Last week I came upon this marbled orb weaver (Araneus marmoreus) face-planted on a leaf in the forest. I propped it up and stuck around to see if something else might be going on…
Sure enough, this two-spotted spider wasp (Pompilidae: Episyron biguttatus) zipped around me, antennating the ground to detect chemical cues and relocate its paralyzed target. Wasps in the family Pompilidae are charismatic hymenopterans due to their parasitoid antics. A female wasp will locate a single appropriately-sized spider and incapacitate it with neurotoxic venom. She will drag the spider down a previously-excavated burrow, where she will deposit a single egg. In order for the host to remain in good quality and prevent decomposition, the pompilid doesn’t kill the spider during the attack, simply ceasing its motor function. A wasp larva will eventually hatch from the egg and begin consuming the hosts tissues, again, keeping vital organs intact to prolong the victim’s demise.
Many large Pepsis wasps will readily take down tarantulas more than twice their own size. Battles between wasp and spider are dire for both parties, and a single wrong movement can result in death… by fangs or by sting. However, smaller pompilids like Episyron have a much less arduous process in acquiring a host. An almost entirely defenseless spider such as this marbled orb weaver (Araneus marmoreus) doesn’t stand a chance of penetrating the wasp’s thick exoskeleton, and escaping from the agile and swift parasitoid is essentially impossible.
09/08/2020: Yesterday I spotted the tiniest spider wasp (Pompilidae) I have ever seen carrying a spider. At just a centimeter in length, lugging the immature jumping spider (Colonus sp.) to its preprepared burrow proved to be quite the effort. The wasp repeatedly stopped along the way to recuperate its strength, and it occasionally stung the spider on the underside of the abdomen to ensure the spider’s paralysis. Many spider wasps will also clip the legs off of spiders before transporting them for the sake of efficiency, and during my time with this wasp it removed the last remaining leg.
All wasps photographed after pursuit 
I’ve been lucky to see pompilids and their spider prey on many occasions. Below are two videos of larger spider wasps— the first from the Osa peninsula of Costa Rica and the second from the beaches of NSW, Australia.