Spiders are very strong viruses that while not often seen, are one of the bigger threats you will face within the levels of Darwinia. These viruses will attack your units by leaping onto them, which will kill whatever unit the Spider is targeting outright. You will usually notice the attack beginning when the Spiders begin to turn in place, as they typically only do this before leaping one they are facing a unit of yours.
After the Spider leaps, is the best possible time to hit the virus back. Since they are immune to lasers, you will have to target them with some sort of explosive weapon, or with the Armor turret. If you are going to use grenades, fire them as soon as the Spider lands, or while it is leaping if you know what unit it is trying to land on. A second option is to use rockets, which are the most effective after their initial upgrade to version 2.0 thanks to their fast fire to hit time. It takes about 5-6 hits with these weapons to kill a Spider in Darwinia.
Trying to use Airstrikes against them is ill advised unless you have them in a small area you can fire the beacons into without being noticed by the Spiders, or if you plan on using a unit as a sacrificial lamb to keep the Spiders busy as the bombers make their way to the target area. Make sure to lay a wide spread to give yourself better chances of taking them out.
Spiders are rarely spawned from Triffid eggs, which can be devastating if launched into an area of Darwinians that don’t have the grenade upgrade or Armor turret. In addition, make sure you have one of these available before sending Darwinians against the Spiders.