A Closer Look

"strength and beauty"

Six-foldout depiction of a flea by Robert Hooke (1635–1703)

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High quality scientific illustration of the flea

Courtesty of the Peter S. Raven Library of the Missouri Botanical Garden, via the Internet Archive

It has long been known to certain serious entomologists that most mature females of this species have body lengths ranging between 1.5 and 5 mm. So if we consider, say, 3.5 mm to be a reasonably average length, and Hooke's engraving is 343 mm (13.5 in) wide, then this image is nearly 100 times larger than the live specimen that was his model.

From its initial appearance in 1665, Hooke's blowup of this nearly microscopic specimen of the order Siphonaptera is said to have "set natural philosophy afire,"1 although the incipient methods of scientific exploration were slower to ignite beneath the muggy mantles of religious prejudice that thrived on fears of René Descartes' Rationalism and Francis Bacon's Scientific Method. It was published in 1665 in a book by the English physicist, natural philosopher, and general polymath Robert Hooke (1635–1703), under the newly coined, tantalizing title—surely one of the reasons for the book's stellar success—Micrographia.2

One of Hooke's biographers, the English antiquarian John Aubrey (1626-1697), described him as a man of "prodigious inventive head," and wrote of him with profound admiration, tempered with just a mite of annoyance. Aubrey vowed that he would "get of him a Catalogue of what he [Hooke] hath wrote; and as much of his Inventions as I can. But they are many hundreds; he believes not fewer than a thousand." Well, sighed Aubrey, it was always "such a hard matter to get people to doe themselves right."3

'Infinite Admiration'

"My faithful Mercury,4 my Microscope."

To see labels, point to the image.

Hooke's microscope: A crude looking syrange using mirrors to illuminate the specimen

Hooke, Micrographia, Scheme 1

In his preface to Micrographia, Hooke pointed out that whereas his microscope required intense illumination by direct sunlight, "the light of the Sun . . . is in a continual variation, and [therefore] many Objects cannot be view'd long enough . . . to be throughly examin'd." Furthermore, he added, "often-times the Weather is so dark and cloudy, that for many dayes together nothing can be view'd: And because also there are many Objects to be met with in the night, which cannot so conveniently be kept perhaps till the day, therefore to procure and cast a sufficient quantity of light on an Object in the night, I thought of, and often used, this Expedient."

Nearly a century and a half before Lewis and Clark's encounter with fleas en masse on the lower Columbia River, and almost another century before the species acquired its reputation as an accomplice in some of nature's most dreadful plagues, the little insect acquired for its kind an almost admirable, if not respectable, reputation, thanks to one man and his own remarkable instrument. The man was a 17th-century British polymath named Robert Hooke; the instrument was his own personal one-of-a-kind microscope. The principles and purposes of the microscope have been traced back more than four thousand years, but its name—from the Greek micron, meaning "small," and skopein, meaning "to view"—was coined in 1624 by the German botanist and curator of the Vatican's botanical garden, Giovanni Faber (1574-1629). In that same year Galileo Galilei (1564–1642), who is credited with having basically improved the instrument, wrote to the pioneer Italian naturalist Federico Cesi (1585–1630) that he had "contemplated very many small little animals with infinite admiration," including a mosquito and a moth—which struck him as beautiful—and a flea, which was "most horrid." In short, he declared, through that instrument "the greatness of nature can be infinitely contemplated, and [we may now see] how subtly and with what unspeakable care she works."5

The long list of other subjects in Hooke's majestic volume included the point of "a sharp small needle," the edge of a razor, sparks struck from a flint or steel, figures seen in bits of sand, gravel in urine, blue mold, an extreme closeup of moss, a poppy seed, a fly's eye, and a fly's feet, an ant, a mite and a louse.

The Beauty of It

And that enormous image of a flea that everyone was talking about! Sure enough, there it was, in Scheme XXXIV (Plate 34). We can imagine the hubbub it provoked. Little children giggled over it. Proper ladies slammed the book shut with shudders and squeals of revulsion. Well-bred gentlemen frowned and leaned closer in morbid curiosity. Hooke himself was awestruck. "The strength and beauty of this small creature," he began, "had it no other relation at all to man, would deserve a description." And so he drew, and then engraved with his own hand, all that he could see through his magical lenses. Finally, he transcribed his insights—a few of them far in advance of his time—into words: "As for the beauty of it," he wrote, it was "all over adorn'd with a curiously polish'd suit of sable Armour neatly jointed," which would help it to resist the claws, paws, or fingers (but not the opposed thumbnails) of any host that might try to brush it off or squeeze it to death. Today that hard external shell is termed, with scientific clarity and precision, an exoskeleton.

