This article states that : “…all spiders are predatory in the strictest sense – without exception they only eat what they can kill.” (italics mine)
I believe there is an exception to this, the known as Acherodes or the “Thief Spider” - this spider finds a ‘normal’ spider’s web and constructs its own ‘tethers’ so it can invade the unsuspecting spider’s web without being detected.
It then locates insects that have already been killed and wrapped up by the host spider, and proceeds to cut the web around them, (again using its own silk to modify the web so that the ‘host’ spider is unaware of its activities) so it can make away with a meal.
This spider is not eating what it can kill, it is eating what a much larger spider can kill, and thus is more of a scavenger than a predator.
David Attenborough’s series “Life in Cold Blood” contains some fascinating footage of this spider in action.
smiling bandit - it is true that an insect that is wrapped up isn’t necessarily dead, this is especially true of spiders that have no venom and rely upon their silk alone to immobilise their prey. Some insects caught by venemous spiders may only recieve enough venom to paralyse, but far more frequently the prey IS killed by the initial envenomation.
There are also spiders that whilst predatory, also eat plant nectar* so to say spiders only eat what they can kill, does apply here either. In captivity, some spiders are also known to eat bananas, marmalade, milk and egg yolk(see same reference below).
…mercurial
*Jackson, R.R. et al. (2001). “Jumping spiders (Araneae: Salticidae) that feed on nectar” J. Zool. Lond. 255: 25–29
I will somehow resist the temptation to wax lyrical about Jackson’s research on the incredible behaviour of Portia spp. (also Salticidae) as it isn’t relevant to the OP.
I agree with everything mercurial has to say on this topic. The thief spider, Argyrodes spp., known as a kleptoparasite, is found over lots of the world.
On the OP, there is also the nursery web spider species, Pisaura mirabilis, in which the male offers a nuptual gift of a wrapped up prey to the female so that he can mate while she is busy feeding. So she is eating something she has not killed.
He will also feign death (thanatosis) if she gets aggressive.
Only one family in 108 don’t have venom, the hackled orbweavers (family Uloboridae). They build orb webs, but they aren’t sticky like other orb weavers, but hackled, like the funnel-weavers. And lots more fascinating stuff. There are 40,000 known species of spiders, and many more again yet to be described - all with their own way of doing things. There is a reason I am obsessed by spiders!