When it comes to venom, assassin bugs are double trouble


The tiny insects are the only animals in the world to produce to distinct substances. Andrew Masterson reports.


An assassin bug of the species, Rhodnius prolixus: not good news for mammals, birds or insects.
An assassin bug of the species, Rhodnius prolixus: not good news for mammals, birds or insects.
VOLKER STEGER/SCIENCE PHOTO LIBRARY/Getty Images

Assassin bugs are little things but, boy, are they nasty. At least, they are if you happen to be an insect they consider to be prey – or, indeed, a rat or bird seeking to convert one into lunch.

The name of the bugs – which covers about 300 species clustered into the family Reduviidae – already implies that they are not to be trifled with. New research led by molecular bioscientist Andrew Walker from the University of Queensland, however, has revealed that they are far, far more unpleasant than previously thought.

In a paper published in the journal Nature Communications, Walker and his colleagues use imaging data to show that the assassin bug has not one but three distinct venom glands. More, it produces two entirely separate types of venom – one to conquer prey, and the other to repel predators.

“We discovered that assassin bugs actually make two different venoms, each containing a unique cocktail of over 100 different toxins,” Walker says.

The way an assassin bug feeds is the stuff of B-grade science fiction. Using its trademark strong proboscis – otherwise known as a rostrum – the bug impales its prey and then injects venom-laced saliva. This serves two gruesome purposes: first, it paralyses the victim, and then it liquefies its internal organs, allowing the assassin bug to suck it all out.

A 1978 study of a US assassin species observed that the strategy was so successful the bug was able to ingest as much as 99% of the live weight of its victim.

And if that simply makes the members of the Reduviidae sound like greedy brutes, consider this. In 2010 a paper in the New Zealand Journal of Zoology provided the world with the first description of a newly discovered assassin bug species, Stenolemus giraffa.

This particular bug makes its living eating spiders. It does so, reported lead author FG Soley from Australia’s Macquarie University, in ways that “emphasise stealth”. These include “slowly stalking the resident spider until within striking range”, a process that involves “breaking silk threads in its path while walking across the rock substrate beneath the web”.

But it’s not only soon-to-be-liquidised insects and spiders that have cause to be wary around assassins.

It’s long been noted that handling a bug in a way that makes it uneasy is a foolish thing to do. An assassin bug bite produces intense, localised pain and, eventually, a small patch of dead tissue.

Until Walker’s team went to work, it was assumed that the discomfort arose because the bug injected the same venom it uses to Magimix its food. It turns out the assumption was incorrect.

The researchers discovered that the bugs produce two quite different venoms and apply either depending on the situation.

The hunting venom is produced in one spot, an area dubbed the anterior main gland. The defensive alternative is produced behind it, in the posterior main gland. Both glands, plus a third auxiliary one, converge on a structure called the hilus, described as a set of muscle-controlled mixing chambers.

When Walker’s team applied the defensive venom to prey insects it had no effect at all – but, boy, did it hurt bigger animals.

The researchers say that as far as is known, the capacity to produce two venoms with different functions is an evolutionary adaptation not found in any other animal.

It may also, Walker says, lead to some useful innovations.

“The hunting venom seems like a good place to look for leads for eco-friendly insecticides, as it contains many different toxins that have evolved for the specific purpose of killing insects,” he explains.

“On the other hand, defensive venoms are designed to cause pain and consequently they are a good source of toxins that can be used to reveal new information about pain sensing in humans.”

Researchers working on the latter subject, we’re predicting, might struggle to find sufficient volunteers.

Explore #Insects
  1. https://www.nature.com/articles/s41467-018-03091-5
  2. https://www.nature.com/articles/s41467-018-03091-5
  3. https://academic.oup.com/aesa/article-abstract/71/4/476/85993
  4. http://www.tandfonline.com/doi/abs/10.1080/03014223.2011.604092
  5. http://www.tandfonline.com/doi/abs/10.1080/03014223.2011.604092
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