When it comes to spiders, people generally know two facts about them- that you swallow around eight of them per year and that their blood is bright blue. Fortunately, the former is absolutely not true at all, the latter, on the other hand, is mostly correct.
While it’s certainly true that spiders have blue liquid in their veins, the reality is a bit more grounded than is sometimes depicted, with the fluid a much more reserved shade of blue-ish green. Also, thanks to fact that spiders are (generally) so tiny and contain very little liquid, you’re not going to see too much of this after smashing one.
It should also be noted that, unlike humans, spiders have what is known as an “open circulatory system.” Essentially, their blood is allowed to mix with all the interstitial fluids within their bodies. The scientific term for this mixture is hemolymph, which is a combination of the Greek word for blood (Haîma) and the Latin word for water (Lymph) and it’s defined as: “The circulating fluid in many invertebrates that is functionally similar to the blood and lymph of vertebrates”
So what in this mixture is causing the fluid in the spiders to turn blue? Well, as you may recall, human and indeed all mammal blood is red because of the presence of the protein haemoglobin. The reason haemoglobin makes blood red instead of say, green is because of the presence of iron as an oxygen carrying pigment. (And to quickly debunk another popular blue-blood myth: No- deoxygenated human blood does not turn blue. It turns dark red. On a related note, the red juice you see in red meat at the grocery store is not blood.)
Spiders and other arthropods don’t have haemoglobin in their bodies, rather they have a protein known as haemocyanin, which contains copper instead of iron. However, haemocyanin isn’t bound to any cells in the creature’s body like haemoglobin is, instead it just grooves around their circulatory system at its leisure. When an oxygen atom binds itself to haemocyanin, instead of turning a deep shade of red, it will instead turn a pale blue-ish green, as copper…
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