Arachno's are found in the sub-sewers deep beneath Freedom City. A nice source of income but also a deadly one. Keep away from until you've learned to play a bit and have outfitted yourself with some nice gear.
Arachno hunting contracts pay out $500 per kill before corp tax.
Make sure you've got defense against stab/slash covering your entire body! Arachno's don't always attack instantly but tend to be fairly aggressive.
Gilmore added our friend to the contract lol. Enjoy trying that out.