Just find all these little guys and shoot or smash them. There is about 333 of these guys so look in every nook or cranny or lockers. They make a buzzing noise if there is one near by. There are 93 Antlion Grubs in Chapter 1 and 240 Antlion Grubs in Chapter 2. We aren't going to list them all but here are what totals you should have.
-When you get to the Vortigaunt saving Alyx I had 93 grubs
-When you get to the weighted elevator I had 142 grubs
-When you get to the elevator with the Thumper I had 226 grubs
-When you get to the Acid-Lion guard (where the guard is first able to kill you) I had 263
The last grub is after the vortigaunts heal alyx.