Unlike most other spiders, Lava Spitters are found in plain sight and do not suddenly descend to the ground. Also, while other spiders augment their attacks with Apply Poison, the Lava Spitter uses Incendiary Arrows - meaning its attacks strike in an AoE and cause Burning instead of Poison.
Farming for both Igneous Spider Leg and Ebon Spider Web trophies for the Canthan New Year can be accomplished by repeatedly doing the Ring of Fire mission, and going northward through the lava caves instead of through the large ether seal protected gates at the fork of the mission. There are a great number of Lava Spitters in this area, including the boss Casses Flameweb. Ebon Spider Webs appear much less frequently than Igneous Spider legs, thus farming the webs will take much longer.