Kafka uses Gregor's preference for decayed food as opposed to fresh food to show that his transformation has not only physically altered him, it has changed him mentally as well.
This preference is displayed in the following passage: "Have I become less sensitive?" he thought, already sucking greedily at the cheese, which had immediately and forcibly attracted him ahead of all the other dishes. One right after the other, and with eyes streaming with tears of contentment, he devoured the cheese, the vegetables, and the sauce; the fresh foods, on the other hand, he did not care for; he couldn't even stand their smell and even dragged the things he wanted to eat a bit farther away."
This takes place near the middle of the novella, after Greta has brought him a wide variety of food to try. Gregor's preference of the more rotten food (the piece of cheese that two days previously Gregor had declared inedible; half-rotten vegetables; leftover bones congealed with a white sauce) over the fresh foods is a sign of a mental transformation into the beast-like creature that he appears to be, physically. The juxtaposition of his introspective "Have I become less sensitive?", more human-like with his greedy, animal-like sucking of the food has the effect of showing that Gregor still retains his human characteristics, even while behaving like and having the instincts of an animal. However, this passage also suggests that just as the fresh food has become distasteful to him, so too has human-ness become distasteful, and his dragging of the rotten foods "a bit farther away" from the fresh foods is symbolic of him distancing himself from humanity and the human aspects of his mind and associating more with the bestial side. The fact that Gregor does not find suitable the white bread and milk (the "fresh foods") that are brought, which on a certain level represent purity, indicates that he is rejecting the "purer" (for that is how humans often view themselves in comparison with other animals), cleaner way of living in favour of a more lowly, base, verminous one. The almost oxymoronic "tears of contentment", with its understated "contentment" juxtaposed with "tears", has the effect of conveying Gregor's relief and happiness and also suggesting that he is confused and cannot make sense of his emotions.
Through Gregor's growing indifference towards both his worries and concerns prior to the metamorphosis and his physical state, Kafka conveys that Gregor's human side is decaying, to be replaced by a more animalistic, detached, insouciant mentality.
This lethargy is illustrated within the following passage: "[...] he too was completely covered with dust; he dragged around with him on his back and along his sides fluff and hairs and scraps of food; his indifference to everything was much too deep for him to have gotten on his back and scrubbed himself clean against the carpet, as once he had done several times a day. And in spite of his state, he was not ashamed to inch out a little farther on the immaculate living-room floor.
Within this passage, Gregor has heard the sound of his sister's violin playing and is so drawn to the music that he has decided to venture out into the living room to hear it better. The dust here is symbolic of the accumulation of animalistic values and instincts which have replaced his previous human-like ones since his initial metamorphosis into a monstrous vermin. The alliteration used in "completely covered" helps to emphasize how indifferent Gregor has become to remaining clean, something that is a very human desire, for insects (assuming that is what Gregor literally is) have no need of keeping themselves clean; this thus shows how the priorities that governed his life prior the metamorphosis have faded for him. The imagery created by "fluff and hairs and scraps of food" and alliteration in "completely covered" emphasize how dirty he has become, and therefore how diffident and less human-like he has become. The fact that he used to clean himself "several times a day" is evidence of an ongoing metamorphosis, in that he is becoming more indifferent as time goes on. Also, the juxtaposition of this description of Gregor as a dirty vermin and the "immaculate living-room floor" leads to the realisation that the family, which is symbolized by the immaculate floor, is clean, he is dirty and full of dust, and therefore not only different from the rest of them but also a burden that soils or taints them as well. Additionally, prior to this passage Gregor had been thinking about his decreasing consideration towards the rest of the family, which is further evidence for his continual metamorphos and for the decay of the human aspect of his mind. However, this is not portrayed in a completely negative light, as being in this form allows Gregor to not care about the mundane worries that governed his life before the metamorphosis - his job, mostly. By becoming more beast-like, he is also becoming more free and unrestrained by the responsibilities he faces in his place in the family and in society, and by not cleaning the dust from his back he is resisting the urge to give in to his human concerns and allows them to pile up, undisturbed, without worrying about them. He has become free of the restraints of human life, and in a way is more fully human than he was when he was simply an automaton, working mindlessly at his job.
The physical decay of Gregor's body also reflects Gregor's changing mental state as he becomes less like the character he was prior to his transformation into a monstrous vermin, and shows his imminent death.
