According to this video, butterflies sometimes drink blood. Do they have the organs necessary to digest blood? I thought they eat only salts and nectar, which are easy to digest compared to proteins.
100mg/dL is the human blood sugar content (I assume it is similar in other animals). Afaik. sugar energy density is about 4kcal/g. A butterfly is about 1g of weight, which means 1mL if we count with water. So it can drink around 0.5mL of blood. That means 0.5mL×100mg/dL×4kcal/g = 0.002kcal = 8.4J. The top butterfly speed is about 12 mph, so I think we can count with 6mph by an average butterfly, which is about 2.7m/s. So it needs 1/2×1g×(2.7m/s)2 = 0.0036J kinetic energy to move and 1g×9.81m/s2×1m = 0.0098J potential energy to keep the 1m height of flight. I think we can guess that it loses this kind of energy in 2secs, which means it needs 0.0134J/2s = 0.0067W power to fly. So the 0.5mL blood means 8.4J/0.0067W = 24925s = 21min of flight time, which is not much compared to nectar (30g/dL$, $2500J$, $100h). Ofc. this is just a very rough estimation (the real flight time is probably much lower) based on data coming from non-scientific sources, but I think you can see the difference.