Life is made mostly of proteins. Protein is about 16% nitrogen. Plants convert sunlight into sugars, and the structures of plants are made of cellulose. Sugars and cellulose don’t contain nitrogen. As a result, plants contain a significantly lower percentage of nitrogen than do animals.
Butterflies can assist the survival of their caterpillars by starting them out with a little more nitrogen than they would normally get. Male butterflies concentrate salts, including nitrogen salts by sipping water from mud. They excrete the water and concentrate salts, which are transferred to the female during mating.
Sometimes butterflies will seek out things like dung and carrion from which they can obtain fairly concentrated solutions of nitrogen salts.
Which brings us to a road-killed frog and a butterfly.
The frog’s loss is the butterfly’s gain. This hackberry emperor will enhance the survival of its offspring with the juices of the frog.