Disagreeable Diptera

Page 5 of 10

Topical Summary: Chloropidae: "Horse drivers"—Hippelates sp. "Eye gnats"—Life cycle—Anonymous bugs—Catching up—An unintended consequence—Are they gone?

"Musquitors verry troublesom, and
in addition to their torments
we have a Small Knat,
which is as disagreeable."

Figure 6

The Eye-Gnat

Hippelates sp.

Small black fly

Photo © J.L. Castner, University of Florida

As of 1985 there were 30 orders of the class of animals we call insects, containing an estimated 751,012 distinct species worldwide; in the U.S. and Canada alone there were 87,107 recorded species. Within the order consisting only of insects with two wings–called Diptera; Di meaning "two", ptera meaning "wings"—there were 108 families comprising a total of 16,914 species.4 The minute distinctions among species alone, not to mention the details distinguishing genera and families, are virtually inconceivable. Naturalists of the generation of Lewis and Clark, when the science of entomology was still in its youth, could only imagine how many insects there were in the world, and it required an uninhibited imagination to come even close.

The photo clearly shows the superficial characteristics of all Diptera–two wings that overlap when at rest, six legs, and three body segments (head, thorax and abdomen). However, the most conspicuous difference between an adult eye gnat and an ordinary house fly (Musca domestica Linnæus), is size. The house fly may be from 8 to 12 mm long, whereas the eye gnat grows up to be between 1.5 and 2.5 mm long. For the most part, the other identifying details border on the microscopic range. Still, it's hard to imagine what prompted Lewis to write (on July 12, 1805) of this nemesis as "a large black knat."

"Horse drivers"

Within the order Diptera in the hierarchy of insects is the cosmopolitan family of "frit" flies1 officially known as Chloropidae (klo-RAHP-id-eye)–from the Greek clorops, meaning "swarm." The Chloropidae consist of about 188 genera representing a worldwide total of more than 149,000 officially described species. Among the genera is one designated Hippelates–a Greek word meaning "horse drivers," underscoring its power to significantly disturb and distract both man and beast–and the sub-genus Liohippelates, the prefix lio (Latin for "plain") having been added to clarify the increasingly complex taxonomy of the genus, which now contains 290 species in North America alone. One of them has been named pusio (Latin for "little boy"), or in everyday terms, the eye gnat.2 The species was first described in 1872 by the German entomologist Hermann Loew (1807-1879).3 The name by which the men of the Corps of Discovery privately knew this particular "Small Knat" was probably unprintable, but the full universal, scientific, and entirely printable designation for the disagreeable little insect is Liohippelates pusio Loew 1872.

Eye gnats are born from eggs deposited in soil that is rich with decaying vegetation or manure. Thus they qualify as members of the large insect population known as "filth flies" which, incidentally, includes the common house fly (Musca domestica Linnaeus). Two species of Hippelates owe their common name to the need of their pregnant females to feed on protein at its most accessible sources–the sensitive mucous membranes at all natural orifices of mammalian bodies, the minute openings around hair follicles, pus and blood associated with open wounds, and the exposed genitals of quadrupeds. They are especially drawn to the eyes and lips of most mammals, including humans.

The normal life cycle of an eye gnat, from egg to egg, may last as few as seven, or as many as twenty-eight days, depending partly on average ambient temperatures surrounding its birthplace. Since the species breeds in disturbed soil that is rich in organic matter, it is likely that eye gnats once were especially numerous in areas where buffalo roamed. On the portage route in June of 1805, Lewis and Clark and their men were stumbling right through the little arthropods' recent nurseries, although apparently they weren't numerous enough during the days of the portage to make history as impediments to the Corps' progress. In any case the problem wasn't mentioned until a couple of weeks later when the Corps was several miles up the Missouri from the Falls. Not at all surprisingly, it took 102 years after Loew classified it for science to figure out maximum range of an eye gnat. And the answer is—a shade under seven-tenths of a mile.5 But of course, swarms of them can leapfrog farther afield to keep in touch with suitable breeding-grounds.

Anonymous bugs

In the first two journal references to "knats," written on June 23 and 24, 1804, on the lower Missouri, they are merely lumped in with mosquitoes and ticks, without any identifying details. The third reference was to "a large black knat" seen at Fort Mandan near the end of March, 1805, and recognized as a sign of spring. The next eleven references to gnats are dated between July 10 and August 7, 1805, from the canoe camp a few miles upriver from White Bear Islands to the confluence of the Wisdom (Big Hole) with Jefferson's River.

Based on that chronological span, it is reasonable to suppose that they all belonged to the same species. Lewis's observation on July 12 included details that clearly identified the species as the eye gnat. "Musquitoes extremely troublesome to me today," he wrote, "nor is the large black knat less troublesome, which does not sting but attacks the eyes in swarms and compels us to brush them off or have our eyes filled with them" (emphasis added).

