Honey bees have what is called a “fat body” which is a section of tissue that holds fat cells, as well as glycogen, protein, and high concentrations of mitochondria and enzymes. The fat body is the equivalent in insect terms of a mix of a liver and fat storage.
During larval state, the fat body covers most of the larvae. The fat body will quickly and efficiently store nutrients during the larval stage to be used during the pupae stage (while the bee is not fed) and the adult bee will emerge with a fat body only in her abdomen.
The fat body is extremely important for winter bees (aka fat bees). Winter bees have a bigger fat body that would allow them to store more energy and nutrients that will last for months. Summer bees have a smaller fat body and in some cases almost non-existent.
Why is fat body important?
Recent studies on the parasitic varroa mite found out that the varroa mite is not actually sucking hemolymph (bee blood) our of the larvae and the honey bee, but it’s actually eating fat body cells, which are higher in nutrients. Different experiments determined that the mite could not extract enough energy by sucking hemolymph (which has a lower concentration of nutrients) and that it would not be as detrimental to bee health to lose some hemolymph.
By taking fat body cells, the varroa mite can take a higher nutritional material that would allow for fast growth and reproduction. Taking fat body is more detrimental to the honey bee colony’s health, especially over the end of the season when the mites feed on what were supposed to become winter bees. The winter bees that fed varroa mites have a smaller fat body and will not be able to survive for as long as the colony intended when raised those bees.
This explains some of the abrupt colony collapses that beekeepers find by the end of fall when stronger hives (big in numbers and resources) end up dying before even getting to winter.