Moreover, its "armour" was beset with "multitudes of sharp pinns, shap'd almost like Porcupine's Quills, or bright conical Steel-bodkins [stilettoes]." The head, he continued,

is on either side beautify'd with a quick and round black eye, K behind each of which also appears a small cavity, L, in which he6 seems to move to and fro a certain thin film beset with many small transparent hairs, which probably may be his ears; in the forepart of his head, between the two foreleggs, he has two small long jointed feelers, or rather smellers, MM, which have four joints, and are hairy, like those of several other creatures; between these, it has a small proboscis, or probe, NNO, that seems to consist of a tube NN, and a tongue or sucker7 O, which I have perceiv'd him to slip in and out. Besides these, it has also two chaps or biters [jawbones] PP, which are somewhat like those of an Ant, but I could not perceive them tooth'd; these were shap'd very like the blades of a pair of round top'd Scizers, and were opened and shut just after the same manner; with these Instruments does this little busie Creature bite and pierce the skin, and suck out the blood of an Animal, leaving the skin inflamed with a small round red spot.8

It seemed impossible to observe how fleas could propagate more fleas simply through a mindless, biologically compulsive act of copulation, so Hooke said nothing of that. Besides, in the second half of the 17th century CE almost everyone still held fast to Aristotle's 4th century BCE thesis that fleas were spontaneously generated out of dust and dirt.9

Power Legs!

Above all, what was so unusual about the flea was that it could leap so far—13 inches (33 cm)—and so high—7 inches (18 cm)—without wings. Hooke was bold enough to assume that the solution to that conundrum could be found through a thoughtful examination of a flea's external anatomy. On that premise, he continued:

the Microscope is able to make no greater discoveries of it then the naked eye, but onely [sic] the curious contrivance of its leggs [DD EE GG HH & I] and joints . . . is very plainly manifested, such as no other creature, I have yet observ'd, has any thing like it; for the joints of it are so adapted, that he can as 'twere, fold them short one within another, and suddenly stretch, or spring them out to their whole length.10

Yes, he was mistaken, but his argument seemed credible enough that it remained unchallenged for another 300 years. Then, in 1967, the truth began to emerge: The energy a flea needs in order to leap high and far is stored in a pad of highly resilient protein called resilin; the pad is located where the legs are attached to the body. But another 44 years elapsed before it was discovered through high-speed imaging, mathematical modeling, and scanning electron microscopy, that the flea applies resilin's force to the ground through its shins (tibiæ; TIB-ee-eye) and toes (See figure above).11

Eventually, Hooke's flea was recognized as a reasonably accurate depiction of a female of the species that specifically prefers human blood, even though nobody in the 1660s had any reason to suppose there were more species in existence. Indeed, almost a hundred years later, Carl Linnæus (1707–1778) knew of only two, the 1-millimeter-long flea native to beaches in Latin America and the Caribbean, now called Tunga penetrans ("sand flea" or "chigger") and one that resembled Hooke's, to which Linnæus assigned the binomial Pulex irritans ("irritating flea") in the Tenth Edition (1758) of his Systema Naturæ. His P. irritans was universally regarded in those days as a bothersome but benign species that vexed every living animal, human or otherwise, and the worst was that it clearly thrived on filth of any sort. 

The taxonomy—scientific naming, formal description and orderly classification—of insects in general grew very slowly during the 18th and 19th centuries. Despite the notoriety of Hooke's microscopial exposé, and the fact that there was hardly a warm-blooded bare skinned, furred or feathered creature on earth that was not intimately acquainted with that notorious one-trick bug, the flea drew little if any serious attention from early entomologists. It was cast aside in favor of the study of more attractive insects such as moths and butterflies, or of agriculturally harmful species such as beetles, locusts, fruit flies and the tiny but devastating Hessian wheat fly. It is small wonder that Lewis and Clark saw no real significance in any of their encounters with mere pests. By 1850 only 15 different species of fleas had been identified—including the ones hosted by dogs and cats—and by 1900 the roster still listed fewer than one hundred. During the last two decades of the 19th century, however, it was discovered that fleas had actually been the carriers, or vectors, of the bubonic plagues which for centuries had decimated human populations worldwide (except in the U.S. up to that time). That was more than sufficient to turn the flea into a worthwhile object of research and discovery, and by 1950 about 1,500 different species had been identified. By the beginning of the bicentennial observance of the Lewis and Clark expedition in 2003, nearly 2,500 varieties had acquired binomial identities, with some 325 of those (classified under 66 genera in seven families) found in the U.S. and Canada alone.12

But his book quickly became a bestseller, so it is safe to assume that most readers back then merely smiled and nodded their agreement with the author's own intense wonder and delight in every secret that his microscope had revealed.

