My answer focuses on how the spelling of mosquito evolved in English dictionaries between 1658 and 1909. In this narrow sense, I’m trying to track the history of the word’s spelling.
Walking with the dictionaries
Here is a chronological journey through the spellings of mosquito that appear in the various dictionaries that I consulted.
Thomas Blount, Glossographia Or a Dictionary: Interpreting All Such Hard Words (1656) is the earliest dictionary I found to address the pernicious bug:
Muscheto, a kind of Gnat. Herb. Tr.
I have not been able to identify the source that Blount cites for his spelling of the word. Searches of William Turner, A New Herball (1551) and John Gerard, The Herball, or Generall Historie of Plants (1597), for example, yield no matches for muscheto.
Two years later, Edward Phillips, The New World of Words (1658) offers a somewhat more nuanced description:
Muscheto, a kind of Insect so called, somewhat resembling a gnat.
Elisha Coles, An English Dictionary (1676) adds an etymology to an Italian root word:
Muscheto, (Moschetta, I. a little fly) a kind of Gnat.
The entry remains unchanged in the 1717 edition of Coles.
The author of Glossographia Anglicana Nova (1707)—not Thomas Blount, who died in 1679—seems to have cribbed Phillips's definition:
Muscheto, a kind of Insect somewhat resembling a Gnat.
Edward Phillips & John Kersey, The New World of Words, sixth edition (1709) has two entries:
Moschetto, a kind of stinging Gnat ; very troublesome in the West-Indies.
...
Muscheto or Moschetto, a kind of Insect, somewhat resembling a Gnat ; which is very common and troublesome in many Parts of Africa and America, as also in the hottest countries of Asia.
The same entries appear in the seventh edition of Phillips & Kersey (1720), and essentially the same entries appear in Kersey, Dictionarium Anglo-Britannicum (1708), continuing through at least the third edition (1721).
Nathan Bailey, An Universal Etymological English Dictionary, second edition (1724) has this:
MOSCHETTO, a stinging Gnat, very troublesome in the West Indies.
...
MUSCHETO. See Moschetto.
Editions of this dictionary through the 21st edition (1775) continue to include both of these entries. Oddly enough, however, Editions of Bailey’s The New Universal English Dictionary, fifth edition (1760) moves the entry for moscheto/muschetto to appear alphabetically between murther and muscles:
MOSCHETO [or] MUSCHETTO (in America, &c.) a very common and troublesome insect, something resembling a gnat.
Thomas Dyche & William Pardon, A New General English Dictionary (1735) has a much expanded definition that focuses on what is now known as the Miskito Coast of Nicaragua:
MOSCHETTOS or MOSKITTOS (S.) A small Indian Nation on the North Side of the Continent of America, near Cape Gracia Dios, who are so dexterous in throwing and avoiding all sorts of Darts, that they will defend themselves from being hurt, tho’ their Enemies throw ’em very thick ; they are generally very kind to the English, and have no form of Government among ’em, but own the King of England for their Sovereign, and learn to use the English Language, esteeming the Governor of Jamaica as one of the great Princes in the World ; also a very troublesome small Insect that afflicts the West-Indians, by stinging them very severely.
This entry persists unaltered through the eleventh edition of Dyche & Pardon (1760).
John Kersey, A New English Dictionary, fourth edition (1739) also has two entries:
A Moschetto, a stinging Gnat in the West-Indies.
A Muscheto, or Moschetto, a troublesome Insect.
The seventh edition of Kersey (1757) drops the entry for muscheto, however.
Samuel Johnson, Dictionary of the English Language (1756) contains no entry for any spelling of mosquito, although (as RaceYouAnytime observes in a note elsewhere on this page) his entry for gnat specifies that the latter word refers to “a small winged stinging insect,” which might be taken as synonymous with mosquito. However, so many contemporaneous and earlier dictionaries do have entries for mosquito in one spelling or another that I doubt Johnson omitted it on the reasoning that having an entry for gnat obviated the need for an entry for mosquito. Whatever the case may be, H. J. Todd’s revision of Johnson’s Dictionary (1818) includes the following entry:
MUSKITTO [or] MUSQUITTO n. s. {musca, Lat} A stinging fly or gnat of the Indies. [First cited example:] They paint themselves to keep off the muskitas. Purchas, Pilgrim[age]. (1617,) p. 1085. [Second cited example:] If in writing voyages you have occasion to send messengers through uninhabited country,—infest them with musquittos. Cambridge.
