Many spiders build webs and passively wait for prey animals to be ensnared in their traps. Others like crab spiders (Araneae: Thomisidae) don’t build webs, but still passively wait to ambush prey. Some spiders, however, are active predators that stalk and kill their meals. Among them are slender crab spiders (Araneae: Philodromidae: Tibellus sp.). Earlier this month I came across one that had hunted down a signal fly (Diptera: Platystomatidae).
Slender crab spiders spend most of their time on low foliage hunting insect prey. They’re often found with their legs stretched out along the shoots of plants as shown here. Signal flies keep their wings in near-constant motion as if “signaling,” and this habit perhaps makes them more obvious prey for these predatory spiders.