A Reason to Jump

By the early 1700s Hooke's Micrographia was long out of print, and as the English naturalist and microscopist Henry Baker (1698–1774) observed, "since a Desire of searching into the minute Wonders of Nature [has] become almost general," and since nearly all of Hooke's engravings were still in existence, a reprint was overdue. A new edition, retitled Micrographia restaurata ("restored"), appeared in 1745, based on 33 of Hooke's 38 engravings. Baker himself revised and brought "up to date" Hooke's old-fashioned, "verbose and disfused [sic] Way of Writing," which he considered "tedious and distasteful." For example, he reworded Hooke's description of how a flea jumps, and then added a new theory as to why. The flea, he explained,

lives by sucking human Blood, or the Blood of other living Animals, which cannot be obtained without inflicting Wounds and causing Pain, which must necessarily produce Resentment, and a Desire of Revenge, it was absolutely requisite the little invader should have some ready Means of Escape; since every Meal must otherwise be paid for with its Life. As therefore it has no Wings, its Safety must be entirely owing to its Legs . . . whereby they commonly deliver the little Animal from the Danger of a Pursuit.13

Furthermore, he continued fancifully, "however pretty they may be in the Microscope, or for these ingenious Purposes, they are certainly very troublesome Bedfellows, and especially to Women and Children, whose Blood they are particularly fond of."14 This and the previous quotation are typical of Mr. Baker's "many Entertaining and Instructive Discoveries and Observations in Natural History" which he promised in his own version of Hooke's Preface. Although his rather entertaining theories were refreshingly free of religious overtones, they were still far from reflecting the rigorous discipline of the Enlightenment's new standards of scientific method, at least as far as the trivial flea was concerned. Nevertheless, Hooke's inimitable, "curiously engraved" plates continued to reappear in print throughout the 19th and 20th centuries, including scaled-down versions in paperback, and facsimiles of the several editions on CD-ROM and, most recently, full-scale editions on the World Wide Web.

  • 1. Brian J. Ford, "Enlightening Neuroscience: Microscopes and Microscopy in the Eighteenth Century," in Harry Whitaker, et al., Brain, Mind and Medicine: Essays in Eighteenth-Century Neuroscience (New York: Springer, 2007), 29.
  • 2. The full title is Micrographia, or, Some Physiological Descriptions of Minute Bodies made by Magnifying Glasses, with Observations and Inquiries thereupon. Several digital copies are available online. The volume used for this reference was contributed to the Internet Archive by the Missouri Botanical Garden, and will be found there in various formats (PDF, EPUB, Kindle, Daisy, DjVu), on Internet Archive, at https://archive.org/details/mobot31753000817897 (accessed 2 November 2013).
  • 3. Oliver Lawson Dick, ed., Aubrey's Brief Lives (Boston: David R. Godine Publisher, 1999), 164-66.
  • 4. Among his several mythical roles, the Roman deity Mercury was the god of communication.
  • 5. Quoted in David Freedberg, The Eye of the Lynx: Galileo, his Friends, and the Beginnings of Modern Natural History (Chicago: University of Chicago Press, 2002), p. 151. Translated, letter from Galileo to Cesi, September 23, 1624, in Galileo, Opere, XIII, pp. 2088-209 and Carteggio, No. 781, pp. 942-943.
  • 6. Hooke was unable to determine the gender of his specimen, so he was using "he" in the gender-neutral sense.
  • 7. Hence the name of the suctorial Order to which all 3,000 species of this insect belong: Siphonaptera. The first half of the name, "siphon," is Latin name of a tube used for drawing liquid, like a straw. The second half, the Latin term "aptera," means "wingless."
  • 8. Micrographia, "Observation LIII. Of a Flea."
  • 9. The abbreviation CE stands for "Common Era," while BCE stands for "Before Common Era." They serve to replace AD (anno Domini, Latin for "in the year of the Lord") and BC (before [the birth of] Christ.) Many professional, scholarly and historical associations have come to favor these designations because they are non-sectarian and cross-cultural, and thus are more appropriate in our current era of globalization than the still-traditional Christological designations, which were introduced early in the 17th century—CE.
  • 10. Ibid., "Observation LIII, Of a Flea."
  • 11. Gregory P. Sutton and Malcolm Burrows, "Biomechanics of jumping in the flea," Journal of Experimental Biology, November 16, 2010. Sutton and Burrows are zoologists at the University of Cambridge. The big payoff is that scientists have already discovered how to manufacture synthetic resilin, and have begun to consider various uses for it in medicine and many other fields. Kara Rogers, "Jumping Fleas: The Biomechanics Behind a Wingless Existence," Encyclopædia Britannica Blog, February 10, 2011. Gordon Gordh and David Headrick, A Dictionary of Entomology (New York: CABI Publications, 2001), s.v. pleural arch.
  • 12. "Historical Notes," in Govnor Brinck-Lindroth and Frans G. A. M. Smit, The Fleas (Siphonaptera) of Fennoscandia and Denmark (Boston: Brill, 2007), p. 3. Ross H. Arnett, Jr., American Insects: A Handbook of the Insects of America North of Mexico, 2nd ed. (Boca Raton, Florida: CRC Press LLC, 2007), 927.
  • 13. Baker, Micrographia Restaurata (1780), 61.
  • 14. Ibid., 63.