The petitioner of this hunt - Niray - can be found in the only (except for Ondore's) mansion in Bhujerba.
Her kids have runaway to Lhusu Mines. What's worse an Antlion has been seen there recently. She asks you to find her children and gives you the Site 3 Key, so you could get deeper.
While in Lhusu Mines run to Site 2 (where Ba'Gamnan was) and at the back of this area you'll find a closed gate. Use the key to open them. Before you enter the area with gate crystal open the giant gate, which blocks the way to the main part of the mine and run to the west.
5x Killer Mantis
About 108 000 HP
Resistant to Libra,
Vulnerable to Wind
Antlion can be found in the most western part of Lhusu Mines. Along with him five other Killer Mantises wait for you. All of them, including Antlion, are vulnerable to wind, so use few Aerogas in the place where there's the biggest concentration of them and after just few seconds you'll be left alone with Antlion.
He doesn't have any special attacks. Just flatten, which deals pretty large damage to one character, but he uses it also pretty rarely. At the end of the battle he'll use enrage, which will raise his attack speed and amount of hits he can pull off in a single combo. His normal attack may cause disable, but only on the attacked character, so there will be always someone to cure the rest.
Bounty: 4300 gil, Bubble Belt, Sickle-Blade (1878g), Site 3 and Site 11 keys.
After the fight the boy will tell you that he had dropped the Site 11 key and it fallen through a breach in the floor and onto the Earth. It seems like looking for a needle in a haystack - but hey, where's the problem?
Remember the guy in Phon Coast which is said the have fallen from Bhujerba? You can find the key just right from him, on the ground.