Many social insects practice trophallaxis (from the Greek: troph(o) means nourishment: all axis means exchange). In this behaviour, food that has been digested by one individual is passed on to another, either by oral-oral contact or even anal-oral contact (in termites, for example).
In giant hornets, the most well-known form of trophallaxis happens between larva and adult workers: The worker hornets bring small pieces of meat into the nest and feed these directly to the larvae. In return, the larvae secrete a nutrient-rich liquid that is ingested (mouth-to-mouth) by retuning workers.
The amino acid content of this secretion is thought to be responsible for the worker hornets' ability to sustain their energy over long distance flights, and there is evidence that it has a similar effect on other animals such as mice, increasing their capacity for endurance exercise. Knowledge of the amino acid composition of this larval secretion has even been used to develop a sports nutrition drink specifically tailored to endurance athletes, VAAM (Vespa Amino Acid Mixture).
But what about the two hornets above? They are both workers. They arrived at this bottle of sugar water at different times, and initially seemed to fight mid-air. We presumed that they had come from different colonies, and were fighting for the food. But, after a few minutes of apparent aerial combat, they initiated trophollaxis, as shown above. Are they really from different nests? Is worker secretion similar in composition to larval secretion? There are many, many questions on this topic that are (as far as I know) yet to be researched!