Critter Corner No. 35 – Snail Killing Flies by George Sly
I shouldn’t be surprised, having spent my adult life as a professional biologist, when a completely novel group of animals comes to my attention. Such is the nature of the tremendous biodiversity harbored on our planet. Despite years of study, field work, and reflection, the natural world will always produce something new and fascinating to contemplate.
The flies that are the focus of this edition of Critter Corner represent such a group. I became aware of them during the first biodiversity survey held at Goose Pond FWA back in 2010. Many specialists were on hand to investigate just what we had here at this recently established fish and wildlife area. One such person was William Murphy, an expert on the biology of snail-killing flies. It was my good fortune to encounter him during his survey work which led to a later, more informative conversation about these interesting insects.
Sometimes referred to as marsh flies, snail-killing flies are one more example of a family of organisms carrying out intriguing life cycles of ecological significance which go totally unnoticed by most of us. Four species were recorded at GPFWA during the 2010 biodiversity survey.
The common name of these insects, which belong to a family known as the Sciomyzidae, is quite accurate. They represent an aquatic (and in some cases terrestrial) snail’s worst nightmare. Adult flies are short-lived — a few weeks typically — and feed on flower nectar. It is their larvae that are the fearsome predators of freshwater snails.
The adult female lays eggs on emergent vegetation such as cattails or in another moist location. The larvae hatch from these eggs and drop into the water. Here they actively pursue their snail prey. They use sharp mouthparts to pierce and enter the soft body of a snail. Here they feed voraciously on the snail’s tissues ultimately causing its death. As the larvae grow, they move on to attack other, larger snails. Mr. Murphy related to me that the record number of snails killed by a single larva is thirty! Like other flies, the larvae form pupae, their final stage of development before adulthood. Depending upon the species, pupation may occur within the snail shell, floating on the water, or lying on the ground.
Given their snail-killing life cycle, one might guess that these flies could act as agents of biological control for snails involved in transmission of parasitic diseases such as liver and blood flukes (aquatic snails serve as intermediate hosts of the larvae of these flukes). In fact, in the 1950s, this was successfully attempted in Hawaii. Snail-killing flies from Central America were introduced to the islands. Cattle were especially hard-hit by infection with giant liver fluke. Snail-killing flies effectively attacked the intermediate snail hosts and subsequently reduced liver fluke infection in the cattle. It should be noted that this, as is often the case, was not accomplished without unintended ecological consequences.
Treating human fluke infections is less amenable to biological control by snail-killing flies according to Mr. Murphy. Human liver and blood fluke infections occur over wider areas — sub-Saharan Africa and Southeast Asia — as opposed to an isolated island. It is essentially impossible to significantly reduce the number of snails that might act as intermediate hosts for the flukes over such a massive geographic area.
At Goose Pond FWA, we’re fortunate to be able to view snail-killing flies and their quarry from a purely ecological viewpoint. Their relationship is an intricate bit of interspecies drama happening right under our noses — yet for most visitors, this remarkable natural drama remains completely unknown.
*snail-killing fly photo by Charles J. Sharp @ commons.wikimedia.org
*Many thanks to William Murphy for providing insight into these fascinating insects.
*If further knowledge of the interaction of aquatic snails and human fluke parasites interests you, this URL provided by Mr. Murphy, though admittedly dated, explains it well.