Dear Straight Dope:
As with most of your readers, I have a bet going with a good friend about insect circulatory systems. Mainly, how do they work? I think they have a very different systems than other creatures, but my friend says that they have normal (albeit smaller) hearts and red blood cells which transfer nutrients and oxygen to all the vital organs. I think he's full of SH*T!! With all of your improbable wisdom, can you set him straight? (I want my money!)
Your friend is full of it. Insects have neither red blood cells nor a closed circulatory system. Nearly all non-aquatic insects deliver oxygen to their cells (and remove CO2) through micro-tubules called "tracheoles" that join up to larger passages connecting to openings ("spiracles") in their body wall. Their “blood” (called "hemolymph") does not carry oxygen, except in some aquatic insect groups.
As for having a "heart," they can be said to have one only in the most general sense. Imagine a flute made out of rubber that you spit liquid into while squeezing it–a flexible tube with numerous holes all along its length. That is the only blood vessel in an insect body, and since it pulses rhythmically, it’s called a "heart." Technically, it’s called the “dorsal aorta," which is a better analogy, really, since it’s more of a vessel than a muscular organ.
So, ultimately, they have neither lungs nor a true circulatory system; nutrients flow freely in the hemolymph, which is basically swirled in the body due to the pulsing of the aorta, and oxygen is delivered mostly by diffusion from the outside, through tubes, with a little supplementary assistance through body movements (e.g., pumping the abdomen to help force air in and out). Yes, there are a few exceptions where the hemolymph carries oxygen, but they are exceptions, and not significant enough for your friend to claim victory.
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