Venom-Spitting Assassin Bug

Two-spotted assassin bugs (Platymeris biguttatus) are the largest reduviid I’ve seen to date. In contrast to many others in the family, they are very fast-moving, shuttling across the substrate like a robotic wolf spider. Today I learned something fascinating about this animal. When I moved my face close to the bug to examine tiny transparent mites, I felt a faint spray hit my right eye. I observed virtually no movement from the assassin bug, but there were drops of liquid on top of the pronotum and also a faint caustic aroma. I began to feel a mild burning sensation and rinsed out my eye, though the vision blurred strongly for only about five minutes. This sort of situation (minus the blurring) has happened countless times with carabids and tenebrionids that I’ve grabbed, though they usually expel chemicals indiscriminately from the abdomen. I had never heard of defensive secretions in reduviids before, but after doing some reading, remarkably, venom spitting has been documented in the Platymeris genus.

These assassin bugs can discharge enzymes from the proboscis up to thirty centimeters away. But how is this action performed? I didn’t notice any body arching or movement at all. Apparently, the flexible rostrum will arch sideways and upwards — sort of like sneakily throwing something behind your back — to fire rapidly, up to five spits a second. Not only that, but the discharge seems to be aimed directly at the eyes of a vertebrate predator, similar to a spitting cobra. No studies have examined spitting accuracy in assassin bugs, though I wouldn’t be surprised if they have also evolved to track and aim at eyed targets. Assassin bugs use their venom for both defense against a predator as well as subduing prey. A cool study last year found that they modify their venom composition depending on the ecological context. Differential secretions from three distinct compartments allow the bugs to produce different conglomerates of proteins to best suit the situation at hand. Although this work was performed on a different species of assassin bug, it wouldn’t be surprising if Platymeris discharges enzymes that induce respiratory paralysis when capturing prey, in contrast to more irritant and pain-inducing saliva when spitting at a predator. If you look closely in this image you can see flakes of a crystallized layer on its body, which is actually the saliva hardening after the bug ejected its venom up at me.

Assassin bug venom is potent for an insect. Many species are able to paralyze vertebrate predators hundreds of times their size and induce respiratory failure in mice in under half a minute. As I fed this assassin bug, it merely touched its proboscis lightly to a grasshopper’s forehead and the grasshopper ceased moving instantaneously. A true soft kiss of death. Targeted studies on reduviid venom have shown that Platymeris venom induces intense contractions of invertebrate heart muscles and loss of synaptic transmission and haemorrhaging in small vertebrates, suggesting cytolytic activity and the presence of neurotoxins.

Photographed after capture under controlled conditions [6]


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