The second specific reference to eye gnats, dated July 24, 1805–the day before they entered the second gate of the Rocky Mountains–is the one in which Lewis described the "trio of pests" as being equally as bad as any of the ten plagues with which Moses intimidated the Pharoah and his people. However, only four days after he complained about that pesky trio, the balance among them shifted somewhat. On the 28th he remarked that "the Musquetoes are more than usually troublesome, the knats are not as much so." Finally, by August 7, "the green or blowing flies [were] still in swarms," but the expedition had left the eye gnats behind.

It is somewhat surprising that Lewis's complaints about eye gnats didn't include any mention of eye problems, since the lips of the eye gnat's mouth have spines that can scratch the membrane lining the eyelid and covering the exposed surface of the eyeball. The resulting irritation is called conjunctivitis, or pinkeye. Moreover, like the nexus between mosquitoes and malaria, the eye gnat's role as a mechanical vector of afflictions such as mastitis and trachoma in animals, and of human pathogens such as yaws (a tropical infection of the skin, bones and joints), remained to be discovered nearly a century later.

Catching up

At the time of the Lewis and Clark expedition little systematic work had been done toward the identification and classification of insects in North America. Even Noah Webster, despite his anti-British sentiments defined "gnat" in his Compendious Dictionary of the English Language (1806), as "a small winged stinging insect"–which could not have included the eye gnat since, lacking a proboscis, or any other penetrating mouth-part, it is incapable of inflicting such a wound. In short, "eye gnat"–or "knat," as Lewis spelled it–simply was not in the etymological lexicon, even at the popular level.

During the eighteenth century a few amateur naturalists had studied some of the insects that damaged local crops, but their accounts were mainly anecdotal and their theories mostly rumors. Had Meriwether Lewis remembered this one from his childhood? Or did he summon its name from his own imagination based on its mode of attack? Or did he learn of it on the road from one of the engagés who might have known it all too well? Even if it had been widely known, it would still have been relegated to the large category of pests–including the mosquito–which everyone simply had to tolerate, since the damage it could inflict on crops, or on human eyes, had not yet been recognized.

By 1810 the famous natural history museum established in Philadelphia in 1794 by Charles Willson Peale contained a collection of nearly 4,000 insects, all carefully preserved and protected from destructive vermin, with some of the smaller ones mounted under rotating magnifiers for viewers' convenience. Most of them were sent to Peale from various parts of North America.5 But aside from its value as a presentation of objects of beauty, and inspiring evidence of "the wisdom of God in His creation," the exhibit's public appeal would have resided chiefly in the exotic specimens from India, China, South America and Europe. Whether there were ever any specimens of the eye gnat among them is not known today, but if so it couldn't have held a candle to the likes of a monarch butterfly.6

The founder of systematic entomology in America, Thomas Say (1784-1834), made use of Peale's collection for his studies, and was in touch with observers of insects from Maine and Massachusetts to Savannah and New Orleans, as well as with some of the leading naturalists in Europe. In his three-volume study American Entomology (1817, 1824, 1828), beautifully illustrated with 54 hand-tinted engravings, he employed the latest version of the Linnæan system of classification. Altogether, in this as well as his numerous other publications in entomology, Say described a total of 1,150 species of the order Coleoptera (beetles), plus 100 species belonging to sundry other groups, and 225 species of the order Diptera—but not one from the family Chloropidae.

Early in the 1890s, as farming flourished in the vicinity of Crescent City, Florida, after the arrival of rail service, a plague of Hippelates occurred, prompting a local flurry of concern over "sore eyes." By 1900 the insect's North American range had been mapped from Puerto Rico and Texas in the south to New Bedford, Massachusetts, in the north. American farmers discovered a problem that had been with them all along. However, it took an Act of Congress to elevate the miniscule eye gnat to notoriety a little more than a century after Lewis expressed his annoyance with it.

Unintended consequence

The Reclamation Act of 1902 was conceived as a step toward solving the West's overriding agricultural challenge. Despite plenty of water from melting mountain snows, too much of it arrived in mountain valleys and the high plains far too soon, often scouring floodplains of their spring seedlings and leaving little or no water for the dry season, which typically coincided with the crops' maturation. One solution was for the federal government to construct large-scale irrigation projects to store spring runoff, and provide irrigation canals and ditches to enable distribution to farmlands throughout the growing season. It worked. Between 1902 and 1907 thirty irrigation projects were in place throughout the West.7 Truck farms and citrus orchards began to flourish in the low deserts of southern California, for example, such as the Coachella Valley.

The unintended consequence was that the buildup of decaying vegetable matter after the year's crops were in, produced a rich habitat that was ideal for the gestation of Hippelates. In 1925 Elizabeth C. Moore, of Indio, California sent a note of alarm to the University of California: "The last five or six years we have had a regular pest of gnats. We never had them before. I know, for I have lived here for twenty-six years. . . . According to other residents the fly was first noted about 1912, but did not become troublesome until 1920." The troubles were, first and foremost, that the pests were so distracting in the fields and orchards that they reduced the efficiency of farm hands. Moreover, mastitis and yaws became rampant among farm animals, and pink eye became a serious problem among children, forcing schools to close for days at a time. Throughout the Coachella Valley those clouds of eye gnats prompted the gestures that became recognizable locally as the "Palm Springs salute," or the "Indio wave."8

Where are they now?

A related species in the family Chloropidae, known as the grass or frit fly (Oscinella frit), is one of many insects that breed in a similar manner. Three broods can be born between March-April and October-November from eggs laid on the stems of bluegrass, bentgrass, and cereal grains. It is a pest on golf courses in California and the southern U.S., although it doesn't rank very high as a nuisance to most golfers. In agricultural zones it can damage crops such as oats, rye, barley and wheat, although it is not currently on the "most wanted" list in the temperate latitude. During the second half of the twentieth century the eye gnat, including Hippelates collusor (Townsend) remained an object of intensive research by Universty of California entomologist E.F. Legner and others, in southern California as well as on islands in the Caribbean Sea. But that field of study has pretty much dried up.

Apparently, the little gnat that assaulted Lewis and Clark and their men for at least a month in the summer of 1805 is no longer on the map of the expedition's journey. The population of Liohippelates pusio Loew has been reduced considerably since the early 20th century, and few people have been bothered by them for many years past. Perhaps the widespread use of mosquito repellents such as DEET, which also fends off gnats, is partly responsible for that. Certainly the radical reduction of wandering herds of bison, deer and antelope has deprived Lewis's tiny "black knat" of a reliable source of protein. Indeed, Missouri River travelers who retrace the Corps of Discovery's route between the Falls of the Missouri and the mouth of the Dearborn River today need not dread Moses' fifth curse.

The "horse driver" is history.


1. Frit fly larvae inflict serious damage when they feed on cereals such as wheat, rye, barley and oats, as well as golf course grasses. Although it does damage some fruits, the frit fly does not belong to the family informally known as fruit flies. Encyclopaedia Britannica Online. http://www.britannica.com/EBchecked/topic/164747/dipteran (accessed July 25, 2008). Etymologically, the verb frit means "to fuse" or harden, coming from the Latin verb frigere, "to fry." Entomologically, it denotes the damage done by this fly's larvae as they feed on growing grasses and grain stalks. Online Etymology Dictionary, Dec. 27, 2010, http://dictionary.reference.com/browse/frit.

2. Ross H. Atnett, Jr., American Insects: A Handbook of the Insects of America North of Mexico, (New York: Van Nostrand Reinhold Company, 1985), 29.94.2.

3. Numerous specimens of American dipterae were sent to Loew in Germany by Osten Sacken, a fellow entomologist who in 1856 was named Secretary of the German Legation to the United States. C. R. Osten Sacken, Record of My Life-Work in Entomology, In Two Parts (Heidelberg, Germany: J. Hoerning, 1902), Part Second, 31-34. Elliott Coues (History of the Expedition, 2:409) and Elijah Criswell (cited in Moulton, Journals, 4:377n) identified that "large black knat" as the buffalo fly. However, the females of that species do bite, because they depend on blood-meals for ovarian production. Texas A&M University System, Texas Agricultural Extension Service; http://insects.tamu.edu/extension/bulletins/uc/uc-019.html (retrieved 11/25/2010).

4. Arnett, p. 8. New species, and even new orders, are still being discovered. A 31st order, named Mantophasmatodea, was identified in southern Africa in 2010. (See National Geographic News for October 28, 2010, http://news.nationalgeographic.com/news/2002/03/0328_0328_TVstickinsect....) That's not saying much, of course. For many years, now, extinctions and threatened extinctions have been outpacing discoveries within all known orders.

5. Charles Coleman Sellers, Mr. Peale's Museum: Charles Willson Peale and the First popular Museum of Natural Science and Art (New York: Norton, 1980), 87-88, 101, 162, 210, 219. See also The Long Room, with cabinets of "treasures of the field" on the wall at right, and The Artist in His Museum, 1822.

6. David F. Williams and Louis C. Kuitert, "Dispersion and Population Densities of the Eye Gnat Hippelates Pusio," The Florida Entomologist, vol. 57, no. 4 (December 1974), 361.

7. The fifth federal project under the Reclamation Act which was authorized in 1905 and completed in 1907, was situated on the north bank of the Yellowstone River a short distance downstream from Billings, opposite Pryor Creek. There were some major flaws in its design and operation, so at first it was not very helpful to the industry.

8. E. O. Essig, A History of Entomology (New York: Macmillan, 1931), 216.

Funded in part by the Lewis & Clark Fort Mandan Foundation