Second is the description of Gregor "hissing." It is true that some insects produce hissing noises. In fact, this detail may be one of the reasons so many people assume Gregor is a cockroach (although some beetles also hiss). However, I believe in this case that the word "hiss" refers to agitated, unintelligible sounds. Gregor is struck in the beginning by the discovery the his voice has become incomprehensible to his family, a fact which upsets him greatly. His father (whose speech is still entirely understandable to Gregor), is also said to be "hissing." Indeed, in the original German, the same word is used for both father and son ( "zischen"/"zischte"). Thus I think it is fair to work off of the idea that Kafka was using "hiss" in a broader sense.
Finally, it is necessary for me to point out that the great writer (and renowned entomologist) Vladimir Nabokov discussed the question of Gregor Samsa's insect identity in a lecture on "The Metomorphosis." His discourse on the subject goes as thus:
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.) Further, he has strong mandibles. He uses these organs to turn the key in a lock while standing erect on his hind legs, on his third pair of legs (a strong little pair), and this gives us the length of his body, which is about three feet long. In the course of the story he gets gradually accustomed to using his new appendages—his feet, his feelers. This brown, convex, dog-sized beetle is very broad.
While it is certainly poetic to presume that Gregor did indeed have undiscovered wings, I believe his sister's comment introduces us to the tragic truth that he is capable of neither a human's nor an insect's escape. Her words, "for he could hardly fly away," drive home how trapped Gregor is in his dismal reality. Thus, despite my respect for Nabokov, my pursuit continues for a flightless Gregor Samsa.
Therefore, my search for the insect Gregor will not include the qualification of hissing, nor that of sticky feet, but will include the inability to fly.