Purchas is Samuel Purchas, and his book is Purchas His Pilgrimage; Or, Relations of the World and the Religions Observed in All Ages and Places ... (1617), which includes this observation about the natives of Dominica (on page 953 of the 1626 edition):
Master George Peercie relateth of the Dominicans (which they visited in their Virginia Voyage) that they paint themselves to keep off the Muskitas : that they weare the haire of their heads a yard long, platted in three plaits, suffer none on their faces, cut their skinnes in diuers Workes or Embroderies: ...
John Ash, The New and Complete Dictionary of the English Language (1775) returns to the complication of there being an insect and a New World region with similar names:
MOSCHETTO (s. in the history of insects) A kind of gnat exceedingly troublesome in some parts of the West Indies.
...
MOSKITO (s. in geography) A country in North America, a part of Mexico or New Spain.
...
MUSCHETO, MUSCHETTO (s. in the history of insects) A kind of gnat exceedingly troublesome in some parts of America.
Evidently, Ashe takes the view that the moschetto is a West Indian gnat, whereas the muscheto (or muschetto) is an American gnat.
Thomas Sheridan, A Complete Dictionary of the English Language (1780) reverts to a single spelling:
MOSCHETTO s. A kind of gnat exceedingly troublesome in some part of the West Indies.
The same entry appears in Sheridan’s second edition (1789) and in Stephen Jones, Sheridan Improved: A General Pronouncing and Explanatory Dictionary of the English Language (1798).
At the turn of the nineteenth century, then, most dictionaries seem committed to moschetto, muscheto, or both. John Walker, A Critical Pronouncing Dictionary, and Expositor of the English Language (1807) doesn’t do much to rock the boat:
MOSCHETO, s. A kind of gnat exceedingly troublesome in some part of the West Indies and America.
But Noah Webster, A Compendious Dictionary of the English Language (1806) does:
Musketoe, n. a vexatious insect
Where did musketoe come from? A Google search turns up just one source for it older than Webster’s first dictionary—Michel Adanson, A Voyage to Senegal: The Isle of Goreé, and the River Gambia (1759):
The same day we arrived before sunset at the Musketoe trading-place ; where was to be the sale of cattle. This being the first port we meet with, in going up the Niger, the same is practiced here, as at sea by those who pass the tropic : the French, the first time they come this way, are bound to make a present to the laptots; and therefore I gave them the usual gratuity. The Musketoe trading-place is only thirteen leagues to the north ¼ north-east of the island of Senegal.
It bears noting that the term Musketoe here refers to a place name in West Africa, making it a less than ideal precedent for the spelling of a gnat widely identified in the early nineteenth century as endemic to the West Indies and America. I think it is more than likely that Webster based his spelling on his personal preference for relatively phonetic orthography. That is to say, he made it up. He persisted in it, too. His American Dictionary of the English Language (1828) has this entry:
MUSKETOE, n. {Sp. Port. mosquito, from Sp. mosca, L. musca, a fly.} A small insect of the genus Culex, that is bred in water; a species of gnat that abounds in marshes and low lands, and whose sting is peculiarly painful and vexatious.
The 1842 abridged version of his complete 1840 second edition dictionary indicates that Webster is relenting slightly:
MUSKETOE, MUSQUETOE, n. {Sp. Port. mosquito.} A small insect of the genus culex, that is bred in water; a species of gnat that abounds in marshes and low lands, and whose sting is peculiarly painful and vexatious.
The 1847 edition of Webster’s dictionary—the first one published following Noah Webster’s death and the Merriam brothers’ acquisition of publishing rights to it—goes a step further, adding a cross-reference entry for mosquito (“See MUSQUITO”) and putting the main entry under the key words “MUSQUITO, MUSKETO,” although it concludes the definition there with a note insisting that musketo is better:
This word has been spelled in various ways, but MUSQUITO and MOSQUITO are most prevalent, though the anglicized form MUSKETO would be preferable to either.
Preferable to Noah Webster, anyway. The 1864 edition of An American Dictionary of the English Language almost entirely gives up the musketo cause, offering instead these two entries:
Mosquito, n. pl. MOSQUITOES {Sp. & Pg. mosquito, from Sp. mosca, Lat. musca, fly; Fr. moustique.} (Entom[ology) A small insect, of several different species, of the genus Culex, having a sharp pointed proboscis, by means of which it punctures the skins of animals and sucks their blood, the minute wounds thus made being often attended with swelling and a considerable amount of pain. ... {Written also musketo, musquito.}
...
