It's not so much a migration.
As stated above. When males fully mature they get their reproductive organs, then go in search for a female.
All those tarantulas you saw were ultimate males looking for some tail.
Most tarantulas will only travel in about 25sq ft their whole life. With a few...
Nah it's a dry process.
The male (left) molts for one final time, in which he gets his sexual organs.
The little "legs" in front that aren't actual legs, but are called pedipalps have hard "bulbs". They use these bulbs(called Emboli) to draw up sperm that they lay down into a special web...