Spiders build spider webs to catch their prey,
and assassin bugs use spider webs to kill the spider.
Spiders know how to figure out what they’ve caught in their webs. It’s how they eat and how they find sex. They do this by measuring the frequency and duration of their web’s vibrations.
Drop a leaf on their web and they ignore it.
The wind blows the web and they don’t care.
Deposit a horny male spider on a web and a horny female spider will definitely care – she’ll respond with her own "characteristic copulatory position."
Drop food on the web – like aphids or flies – and the spider gets to eat.
But place an assassin bug on the web, and the assassin bug deceives the spider.
First, the assassin bug purposely plucks the web to mimic the vibrations of a bug that’s been caught in the web.
This is easy to do, since the assassin bug has feet that don’t stick to the web.
The assassin bug pretends to be an exhausted bug – like an aphid or a fly – that can’t struggle any more and is ready to be eaten.
The assassin bugs, say researchers who studied them:
"…slowly approached the prey spider until within striking range, severing and stretching threads of silk that were in the way."
The unsuspecting spider feels the vibrations, thinks there’s an easy meal, and approaches.
Once within striking distance, the assassin bug will start tapping the spider from above, which somehow reduces the spider’s ability to respond, in a sort of assassin bug hypnosis.
And then… a scientist explains:
"The assassin bugs that stalk the spiders will tap the spider gently and very carefully with their antenna while they slowly move into position above the spider’s body to attack… Once in position, the assassin bug will quickly stab the spider with its mouth part, called a proboscis or rostrum… The spider will usually stop struggling within 10 seconds of being stabbed by the assassin bug."
Another scientist continues:
”The spiders just drop dead like a bag of potatoes. It is quite frightening to watch."
And just so you understand that the assassin bug has won:
"Usually the spider stops struggling within a few seconds. Then the bug starts to suck out the insides, kind of like a spider milkshake."
Appropriately, this process is known as "aggressive mimicry."
Assassin bugs trap spiders in web of deceit, Sydney Morning Herald>>
Assassin bugs deceive hungry spider, Discovery News>>
Predatory behaviour of an araneophagic assassin bug by Anne E. Wignall and Phillip W. Taylor, at SpringerLink>>
Assassin bugs from the genus Stenolemus (Heteroptera, Reduviidae) are predators of web-building spiders. However, despite their fascinating lifestyle, little is known about how these insects hunt and catch their dangerous prey. Here we characterise in detail the behaviour adopted by Stenolemus bituberus (Stal) during encounters with web-building spiders, this being an important step toward understanding this species’ predatory strategy. These bugs employed two distinct predatory tactics, "stalking" and "luring". When stalking their prey, bugs slowly approached the prey spider until within striking range, severing and stretching threads of silk that were in the way. When luring their prey, bugs attracted the resident spider by plucking and stretching the silk with their legs, generating vibrations in the web. Spiders approached the luring bug and were attacked when within range. The luring tactic of S. bituberus appears to exploit the tendency of spiders to approach the source of vibrations in the web, such as might be generated by struggling prey.