The point of flies

Originally published 10 July 1989

Grab the swat­ter! Here comes Mus­ca domes­ti­ca, the com­mon house fly. Cos­mopoli­tan. Ubiq­ui­tous. From Polar Cir­cles to the Equa­tor. On every con­ti­nent. The Typhoid Mary of the insect world. Irri­tat­ing, filthy-foot­ed, sum­mer­time pest.

Ogden Nash got it exact­ly right: “God in His wis­dom made the fly, And then for­got to tell us why.”

Our pre-sci­en­tif­ic ances­tors had an answer. The fly was the Dev­il in dis­guise. Beelze­bub in Hebrew means “lord of the flies.” Medieval writ­ers claimed to hear in the fly­’s both­er­some buzz the nat­ter­ing voice of the old Tempter him­self. The only knight­hood believed to exist in Hell was the Order of the Fly, award­ed for sin­gu­lar ser­vice to Satan. In Spenser’s The Faerie Queen, Archima­go sum­mons spir­its from Hell in the shape of flies.

Only the holi­est per­sons could com­mand the fly. It is said that a fly used to walk up and down on the page of Scrip­tures as St. Col­man sat read­ing; when the saint had to go about oth­er busi­ness, he ordered the fly to sit upon the line where he stopped read­ing to hold his place.

A gem of evolution

Alas, we less­er mor­tals have no such pow­er. Despite our per­sis­tent efforts to screen, cov­er, and close, the fly vex­es. Cen­turies of san­i­tary engi­neer­ing and decades of chem­i­cal war­fare seem only to have made the domes­tic fly more rugged and resource­ful. Clear­ly, our harass­ing cohab­iter is won­der­ful­ly adapt­able, an iri­des­cent lit­tle gem of evolution.

The point of flies is to make more flies. Flies fly to mate. They get the ener­gy they need for flight from oxi­diz­ing car­bo­hy­drates, which is why flies are so pow­er­ful­ly attract­ed to our sug­ar-filled kitchens. They can’t chew or bite; they taste with their feet and feed by dis­solv­ing, regur­gi­tat­ing enzymes that liq­ue­fy food.

The fly is a crea­ture that only a moth­er could love. And fly moth­ers have lots of fly babies upon whom to admin­is­ter mater­nal affec­tion, such as it is. A female Mus­ca domes­ti­ca lays upwards of a thou­sand eggs in a sea­son, in four or five batch­es. She starts in April, after sur­viv­ing the win­ter in some snug place, and if noth­ing lim­it­ed the matur­ing and mat­ing of her off­spring by mid-Sep­tem­ber her prog­e­ny might num­ber in the bil­lions. For­tu­nate­ly, not all eggs become mag­gots, not all mag­gots become adults, and not all adult females mate. Still, even in the tidi­est house­holds, the prodi­gious­ness of the process pro­duces plen­ty of flies to swat.

Flies are sus­pect­ed of dis­sem­i­nat­ing all sorts of dis­eases, from tuber­cu­lous to con­junc­tivi­tis, but the rap has been hard to pin. His­tor­i­cal­ly, most of the evi­dence for fly-borne dis­ease was cir­cum­stan­tial, but any crea­ture that pass­es its ear­ly life in dung, garbage, and decay­ing flesh, and its adult life at our din­ner table is unlike­ly to get off scot-free when epi­demi­ol­o­gists start assign­ing blame.

Bernard Green­berg, who lit­er­al­ly wrote the book on flies and dis­ease, adopts a cau­tious atti­tude to the ques­tion of fly guilt. Fin­gers, feces, food, and flies all play a part in con­t­a­m­i­na­tive dis­eases, and an overem­pha­sis on one fac­tor at the expense of the oth­ers is, accord­ing to Green­berg, too easy and untrue. Most dis­eases asso­ci­at­ed with flies would con­tin­ue to exist even if flies were exter­mi­nat­ed, unlike the appar­ent­ly nec­es­sary link between malar­ia and mosquitoes.

They are like sponges

How­ev­er tight or loose the link between flies and dis­ease, flies are air­borne pack­ages of pathogens. A house fly can car­ry as many as 500 mil­lion organ­isms on the sur­face of its body, many of them undoubt­ed­ly capa­ble of caus­ing dis­ease. The sticky bris­tles, chiti­nous cor­ru­ga­tions and grooved pro­boscis are per­fect for stow­aways. Flies are like sponges; they sop up microor­gan­isms from what­ev­er nasty place they stroll. The diges­tive tracts of flies are so jam-packed with pas­sen­gers its a won­der their tiny sug­ar-pow­ered wings lift them off the ground.

But a good nat­u­ral­ist should refrain from pass­ing moral judg­ment upon crea­tures that are just try­ing to make a liv­ing, albeit a wretched one by human stan­dards. Not even the house fly, that germ-rid­den Beelze­bug, deserves our unmit­i­gat­ed dis­ap­pro­ba­tion. As Shake­speare reminds us in Hen­ry V, “There is some soul of good­ness in things evil, Would men observ­ing­ly dis­till it out.”

For­get for a moment the house fly­’s dungheap habi­tat and filthy habits. For­get the invis­i­ble load of bac­te­ria that falls into your soup as the fly sits preen­ing on the rim of the bowl. For­get the enzyme-laden kiss by which the fly reduces your care­less­ly uncov­ered din­ner to a kind of diptere­an Kool-Aid. For­get all that and lis­ten to ento­mol­o­gist V. G. Dethi­er, who is a friend of flies if ever a fly had one:

“…a thing of beau­ty, a jet jew­el, from whose pol­ished sur­face the sun coax­es an iri­des­cent dis­play to rival that of the soap bub­ble, a jew­el whose diaphanous wings bear it aloft with con­sum­mate skill, the cur­va­ture of whose eyes flows in smoothest arc, whose faceted design rivals the hon­ey­comb in hexag­o­nal per­fec­tion, whose hairs curve in mar­velous­ly flut­ed columns rival­ing the best of goth­ic archi­tec­ture. And pri­vate­ly, with­in, its soft­er self is laced with an exquis­ite sil­ver fil­i­gree of air-filled tra­chea. There is per­fec­tion in its parts and grace­ful­ness in all its movements.”

Swat!