Clearly a waste: the bee digestive system
The first line in the Huffington Post story reads, “An amateur photographer has captured an amazingly rare sight in his own back garden—a bee urinating.” No doubt, the photo by Mark Parrott is awesome, but is the bumble bee actually urinating?
In fact, the bee digestive system does not divide waste into solids and liquids—instead, all of it is collected in one place. The bee digestive system is more or less a straight line.
The bee digestive system
The mouth is connected directly to the esophagus, and the esophagus extends through the head and thorax all the way back to the abdomen. In honey bees, after food goes through the esophagus it travels into the crop (or honey stomach) where it is stored for transport back to the hive.
At the end of the crop is a one-way valve known as the proventriculus. Anything that passes through this valve moves on to the ventriculus (also known as the true stomach or digesting stomach) where it is digested. But anything that goes through the one-way valve cannot go back the other way. So food that is digested cannot re-enter the crop, and this is why nectar is not bee vomit. Nectar that will be used to make honey never makes it into the digesting stomach, only the crop.
The ventriculus is lined with cells that secrete enzymes that digest any nectar and pollen that has passed through the one-way valve. At the far end, the ventriculus is attached to the ileum, which is like a small intestine.
Right where the ventriculus meets the ileum, about one hundred malpighian tubules connect to the digestive tract. Malpighian tubules act like our kidneys. Just like our kidneys filter waste products from our blood, the malpighian tubles filter waste products from the bee’s hemolymph. This liquid waste, which is analogous to urine, is dumped into the ileum where it joins the solid waste from the ventriculus.
The ileum removes nutrients from the digested food and moves the waste further along the digestive tract. From the ileum, the waste products from both the ventriculus and the malpighian tubules move into the rectum where it is stored until the bee can defecate through the anus.
The waste stream of a honey bee
All bees are built in a similar way, but the crop is more developed in those species that carry nectar back to the nest. The flow of food and waste through a honey bee looks like this:
mouth↔esophagus↔crop (honey stomach)→proventriculus (one-way valve)→ventriculus (digesting stomach)→ileum (intestine)→[waste from malpighian tubules joins food waste in the ileum]→rectum→anus
So back to the photo, I would say the bee was defecating rather than urinating. But clearly she had a lot to drink that day. Who knows? Maybe she was getting ready for a mandatory drug test and was trying to flush away all the poppy nectar she drank.