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Resin bugs and other intriguing behaviour of assassin bugs in the Sabangau Forest, Kalimantan.

One of the most exciting discoveries for me when I was working with the Borneo Nature Foundation in the Sabangau Forest (peat-swamp) last year was some rather unusual behaviour involving two different species of assassin bugs that have always interested me around the world. In both instances I was accompanied by Hendri, a Dayak member of the local BNF field crew who specialises in plants, when I encountered them on different days. The first instance involved an assassin bug laying on its back on a leaf that appeared to be being attacked by ants. But on closer inspection with my glasses on I could see that the ants were in fact feeding off what looked like honeydew exuding from the rear end of the bug, something that is normally manifested in the symbiotic relationship between ants and sap-sucking bugs. But the biggest surprise was revealed when Hendry said that there was another assassin bug on the underside of the same leaf. When he inverted the leaf it appeared to be clinging to the one on top of the leaf that was “feeding” the ants, and apparently holding it in position. It was one of the most bizarre and baffling behavioural situations that I’ve ever witnessed with bugs anywhere. I can’t think of any other explanation except that the assassin bug was indeed feeding the ants. Someone suggested that perhaps the one on its back on top of the leaf was dead, and the ants were feeding off its secreted body fluids, but what was the other one doing beneath it and holding on to it? And they couldn’t have been mating with the leaf between them.






The next surprising encounter with unusual behaviour involving assassin bugs was when Hendri spotted a different species on a dead tree stump alongside the boardwalk that extends beyond the Lori track (old logging camp railway) from the LAHG Research Camp. Again I could see that there was something odd about what I was looking at because the front legs of the bug appeared to be swollen like Popeye arms and yellow in colour. Then attached underneath the same tree I spotted what appeared to be the exuviae (old moulted exoskeleton) of the same bug, transparent apart from the enlarged yellow front arms. Then we quickly deduced from what we were looking at both the bug and the tree that the yellow substance that was attached to the front arm was in fact some of the resin that was seeping from the tree! It got even odder when we examined the source of the resin on the underside of the leaning stump and discovered two smaller nymphs of the same species also with resin coated front arms. And if that wasn’t curious enough, on a subsequent visit we discovered that one of those smaller nymphs had something stuck on the resin coating one of its front arms. With much closer inspection we could see that it was a tiny dead nymph that also appeared to have resin coating its front legs, but also on its abdomen so it could have just been an accidental entrapment in the resin, but how did it end up being stuck on the front leg of the much larger nymph? As someone who panics with sticky hands this particularly odd lifestyle is an absolute anathema to me and looking at these photos makes me cringe!







This was obviously highly unusual behaviour that we were both very excited and intrigued about. I was eager to search for it on the internet at the next opportunity and discovered that these “resin bugs” are a group of reduviids which collect viscous substances from plants with their legs and use it as an aid in capturing prey and protecting their eggs against predators. Most of the descriptive references that I was able to find involve the North American genus Apiomerus, which contains species known commonly as bee assassins or bee killers. Species of Apiomerus frequent flowering plants, where they coat their legs with sticky plant resins and wait for their prey. The sticky resins allow the assassins to readily capture other insects, particularly bees that are attracted to the resin. Plant resins also appear to play a role in maternal care among certain Apiomerus species. Resin bugs include the following tribes in the subfamily Harpactorinae: Apiomerini, Ectinoderini, and Diaspidiini. Resin bugs in the tribe Apiomerini (Reduviidae: Harpactorinae) are restricted to the New World and include species such as Apiomerus pilipes, which are natural enemy of stingless bees in Brazil. The Ectinoderini are restricted to the Oriental region and appear to be much less well-studied than their New World cousins. This tribe also includes species such as Amulius malayus, which dip their setae covered forelegs legs into tree resin and use it to trap stingless Trigona bees.




From some other images that I’ve been able to find of the same species that we observed I’ve been able to identify it as probably being from the genus Amulius, tribe Ectinoderini and subfamily Harpactorinae. But there seems to be very little if any information, observations or research about how the resin is used in its hunting strategy; if it just attracts its prey like bees, traps them or quite possibly both. Anyway from what I’ve been able to find on the internet it appears that I have a very good set of photos that show this unusual behaviour, including the exuviae with resin attached to the front legs and the different sized individuals including the tiny nymph stuck to the larger one. That one resin producing stump that these particular assassin bugs are attracted to would be an excellent place for further study, but the challenge would be in following an adult like the one that we first observed, and must have just gone there to slough its skin and then get a another coat of resin because we didn’t find it again on subsequent visits, just the immature nymphs. It would indeed be fascinating to be able to find one of these resin bugs in the act of catching its prey using the resin.





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