From this lecture:
Now what exactly is the "vermin" into which poor Gregor, the seedy commercial traveler, is so suddenly transformed? It obviously belongs to the branch of "jointed leggers" (Arthropoda), to which insects, and spiders, and centipedes, and crustaceans belong. If the "numerous little legs" mentioned in the beginning mean more than six legs, then Gregor would not be an insect from a zoological point of view. But I suggest that a man awakening on his back and finding he has as many as six legs vibrating in the air might feel that six was sufficient to be called numerous. We shall therefore assume that Gregor has six legs, that he is an insect.
Next question: what insect? Commentators say cockroach, which of course does not make sense. A cockroach is an insect that is flat in shape with large legs, and Gregor is anything but flat: he is convex on both sides, belly and back, and his legs are small. He approaches a cockroach in only one respect: his coloration is brown. That is all. Apart from this he has a tremendous convex belly divided into segments and a hard rounded back suggestive of wing cases. In beetles these cases conceal flimsy little wings that can be expanded and then may carry the beetle for miles and miles in a blundering flight. Curiously enough, Gregor the beetle never found out that he had wings under the hard covering of his back. (This is a very nice observation on my part to be treasured all your lives. Some Gregors, some Joes and Janes, do not know that they have wings.)