Sticky traps, stealth and aggressive mimicry: Assassin bugs, worthy of the name (Part 1)

The world is full of charismatic predators; big cats, wolves and crocodiles, which are commonly the focus of TV shows and documentaries, but some of the most interesting and successful predators are often overlooked. As far as numbers go, the predatory beetles are the most successful terrestrial predators, but we’ll forget that for a moment as they are not the focus of this post. Instead, we will be looking at the fascinating variety and ingenuity of the hugely successful predatory Hemipteran group: Reduviidae.

The bugs belonging to the Hemipteran family Reduviidae are commonly known as assassin bugs and this is certainly a worthy name. Almost all are terrestrial ambush predators that are packing a variety of formidable weapons. Whilst many Hemipterans use their rostrum to extract sap from trees and plants, assassin bug use it to inject a venomous saliva into their prey and suck out the soup that results from its digestive properties. This needle-like appendage and toxic cocktail saliva packs a huge punch, giving them the ability to inflict nasty bites to animals of all sizes, including humans. However, whilst their bite is impressive, there are numerous invertebrates with a venomous weapon (Hymenopterans, spiders, scorpions etc…) so it’s nothing especially exciting. It is their variety of ingenious predatory techniques that make them worthy of the assassin title.

Horrid king assassin bug
The horrid King Assassin bug (Psytalla horrida). Not featured in the post, but the only assassin bug I have met in person. A formidable looking insect.

 

*Quick thing to keep in mind, not every assassin bug possesses all the traits I am about to talk about, this is simply a look at some of the most fascinating of the predatory attributes seen across the family Reduviidae.

Sticky Trap Predation

The first weapon in the arsenal of these mighty predators that I’d like to talk about, is their use of sticky substances to increase their hunting success. Sticky trap predation refers to the use of adhesive substances to aid in subduing prey and whilst uncommon, is seen in various lineages, including flat worms, mites, myriapods and more (Zhang et al., 2015). Sticky trap predation can manifest in two fundamentally different forms, the more common type involves the use of a sticky substance produced from glands located on the animal, known as endogenous sticky trap (Zhang et al., 2015). The second form involves the use of sticky substances produced by plants and not the animal itself (exogenous sticky trap), this is far less common (Zhang et al., 2015). Assassin bugs are the only arthropod to evolve both endogenous and exogenous sticky trap predation, found exclusively in the sub-families Harpactorinae and Bactrodinae (Zhang et al., 2015). Studies have shown that by coating their limbs in a sticky residue, either produced by them or acquired from plants, they significantly increase predation success (Law and Sediqi, 2010). It essentially just makes it much harder for their prey item to escape the assassin bugs grip, giving the predator more time to make killing blow.

Araneophagic Behaviour

Spiders are known for being voracious predators of insects, but sometimes the tables are turned. Several assassin bug groups are araneophagic and have mastered different techniques in tackling the dangerous task of hunting a spider. The assassin bug genus Stenolemus comprises of a group of species that display araneophagic behaviour which, again, manifests in different ways. Firstly, we’ll look at the web-invading species Stenolemus giraffa, a large assassin bug from Australia, that snatches spiders from their webs without being detected (Soley et al., 2011).  S. giraffa is a bizarrely proportioned creature with a significantly elongated pronotum (earning it the name giraffa) and possess forelegs shaped similarly to those of a praying mantis. These features are thought to have evolved to increase their success at hunting spiders on webs. For starters, their large size, elongation and leg span enable them to spread their weight over a large area of the spider’s web, therefore reducing localised web vibrations that could alert the spider to its presence (Soley and Taylor, 2012). Secondly, their mantis-like, raptorial forelegs make it possible for them to snatch spiders from a distance (Soley et al., 2011). This is particularly important, because whilst they display numerous methods of navigating a web without alerting the spider, for example by spreading their weight and moving in certain ways, or sometimes even by cutting some the threads of the web (Soley and Taylor, 2012). It has been shown that given the option, S. giraffa will avoid entering the web at all, and instead approach the spider from a nearby surface, such as a rock (Soley and Taylor, 2012). Their elongated body then enables them to lean across the web and snatch the spider without ever having touched the silken trap (Soley and Taylor, 2012). Not only is this effective, but the fact that when presented with options they will explore those that don’t involve entering the web before attempting to invade the web, shows a degree of predatory risk assessment (Soley and Taylor, 2012). Furthermore, it is not uncommon for S. giraffa to abandon a hunting oportunity if web invasion is the only option, showing just how risky the maneuver is.
Although, hunting venomous, insectivorous, web-dwelling spiders is not without its risks and a wrapped-up S. giraffa in a web is not an uncommon site (Soley et al., 2011).
S. giraffa aren’t the only Stenolemus species that exhibits these behaviours, others such as S. bituberus use similar tactics (Soley and Taylor, 2012).

