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Cockroach eggs vary in size depending on the variety but many of them are the size off small beans, others the size of rice grains and some about the size of grains of sand. But they're all bigger than flea eggs, which are like very small grains of sand, but you can see them with the naked eye and they look just like "eggs" like in chicken eggs, but only very tiny. And when you break them, you can see they look like a broken chicken egg, just without all the yellow yolk inside. Roach eggs are big enough to crush between your fingernails, but they're pretty sturdy.
Would you notice grains of sand on the envelope you're liking? I think so!
So for cockroach eggs to be "stuck" to envelope glue, you would be saying, "What are these bumps on the envelope?" -- especially if you're LICKING them.
Now anthrax or small pox spores, or plague cells, that might be a problem!