Fleas or Lice?

Two Hexapods Compared1

In the fair copy of his journal entry for 26 October 1805, Clark reiterated the facts concerning the onset of their new curse three days previously: "The Flees—he underlined the word for emphasis, as if someone in the company had challenged him about their true identity. "The Flees," he insisted, "which the party got on them at the upper & great falls, are very troublesom and dificuelt to get rid of," etc. Two hundred years later Professor Gary Moulton, editor of the University of Nebraska Press's new edition of the journals, might have been siding with Clark's anonymous disputant when he expressed a still common error in his behalf: "It seems more likely," editor Moulton wrote, "that the 'flees' are lice, since fleas do not attach themselves to hosts for any length of time."2 His citation refers to the 7th edition of Entomology in Human and Animal Health, by Robert F. Harwood and Maurice T. James.3 The twelve pages are devoted to two kinds of lice, "sucking" (pp. 129-38) and "chewing" (pp. 138-41). But neither that passage nor their discussion of fleas (pp. 319-24), contains any direct comparisons between the two species, except regarding their respective anatomies (p. 322). "No part of the external anatomy of an adult flea," Harwood and James insist, "could possibly be mistaken for that of any other insect. The head, the mouth parts, the thorax, the legs, the abdomen, the external genitalia, all present features that are not elsewhere duplicated among the hexapods [six-legged arthropods]."

Cat Flea, Ctenocephalides felis, Linnaeus, 1758

Comb-Headed Cat

extreme close-up of a flea with a vertically flat body

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Scanning Electron Microscope image (SEM)

As adults termed imagos (pronounced ih-MAY-gohz; scientific Latin for adults) the heads and bodies of nearly all the species of fleas in the U.S. and Canada are laterally flattened or narrowed, to ease travel through the thick fur or feathers of their usual wild and domestic hosts—the latter mostly dogs and cats—and the relatively heavy clothing of humans.

One of the most obvious distinctions between these two insects is the shapes of their respective bodies—the flea's is somewhat hunch-backed, with a small head, whereas the louse's is a long bumpy oval. At life-sized scale, however, they are so nearly equal in length that it would take extremely close examination to be sure of their respective identities on that basis.

Notice the broad-toothed ctenidia (sten-ID-ee-uh), or comb, behind this species' head, by which it grips its host's fur. It also distinguishes the cat flea from Hooke's P. irritans.

There is no evidence in the journals that domestic cats were ever observed anywhere on the expedition, although fur trappers and traders often took them along to kill mice and rats that invaded their fur bales.

Head Louse, Pediculus humanus capitis, De Geer, 1767

Small-footed human head louse

extreme close-up of a bug with a cartoonish figure

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Scanning Electron Microscope image (SEM)

Nearly all 56 species of lice in North America are flattened back-to-belly (dorso-ventrally, in entomologists' terms). A head, body, or pubic louse (the last being the shortest of the three at 1–1.5 mm) will take up residence within a human host's clothing, leaving only for blood meals on adjacent flesh.

A louse's feeding may last from a few hours to several days, with the host scarcely aware of the parasite's presence. An adult male louse is about the size of a sesame seed, the female a tad larger.

Body lice, (P. humanus corporis) are known vectors of several serious diseases, including typhus, but head lice have a better reputation. They're benign.

Given the great numbers of Indian dogs they met all along the Columbia River and near Fort Clatsop, Lewis's dog Seaman may have suffered considerably from the bites of fleas as well as the chewing louse, Trichodectes canis.

Another distinguishing characteristic—which is not accurately represented in these two SEMs—is their respective colors. A flea is typically medium brown, whereas a louse is usually dark gray.

Sensitivity to Temperatures

As Harwood and James have pointed out, consistently warm temperatures are critical for the survival of louse populations: "The optimum for the adult body louse is approximately the temperature of the normal human body. A rise of 4 to 5 degrees is fatal to them within a few hours. Temperatures below the optimum are much less critical, although prolonged exposure to 20° C [68° F] or lower may result in death."4 Therefore, since the average winter temperature in the vicinity of nearby Astoria, Oregon, is somewhere in the 40s Fahrenheit, the likelihood of lice prevailing there at all in that season is only marginal.

Fleas, however, are hit-and-run parasites. As soon as the host leaves its bed, most fleas will leap off their hosts' bodies and hide for the day in the seams and creases of the bedding. The captains must have recognized that conduct from ample experiences, for as Clark informs us, as soon as they settled into their huts at Fort Clatsop, it became necessary to assign an enlisted man daily to clear the captains' beds of fleas. Body lice, however, take up permanent residence in or on the host's clothing, leaving only for blood meals on adjacent flesh that may last from a few hours to several days. Meanwhile, the host may be unaware of a louse's presence until its engorged abdomen comes to his attention during a casual scratching of a mild itch.

  • 1. This topic has been included merely to illustrate the major observable differences between the louse (Pediculus humanus L.) and the flea, relying on Robert Hooke's enlarged microscope images of them, published in 1665. The only references to lice in the University of Nebraska Press edition of the Journals of Lewis and Clark are in footnotes. Not one of the expedition's six journalists ever mentioned them, although it is reasonable to suppose that they—or at least Seaman—encountered a few of the little suckers from time to time, although not likely from Fort Clatsop.
  • 2. Journals, 5:344n.
  • 3. (New York: Macmillan, 1979), pages 129-41.
  • 4. Harwood & James, 132.