Yeah, you know this gifting event already.. I have posted few articles about it.
And now here is the official guidance from 411 guy!
I will just copy/paste without further ado.. as you know the drill! 

Arachnophobia 411 & FAQ

Updated 04/15/2011 at 20:26

Arachnophobia 411 &FAQ There are several ways for you to collect Harmless Spiders through the following in-game features:  Gifting  Complete jobs, win fights, and rob properties  Playing Arachnophobia Mini-Game!  Through posting and clicking on wall posts  Purchasing directly from the Marketplace for 1 Reward point each A Harmless Spider mini-game will also be available to increase the chances of collecting extra Harmless Spider! This mini-game may randomly appear when completing jobs, fights, or robbing and you will be presented with the option of selecting one of four briefcases for the chance to win up to five Harmless Spider.        One mini-game can be played every 24 hour period for free. There is also the rare chance of finding a Poisonous Spider behind a secret door! Each time you participate in the Mini-Game you are also given the opportunity for a replay at the cost of 1 Reward Point. Good Luck!        Maximum Collection limits for the Arachnophobia Loot event 10 Harmless Spiders can be looted each day from Job completion.  Each Harmless Spider has a very rare chance of being aPoisonous Spider, which equals 5 Harmless Spiders. (The rare chance of collecting a Poisonous Spider can only be achieved once per day from completing Jobs.) Please note:  The Double Loot Mastery from your Crew bonus does not apply to the Harmless Spiders.     10 Harmless Spiders can be looted each day from Fighting.  Each Harmless Spider has a very rare chance of being a Poisonous Spider, which equals 5 Harmless Spiders. (The rare chance of collecting a Poisonous Spider can only be achieved once per day from Fighting.) 10 Harmless Spiders can be looted each day from Robbing player’s properties.  Each Harmless Spider has a very rare chance of being a Poisonous Spider, which equals 5 Harmless Spider. (The rare chance of collecting a Poisonous Spidercan only be achieved once per day from Robbing player’s properties.) Please note:  The Double Take from the Crew bonus does not apply to the Secret Identities. 40 Harmless Spiders can be collected each day from free gifts. All Harmless Spiders collected after you have reached the daily maximum of 40 will be converted into a Mystery Bag. There is a maximum combined amount of 40 Harmless Spiders that can be collected each day from the combination of Gifting, Robbing, Jobs, Fighting, and Wall Posts. 5 Harmless Spiders can be collected each day from the Harmless Spider wall posts.  There are not any limits to the number of times that you can post an ‘ask wall post’.  The first five friends who click on your post will send one your way  You cannot click to collect from your own wall post - "Nice try, but you cannot collect from you own feed." 10 Harmless Spiders can be collected per day from clicking on a friend's wall post. • There are two different types of wall posts (ask and share) and you can collect up to 5 for each type. • When you click on an ask wall post, it will also send one to your friend. • If you have maxed out at 5 for type of wall post, you will get a "You collected the max number of Harmless Spider from               this type of feed. Try again tomorrow." • If 5 people beat you to the wall post, you will get a "You are too late. All the Harmless Spider have been claimed. Look out         for more."  5 Harmless Spiders can be collected per day from clicking on a friend's Poisonous Spider wall post. • You will receive a Harmless Spider from this post, not a Poisonous Spider • If 5 people beat you to the wall post, you will get a "You are too late. All the Harmless Spiders have been claimed. Look             out for more." There are no limits to the number of Harmless Spiders that you can collect from playing the Harmless Spider Mini-Game, however, it will cost 1 Reward point each time that replay the Mini-Game. Harmless Spiders may be purchased for 1 Reward Point each through the Marketplace. When enough Harmless Spiders have been collected you will be able to redeem them for the following items: 25   Harmless Spiders –  Spider Eater 50/28 50   Harmless Spiders --  Black Widow Armor 68/91 100 Harmless Spiders --  Spider's Fang 89/64 150 Harmless Spiders --  Spider Squisher 80/103 "Oh what a tangled web we weave, When first we practice to deceive", Sir Walter Scott

Source : Zynga Customer Support 411