Transforming into a bug highlights Gregor's relationship to food, which in turn allegorizes his relationship to his loved ones and his profession.

1. Gregor hungers for . . . 

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  sex (cf. p. 1, 32). 
  
• music and his sister. "Was he an animal, that music had such an effect upon him? He felt as if the way were opening before him to the unknown nourishmenthe craved" (Kafka 44-45). 
• the family's garbage.

2. The "unknown nourishment" Gregor really seeks is . . . 

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  to be understood and not starve (cf. p. 41). 

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  attention.

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  "At other times he would not be in the mood to bother about his family, he was only filled with rage at the way they were neglecting him, and although he had no clear idea of what he might care to eat he would make plans for getting into the larder to take the food that was after all his due, even if he were not hungry" (my emphasis; Kafka 39).
  

3. The Apple Scene (cf. p. 35)

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  His father chases him in circles (symbol of inextricable self-involvement of Gregor's fate). 
• 
  His father throws apples at him, choosing a round object to put an end to Gregor's circular wanderings. 
  
• 
  Apples allude to the tree in the Garden of Eden, paradise lost, love, knowledge, and sin.