S. giraffa
“Late instar nymph of Stenolemus giraffa pursuing a Trichocyclus arawari in its web, East Kimberley Region, Western Australia. The nymph has its antennae extended forwards and oriented towards the spider while hanging from the web with its middle and hindlegs, as well as its right foreleg” (Soley et al., 2010).

However, S. bituberus has also been observed using a different tactic to tackle their dangerous prey. Rather than sneaking onto the web and taking measures to reduce detection, they act like every gamer on missions where stealth is optional and throw caution to the wind. S. bituberus has been observed using aggressive mimicry to lure the spider towards them before launching their attack. Web-building spiders rely on vibration queues to alert them to prey, and this is a very effective method, hence the overall success of these spider groups. However, it does leave them open to exploitation by clever predators (Wignall and Taylor, 2010). S. bituberus approaches the web and then uses its forelimbs to pluck the silk to attract the spider towards them, until they are close enough to strike (Wignall and Taylor, 2010). The experiments carried out by Wignall and Taylor (2010) show that S. bituberus does not mimic the full range of prey vibrations but instead is a general mimic, able to replicate a set of vibration queues that sit within the range of vibrations that a spider identifies as a prey. By plucking the web in this way the spider is convinced that it has caught itself some dinner and sets off towards the location of the vibration, not known for their eyesight, this involves getting quite close to the source of the vibrations. This enables the assassin bug to wait for its prey to arrive without having to actually enter the web. Once the spider is close enough, S. bituberus uses those raptorial front legs to snatch the spider and then stab it with its rostrum. Before long the spider is dead and already digesting into a tasty soup for the assassin bug to enjoy.

This kind of aggressive mimicry has been observed in numerous araneophagic spiders, such as the Pholcid and Salticid spiders. However, this isn’t particularly surprising considering that these spiders have an evolutionary history of web/silk interaction and therefore already possess the necessary adaptations for traversing across the silken traps (Wignall and Taylor). It is a far more impressive and surprising feat of evolution that an insect has managed to evolve the ability to exploit the reliance on vibration queues by web-dwelling spiders.  

 

The use of adhesives, stealthy web invasion and aggressive mimicry are just a few of the deadly techniques in the arsenal of these master assassins. In fact, there are two other behaviours displayed by Reduviidae bugs that I wanted to cover, but this post has already exceeded 1,000 words and I don’t like to have my posts to be much longer than that. Therefore, I have decided to split the topic into two parts, so keep an eye out for part two where corpse camouflage and termite fishing will be covered, Regardless of the two uncovered topics, I think it is safe to say that these voracious bugs are worthy of the name assassin!

I hope you have enjoyed this post about one of my favourite groups of insects. As mentioned above stay tuned for part two for more assassin bug goodness! Thank you for reading and, as always if you’ve enjoyed it, a like is hugely appreciated. And don’t forget to find me on twitter, Matthew Woodard @ZoologyNotes and if you want to be notified when a new post is out just hit follow.

Thanks for reading, until next time.

I could’ve so easily written more about each of these techniques, it was difficult to compress, I definitely recommend reading the papers listed below. Assassin bugs are incredible and there are some fascinating papers out there, here are but a few.

References

Law, Y.H. and Sediqi, A. (2010). Sticky substance on eggs improves predation success and substrate adhesion in newly hatched Zelus renardii (Hemiptera: Reduviidae) instars. Annals of the Entomological Society of America, 103, 771-4.

Soley, F.G., Jackson, R.R. and Taylor, P.W. (2011). Biology of Stenolemus giraffa (Hemiptera: Reduviidae), a web invading, araneophagic assassin bug from Australia. New Zealand Journal of Zoology, 38(4), 297-316.

Soley, F.G. and Taylor, P.W. (2012). Araneophagic assassin bugs choose routes that minimize risk of detection by web-building spiders. Animal Behaviour, 84(2), 315-21.

Wignall, A.E. and Taylor, P.W. (2010). Assassin bug uses aggressive mimicry to lure spider prey. Proceedings of the Royal Society Biology, 278(1710).

Zhang, J., Weirauch, C., Zhang, G. and Forero, D. (2015). Molecular phylogeny of Harpactorinae and Bactrodinae uncovers complex evolution of sticky trap predation in assassin bugs Heteroptera: Reduviidae). Cladistics, 1-17.

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