The traditional (non-simplified) Chinese character 風 *prəm > pjuwng > fēng ‘wind (n.)’ consists of 凡 *[b]rom > bjom > fán as phonetic, plus an insect 虫 inside it, in a position that could suit a signific. Such is the analysis implied by the Shuowen Jiezi 說文解字 c. 100 CE, which says [ 從蟲凡聲 ] “with phonetic 凡, and signific 蟲”. The rationale for linking the wind with insects involves numerology: the number ‘eight’ governs both the eight winds and the insects (since these are said to need eight days to transform from the larval stage).
The true reason for the presence of 虫 is more interesting. The evolution of the graph for ‘wind’ was told by Jì Xùshēng 季旭昇 (2002 , vol. 2 p. 225). According to him, the 虫 element in the modern character appears graphically (at first in its full form 蟲), as part of the character for ‘wind’, in Zhan Guo 戰國 times in palaeographic material from Shuihudi 睡虎地. Graphically it continues a triple pattern on the feathers of 鳳 *[b]r[ə]m-s > bjuwngH > fèng ‘phoenix’, a character used for 風 “wind” as a loangraph (or jiajiezi 假借字) in the oracle bone inscriptions. This 鳳 character itself was a drawing of a bird with ornate feathers, sometimes accompanied by 凡 in a phonetic capacity. [Added Feb. 5, 2021: Ma Kun 马坤 informs me that the explanation of the graphic history of the character goes back to Zeng Xiantong 曾宪通 (1996). Chu Wenzi shi cong (wu ze). Zhongshan Daxue xuebao 3:58-65. ]
Phonologically the character’s pronunciation is recounted in Baxter and Sagart (2014:310-311). They reconstructed the OC form of ‘wind’ as *prəm. In the evolution to Middle Chinese, Old Chinese words ending in *-əm underwent a special development when their initial was a labial: *-əm was rounded to *-um under the effect of the initial; upon which the -m ending dissimilated to *-ŋ. In that way 風 “wind” evolved from OC *prəm to *prum, *pruŋ and finally MC pjuwng. The word “bear” 熊 followed the same evolution, from OC *C.[ɢ]ʷ(r)əm to [ɢ]ʷ(r)um to [ɢ]ʷ(r)uŋ and finally to MC hjuwng.
Now back to our original question, what is an insect 虫/蟲 doing there ? In the Baxter-Sagart system, 蟲 is Old Chinese *C.lruŋ > Middle Chinese drjuwng. After the evolution *prəm > *prum > pruŋ, 凡 (*[b]rom in OC) could no longer be recognized as a phonetic, and there would be a need for phonetic remotivation of the character. Getting a 蟲 to appear in ‘wind’, out of an earlier graphic detail, would provide a phonetic clue to the string -ruŋ in 風 at the *pruŋ stage. Imperfect, because of the lack of any labial before -r-, but still informative. In other words, 蟲 occurs in ‘wind’ as an (imperfect) phonetic element.
In the history of the Chinese script, it is not rare for graphic elements originally playing no phonetic role to be reconditioned as phonetics, a process known as “phonetic remotivation”, sometimes called 音化 “phoneticization” in Chinese. As here, such phonetics tend to display imperfect adequation to the character’s pronunciation, evidently due to the strong graphic constraints on the process: 音化 phonetics are selected, not out of the entire collection of available phonetics, but out a narrower collection of phonetics bearing some graphic resemblance to a part of an earlier character.
In the case of 風, we may turn things around and take the first appearance of 蟲 in feng 風 as a terminus post quem non for the late OC *pruŋ stage: Warring States, then.
Baxter, William H. and Laurent Sagart. 2014. Old Chinese: a new reconstruction. New York: Oxford University Press.
Jì Xùshēng 季旭昇. 2010 . Shuōwén xīn zhèng 說文新證 [New evidence on the Shuōwén]. Fúzhōu 福州: Taipei: Yee Wen. (In Chinese)