Ant Stacking Assassin - Observation of the Week, 3/5/24

Our Observation of the Day is this ant-encrusted assassin bug nymph (in the genus Inara), seen in Malaysia by @victor0001!

Victor Heng, a teacher at the Natural History Museum in London, got going with iNaturalist (as well as iRecord) in earnest during the lockdowns of 2020. 

I started using it to record everything I was seeing in my local parks. I started using the maps to look for places with few records in my area and set myself a mission of characterizing the plants and invertebrates in those blank areas. Since then I've started to look more for phenology, recording flower plants at early and late points in the season.

For Lunar New Year Victor and his partner visited Victor’s family in Malaysia, where he took the photo you see here.

This was only the 3rd time I've been to Malaysia, and the first time I'd been since picking up the iNaturalist habit, so I was very excited to have an opportunity to explore rainforests with this new perspective. A few of my relatives live in Penang and, knowing that my partner and I like nature, took us up to Penang hill to experience the canopy walkway.

Several of my cousins and younger relatives were along for the walk, and I was inevitably at the back. My photographing of things encouraged them to also take a closer look at what was around, if only to point it out as something I might want to take photos of. The assassin bug was one of these spots. My cousin called me over and pointed it out, wondering if this small black thing was a bit of poop or actually an animal. I initially thought it might be a spider, but once I saw it through the camera lens I realized what it was, and was amazed at all the ants it had stacked on its back. I'd still like to know how it reaches back to get them stacked up, and how the ants stick together up there.

Yunhan (@eggshe11), who works on identifying assassin bugs in Asia, came across Victor’s find and identified it as being in the genus Inara. I contacted Yunhan about the ID and any info they might have about this behavior. 

To my knowledge, there are 2 common genera of carcass-stacking assassin bugs in southeast Asia - Acanthaspis and Inara. Where I'm from (Singapore), both genera have been relatively well documented (see this page on Inara and this one on Acanthaspis for example). As the photographer Nicky Bay (@nickybay) mentions on his page, Inara nymphs are identifiable by being less concealed overall and having a “neater” ant stack. Acanthaspis nymphs tend to be covered in debris including their heads, legs, etc (example, also visible in this popular observation) while for Inara you will always be able to see the bare legs and the head complete with large eyes resembling the adult (for example, this observation). 

For Inara, post-feeding, a nymph will use its front and mid legs to adjust the carcass to a satisfactory shape, subsequently transferring it underneath the existing stack onto the dorsal surface of the abdomen using its hind legs. The abdomen possesses glands which secrete a sticky substance, and these secretions also hold the stack together. After ecdysis, nymphs will transfer the stack off their molt, rather than start a new one. I'm not sure why they keep a stack, although I think the main reasons lean towards (1) attracting/pacifying ants via pheromones from the carcasses (2) predator avoidance/deterrence. 

Victor (above, in Kuala Lumpur), traces his interest in nature back to his childhood, when he’d collect rocks or bring home insects on sticks. Later in life he became interested in social science and teaching, and is happy he can combine those interests in his current position. He’s currently developing the museums new outdoor program, to be held in its redeveloped gardens. One project involves the students analyzing “bycatch” from moth traps, that represent insects from hypothetical farms. 

The early pilots of this session have been really rewarding. Students often have strong reactions when they first put their petrie dish of dead insects under the microscope. Mostly a mix of surprise and fascination, but occasionally disgust. But in the vast majority of cases, all their reactions drift towards fascination at the variety of insect forms. Its been great to be able to see their shift in attitude towards insects, and appreciate their importance to ecology. And understand the importance of ecology to agriculture and humans.

He’s been using iNaturalist in this work with the museum, tracking which invertebrates are found on which plants in the gardens, for example. 

The process of submitting observations, roughly evaluating the accuracy of iNat's suggested identifications, and being able to quickly look up the taxonomy, I've also found to be a great way of getting a feel for the characteristics of different families. I really like going to someplace I've never been, and recognizing plant and animal families. It’s a nice feeling to be walking somewhere and think to myself “I don't exactly know these ones, but I think I know your relatives.”

(Some quotes have been lightly edited for clarity.)

- check out Victor's Knowing Nature podcast, about enviromental education!

- here's some footage of an assassin bug in action.

- take a look at the amazing panoply of assassin bugs on iNat!

Posted on March 5, 2024 11:27 PM by tiwane