By Lisa Gonzalez
Today’s parasitic fly marvel comes in the form of an absurdly cute group of round, woolly bodied insects known as the small-headed flies (family Acroceridae). A handful of specimens of Turbopsebius diligens, the only species known west of the Rocky Mountains, turned up in only two of our BioSCAN traps, in Hollywood and University Park, an area just north of the USC Campus. At first glance, T. diligens might look like an oddly shaped bee, but to my eye, it’s as if someone took two craft pom-poms to make a miniature snowman, stuck a small fly head with giant fly eyes on top, added 6 legs and voila!
To add to this bizarre image, picture this little fuzzball in motion, as humorously described by entomologist F.R. Cole: “(T. diligens) has a floating sort of flight, rather undulating and uncertain. It has the habit of buzzing around in circles when it falls over on its back on a smooth surface, often doing this for some time before it can regain its feet; most of the time it is making a high, thin humming sound.”
Delightful as that may be, small-headed flies do not just clown around town amusing those lucky enough to spot one; they are diligently on the hunt for other organisms that they can use as a resource to carry on their species. Their target is a spider, specifically spiders known as funnel weavers such as the common Hololena curta and Rualena sp.
As you may now know from previous posts about parasitoid behavior, often the mother will inject a single egg or multiple eggs into the host using a modified structure at the end of her body called an ovipositor, but not the small-headed fly! They do things a bit differently. Instead, the fly mama will lay her eggs nearby the spider, even on the “front doormat” of the entry way to the spider’s funnel. When the eggs hatch, the emerging larvae are free-living, so called planidial larvae, meaning they need to wiggle their way around the web and penetrate the spider’s body, often mainly through the leg joints. Once inside, the larva moves to the spider’s lungs where it can then enter a dormant stage , waiting it out (sometimes for several years!) until the spider is an adequate size for its parasite’s development to be complete.
Once the spider is large enough, the larva itself begins to grow and reach its final molt. At this point, the spider suddenly falls under the spell of the small-headed fly’s Jedi mind tricks and spins its final web around itself, forming a protective cocoon so that the larva can consume the spider and finish its final stage of metamorphosis in peace and safety. Eventually a new generation of bumbling fuzzballs will emerge, and the story of the small-headed fly will carry on.
Special thanks to Chris Borkent for the identification and information, and Emily Hartop for her impressive alliterative skills.
Cole, F. R. 1919. The dipterous family Cyrtidae in North America. Transactions of the American Entomological Society 45(1): 1–79.
Marshall, Stephen A. 2012. Flies: the natural history and diversity of Diptera. Buffalo, NY: Firefly Books.