Yesterday scientists and photographers took to Twitter to celebrate International Invertebrate Butt Day, or #invertebuttfest. Some of them are fuzzy. Some are wet and slimy. Many are literal hard-asses, strengthened by one of nature's toughest materials called chitin. The niche trend is also taking place in honor of biologist Maureen Berg's birthday. (Berg has previously given talks about invertebrate butts, and has previously called for June 9th to be Invert Butt day.)

"The whole idea is that, we love butts, especially invert butts, and decided that there should be a dedicated day to celebrating them," Berg says. "Invert butts should be celebrated because they have a wide diversity in form and function, and because they bring us all joy."

We at Massive have a soft spot for butts. We, in fact, dedicated a whole month to them. That's because butts are fascinating. Humans have evolved anatomically interesting ones, and we use their deposits to track disease. Other animals evolved to literally be a butt's output, to escape predators. And butts can be important tools for animal scent-based communication. 

Invertebrate butts are of course, nothing like ours. Spiders aren't fleshy and bees don't have anuses. But there's no reason to identify part of their anatomy as a butt in any case. The precise definition of a butt is a bit arbitrary. In an interview last year, ecologist and butt science author Dani Rabaiotti told Massive: "I usually just refer to any end of an animal that’s opposite its head as a butt." So there you have it, scientific validation for the ubiquity of butts. Here are some of our favorites invertebutts: