sure, it’s a pretty simple answer!

to recap: 

hemoglobin is a protein found in the blood cells of vertebrates that binds to oxygen and carries it around the body, and also gives vertebrate blood that fun shocking ruby color. 

hemocyanin, on the other hand, is a different-but-related protein that performs the same task for many arthropods, and gives arthropod blood that pleasant milky-blue hue.

now hemoglobin, our fun protein friend that’s delivering oxygen to your hungry cells right at this very moment and second and keeping you alive while you read this, is capable of binding four oxygen molecules at a time (at the blue bits): 

this might not seem like a lot, but when you consider that each individual red blood cell contains thousands and THOUSANDS of individual hemoglobin molecules, the math starts to add up!

but hemocyanin is several orders of magnitude larger than hemoglobin, and is capable of binding over 90 oxygen molecules at a time! (at the red parts):

this might seem like a more effective setup, until you realize that hemocynanin is so FUCKING HUGE that it can’t be tetrised into a blood cell effectively!

so instead, arthopods don’t contain the hemocyanin at all and just kind of let it free-roam around their blood, bonking around where it will and delivering oxygen kind of haphazardly like a runaway blood roller coaster. whee! 

and this works fine for organisms with simple circulatory systems, but once you scale up to vertebrate complexity you really need something more directed and efficient. which is where these little dudes come into play!

some tiny blood friends who, once again, are keeping cellular respiration going in your brain even right now as you read these words. try not to think about it too hard.

and that’s why!