BY ALPER ÖZKAN (MSN/PhD)
d_ozkan@ug.bilkent.edu.tr
I am currently sharing my room with a pair of fruit flies—I do not know whence they came, but they probably hitchhiked on some fruit or vegetable, and I sure am not letting them out in this weather. Clever little things, these are—they drink alcohol to purge themselves of their internal parasites, lay their eggs in alcohol-containing environments to protect their larvae from the same and are attracted to the smell of beer because they feed on (and, in return, transport around) brewer’s yeast, and when a male fruit fly is rejected by a female, he again hits the tavern to forget the encounter (yes, they really do this). Although I never drink (…wine!), I can certainly appreciate the sheer ingeniousness of drowning parasites in alcohol—and so, in honor of these little boozehounds, this column will be about them and their kin.
Easily distinguished by the modification of the hindwings into balance organs called halteres, flies are one of the more populous orders of insects—and that is saying quite a lot, given that insects are the single most populous class of animals. They are broadly divided into two groups—suborder Nematocera contains flies with long, feathery antennae, such as mosquitoes, midges, gnats and craneflies, while Brachycera is the suborder for flies with shorter antennae, the majority of which can also inflate their heads with their own blood, like tiny water balloons.
This particular group of brachycerans pupate inside their own larval skins, which are usually thick and difficult to get out of, except through a weak point on one end. The newly metamorphosed adult fly, however, is not strong enough to bash its way through this region, so it instead expands a sac on its head, called the ptilinum, in order to escape. Although the ptilinum inverts back into the head after use, the act still splits the face of the fly and creates an upside-down U-shaped gap just above the antennae—and so this group is called Schizophora, the split-bearers.
Although the ptilinum is generally not employed beyond this function, it does see another use in a nest-parasite: the fly Cacoxenus indagator infests the brood chambers of mason bees, and although the prospect of feeding on the nest provisions (or the helpless bee larva that the provisions are intended for) is enticing, the strong walls that separate the individual chambers certainly are not—mason bees create their nests out of dried mud, and while the adult fly can sneak in and lay its eggs before the walls are sealed, the larvae themselves are locked within the nest. But the crafty parasite has a solution: by inserting its head into depressions in the nest wall and expanding its ptilinum, the fly can apply enough pressure to create a crack in the wall, through which it squeezes itself to earn its freedom.
While flies are well-known for parasitism (and especially as inquilines: in that role they are nearly as famous as rove beetles, and nest inquilines are without exception weird—there you find the likes of meat-eating caterpillars, parasitic slave-collars, armored snail-maggots, foot-replacing bionic mites and a beetle that has modified its abdomen to resemble an entire termite worker, and by the way, anyone who mails me the names of these animals will get a prize from me), there are also mutualists among their number: Fergusonina is a genus of gall-flies that deposit their eggs on plants such as the eucalyptus, but the maggots cannot by themselves form their galls—that task falls to their benefactors, nematodes of the genus Fergusobia, which live in the female fly and are passed onto her eggs. Once the gall is produced, both the maggot and the worm reap its benefits.
Another fly-intensive mutualism is that of pollination—a great number of plants produce foul-smelling flowers to attract flies, while others attract flies with nectar, and yet others pretend to be mushrooms in order to draw their intended pollinators. Darwin once predicted that a particular orchid with a nectary hidden beneath a very long spur would be pollinated by a moth with a very long proboscis (the moth itself was discovered some forty years later)—a similar interaction exists between African flowers and the tangle-veined fly Moegistorhynchus, which, unlike moths, does not even have the saving grace of a curling proboscis: its mouthparts, reaching up to 10 centimeters in length, form a “tail” of sorts behind the animal when not in use. Mushrooms, too, can be “pollinated” by flies, and a special relationship exists between Phorbia flies and fungi of the genus Epichloë: the former feeds and oviposits on the latter, but the fungal spermatia survive digestion and are delivered to other Epichloë by the fly—which will draw spiral patterns on the fungi with its abdomen, ensuring the deposition of the gametes.
There are more to flies than this, of course—there are bot flies and bat-flies, sheep-keds and wormlions, ocean-faring flies and petroleum-dwelling flies (of course there’s a fly that lives in petroleum), those that swim in pitcher plant fluid and those that throw their eggs as ammunition. Their diversity is truly impressive, and disgusting though some might be to us, the world would be a worse place without them.
…So next time you see a housefly or two loitering around on your windowpane, do think to open the window and let them go on their merry dipteran way.