"The Man in the Ant Hill", which spawned Ant-Man, was not Timely/Atlas/Marvel's first ant-themed tale...
...but I'm betting this never-reprinted tale from Adventures into Terror #43 (1950) is the first!
So there's elements of what would later be "Man in the Ant Hill" as well as the 1957 novella "The Fly" by George Langelaan, which became the basis for movie series in 1958 and 1986!
GCD attributes the art to Mike Sekowsky, but it doesn't match his work on Speed Carter: Spaceman only a couple of years later as shown HERE, HERE, and HERE.
Either he radically modified his style within a very short timeframe (which is possible), or the guys at GCD got it wrong, which happens occasionally.In any case, I'm going to stay with both the writer and artists of the story as "unknown", until somebody can provide proof otherwise.
Though this issue of Adventures into Terror was listed in the indicia as #43, it's actually the first issue since the book was previously-known as Joker Comics!Apparently, the Post Office caught on, since by the third issue, the numbering was corrected to #3, indicting a new second class mailing permit had been issued.
The classic example of this sort of bait-and-switch by comics publishers to avoid paying for a new second class mailing license (which each periodical needed) was EC's Moon Girl series.
The first issue was Moon Girl and the Prince.
As of #2, it became just Moon Girl.
When #7 came out, it became Moon Girl Fights Crime, adding true-crime tales narrated by Moon Girl. (The lead stories were still Moon Girl adventures.)
Two issues later (#9), the book became a romance title, A Moon, A Girl, Romance! (The final Moon Girl story appeared in the back of #9.)
Finally, as of #13, the book shifted gears into science fiction and became Weird Fantasy which ran from 13-17.
Then, since it had five issues under the Weird Fantasy title, the Post Office forced EC to buy a new mailing permit for the series, and continue the numbering with #6.
It ran until #22, when it merged with Weird Science into Weird Science-Fantasy.
(This explains why Weird Fantasy has two #13, #14, #15, #16, and #17 issues a couple of years apart!)
Since both Science and Fantasy ended with #22, it's uncertain which series' mailing permit was used from that point until Weird Science-Fantasy became Incredible Science Fiction as of #30!