaBella & Lloyd1 cited some old references that claimed that the firefly larvae inject into their prey a poison to stun it. Symondson2, on the other hand, stated that the snail-eating larvae of several species of fireflies were not believed to inject poisonous substances into their prey. My observations of the slug mentioned above support the latter position, for the slug kept on moving and struggling even after the larva had bitten it.
The larvae of all fireflies (Lampyridae) are predaceous. This time of year I always get fireflies in my backyard. I don't know what their larvae eat though. I don't have any large snails, but there are lots of slugs. Some species are also known to eat earthworms and the larvae of other insects.
1. LaBella, D.M. & Lloyd, J.E. 1991. Lampyridae. In Immature Insects (Stehr, F.W., ed.), II:427.
2. Symondson , W.O.C. 2004. Coleoptera as predators of terrestrial gastropods. In Natural Enemies of Terrestrial Molluscs (Barker, G., ed.).