An example of this decay can be found in the following passage, which occurs just before Gregor's death: "He had pains, of course, throughout his whole body, but it seemed to him that they were gradually getting fainter and fainter and would finally go away altogether. The rotten apple in his back and the inflamed area around it, which were completely covered with fluffy dust, already hardly bothered him."
The most eminent contrast between his mental state here and his mental state at the beginning of the book is how peaceful he is. At the start of the book, Gregor had upon waking up moaned endlessly to himself about his worries, all of which were very mundane and related to his job or lifestyle; by this point, however, he is much calmer, and the peaceful reflection that he is undergoing at this point is a stark contrast to the anxiety-filled ramblings at the beginning of the novel. Kafka's use of the word 'fluffy', with its connotations of light and airy, reinforce this atmosphere of tranquility, and lend to it a touch of contentment - i.e., that he is finally content, and no longer plagued with mindless, grueling complaints. This is also supported by the phrase "hardly bothered him", which indicates that his situation is no longer troubling him. The pains that plague his body are also symbolic of these worries. The repitition of the word "fainter" in referring to these pains is used to great effect in this passage and suggests that life - particularly the life he lived prior to the transformation - is growing fainter as well, and is fading away from him as he approaches his death. The sentence structure emphasizes this point as well, as the first sentence begins with commas that separate the sentence into short sections, but these get progressively longer, with the last section being the longest; this is reminiscent of life fading away for Gregor. Within the next sentence, the imagery about the rotten apple, the inflamed area and the fluffy dust all contribute to an effective and vivid description of the decay he is undergoing. Kafka's use of the word rotten, with its negative connotations, indicate that Gregor's body has also become "rotten" and decayed, in a sense foreshadowing his impending death. The apple that is embedded in his back can be quite symbolic, and is an allusion to the fruit in the Garden of Eden of the Bible (which is commonly regarded as being an apple). It may represent knowledge, wisdom, and passion, but in this specific context it appears to represent the guilt Gregor has as a result of usurping his father's role; the father's usage of the apple as a weapon appears to indicate that the father is ready to reclaim his role, and to punish Gregor for having taken it from in the first place (despite the fact that Gregor did it unwillingly, and for the good of the family).The fact that his physical state is not bothering him points to a growing indifference about life, and also a decay of his human characteristics that would be more likely to cause him to maintain his cleanliness.
Kafka also uses the deterioration of Gregor's room as a way to show that his place in the family is decaying, and that the other family members are becoming less tolerant of him.
This is shown through the following passage, referring to Greta's cleaning of Gregor's room after she has taken on a job: "The cleaning up of Gregor's room, which she now always did in the evenings, could not be done more hastily. Streaks of dirt ran along the walls, fluffs of dust and filth lay here and there on the floor."
The imagery found in the "streaks of dirt", "fluffs of dust" and "filth" that accumulate within the room is firstly representative of the level of care with which Greta cleans his room, because it is her hasty cleaning of his room that caused their accretion. As her main reason for taking care of Gregor and cleaning his room is to ensure that when he has returned to his human form he will look upon her in favour as a way of thanking her for having taken care of him in his transformed condition, this hastiness would result from her diminishing expectations that he will revert to his former self (and also partially because she is more busy now, because of her work). Therefore, she is no longer putting as much effort into ensuring that he is comfortable, because for her, there is no point, and this job has become tiresome burden. This is also symbolic of the decay of Gregor's place in the family: although it was hinted through Gregor's locking of his bedroom door (which represents both the power that he holds within the family and his desire to 'keep the family out', or to keep himself separate from them) that he viewed himself and was viewed by the others as not being completely a part of the family, the initial metamorphosis he undergoes at the beginning of the novel has separated him even more, and as time goes by he becomes even more apart from them, not only physically (for he is almost always inside his room) and mentally (as he no longer can communicate with them) but also in terms of their relationship. Because he has been shunned as a burden to be endured (but nothing more), his perhaps odd but usual position as the head of the family is slipping further and further away from him, and so is the rest of the family. He is no longer truly part of the family, and while the family is initially in disarray from having lost their dominant figure (Gregor), once the father has reclaimed that role the family unit becomes more cohesive and, at the same time, exclusive of Gregor. Kafka also implies this through his diction in his choice of pronouns to refer to the family - "the family", usually, is what is used when referring to the mother, sister, and father both before and after Gregor's death, suggesting that the three of them together are what comprise the family and that Gregor is unnecessary.