It is a good thing we weren't around 400 million years ago. If we had, we would probably have been afraid of being squashed by bugs, rather than the other way around.
Scientists in Germany recently discovered a fossilized claw (preserved remains of a nail), of a sea scorpion that was a foot and a half long. Reconstructing an image of how big the creature might have been, led scientists to realize, that this was probably the largest bug ever discovered. They estimated that this creature, which lived over 250 to 460 million years ago, was, at least, eight feet tall.
The discovery of the claw has also led scientists to believe that in the past, the family of arthropods - animals with hard external skeletons, jointed limbs and segmented bodies (spiders, scorpions, insects, crabs, etc.) - were a lot larger than even most humans today.
There is a lot of debate of how these creatures got so big. One theory is that it was due to the higher levels of oxygen in the atmosphere. However, others believe that since these were sea creatures, it was because they were at the top of the food chain with little competition. They ate everything in sight, including other sea scorpions, and hence ended up larger than the rest of the creatures.