Just guessing here, but the distinction between winter and summer bees is a physiological one rather than anatomical. The physiological changes lead to increased fat body in the winter bees, which means they have larger stores of vitllogenin, or "egg yolk protein".
I have read that this is in response to decreasing amounts of pollen going into the hive in the autumn. Presumably the revers happens when the pollen starts to come in, in the spring. So gradually the number of winter bees drops off as the old die.
The pollen and fat body contain the protein essential for feeding to brood and young bees. Pity it got called the fat body ( it does look like fat) rather than protein body.