Musquito, n. {Sp. & Pg. mosquito, from Sp. mosca, Lat. musca, fly; Fr. moustique.} A small insect of the genus Culex, that is bred in water; a species of gnat that abounds in marshes and low lands, and whose sting is peculiarly painful and vexatious. {Written also mosquito.}
Webster’s International Dictionary (1890) took the unusual step of reintroducing an entry for musketo; but it, like the entry for musquito, now said merely “See MOSQUITO.” Finally, in Webster’s New International Dictionary (1909) all mention of musketo and musquito vanished.
Interestingly, Joseph Worcester, A Universal and Critical Dictionary of the English Language (1850) endorsed mosquito well before his enemy Webster did:
MOSQUITO n. (mosquito, Sp.) pl. MOSQUITOS. A very troublesome insect, of the genus culex ; a large kind of gnat. It is variously written musquito, musquetoe, moscheto, moschetto, mosquetoe, mosquetto, muscheto, muschetto, musketoe, muskitto, musqueto, and musquitto.
In fact, Worcester had included mosquito as one of two valid spellings of the word in his Comprehensive Pronouncing and Explanatory Dictionary of the English Language (1830), before Webster acknowledged the spelling at all:
Mosquito, n. a troublesome insect.
...
Musquetoe, n. a troublesome insect. See Mosquito.
What are we to make of all this?
The key developments chronologically are as follows:
1658 Muscheto first appears
1708 Moschetto emerges as an alternative spelling.
1735 The spellings Moschettos and Moskittos appear in the context of the Miskito region of Central America—and of biting insects.
1760 The variants moscheto and muschetto appear.
1775 Moskito appears as the name of the American region.
1806 Noah Webster introduces musketoe.
1818 Muskitto and musquitto appear as the two options in Todd's revision of Johnson's dictionary.
1830 Joseph Worcester lists mosquito and musquetoe as valid spellings.
1840 Webster acknowledges musquetoe as a variant of musketoe.
1847 Webster’s changes the two approved spellings to musketo and musqitoe, adds a cross-reference entry for mosquito, and concedes that musquito and mosquito are the most common spellings.
1850 Worcester lists mosquito as the primary spelling of the word.
1909 Webster’s drops entries for any spelling but mosquito.
On this record, it seems to me that by 1800 moschetto and muscheto were fairly strongly established in dictionaries as the preferred English spelling of the word. The case for either or both would have been stronger if Samuel Johnson had endorsed it or them, but he demurred, for reasons unknown.
Then came the upheaval of 1806, when Noah Webster, as part of his simplified spelling scheme, introduced musketoe, essentially out of the blue. Although this innovation began to fade after less than half a century, it seems to have helped knock out both moschetto and muscheto. They don’t appear as entries—even as cross reference entries—in any of the U.S. dictionaries I checked from the period 1806–1909.
One inexplicable shift that occurred soon after the debut of musketoe was Todd's replacement in 1818 of the perennial English favorites muscheto and moschetto in his revision of Johnson's dictionary with muskitto and musquitto. It isn't out of the question that he was influenced by the debut of musketoe in Webster's 1806 Compendious Dictionary, but I think it's rather more likely that the impetus came from somewhere else—I just don't know where. Webster's serious claim to international fame was his 1828 An American Dictionary of the English Language—ten years after Todd's revision of Johnson appeared. The demise of muscheto and moschetto in England is not yet satisfactorily explained.
A major factor that I didn’t examine in any detail may be the Linnaean revolution in species naming. Linnaeus published Systema Naturae in 1735, but the interest in species categorization grew stronger and stronger as science advanced in the late eighteenth and early nineteenth centuries. With the differentiation of scientific names came a stronger motivation to distinguish the common names of similar-looking species. The effect in this case, I suspect, was to give serious prominence to the word mosquito at a time when the traditional spellings of the English word had gone into eclipse and yet no one was especially enthusiastic about the new, semi-phonetic options musketoe and muskitto.
Presented with a vacuum in preferred spelling, the entomologists who took on the serious business of naming species of Culex and assigning them distinct common names seem to have found the imported spelling mosquito appealing, and they gravitated to it. In any event, adoption of the new spelling seems to have happened surprisingly quickly: by 1830 Worcester was listing mosquito as the primary spelling, though it seems to have been virtually unknown thirty years earlier.
