Spiders and Bees and Flies: Oh My! - Observation of the Week, 9/23/19

Our Observation of the Week is quite a scene! A Thomisus crab spider eating an African Honey Bee while jackal flies take advantage of the situation. Seen in South Africa by @richardgill.

“I am a self-taught amateur plant-lover that began with a love of trees when I was 19,” says Richard Gill, “I purchased the Palgraves book of southern African trees and started trying to key out any tree I could find - with one hand on the Glossary page to start learning what all the botanical terms meant.” 

His interest in trees sparked Richard’s curiosity about other taxa, and he explains “this journey has had me delving into particular favourite areas such as orchids, bulbs, grasslands, wildflowers and more. The more I learn the more I am fascinated by how interconnected things are, and how little we know about the precious ecosystems we are destroying.”

I’ve personally exulted in my own luck when finding a pollinator perched perfectly on a flower, only to quickly (or sometimes not so quickly) realize the insect was being held in the chelicerae of a crab spider (family Thomisidae), and Richard tells me the spider above was found in just the same way.

The crab spider observation occurred when a nurseryman friend was showing me a beautiful natural hybrid he was impressed by. The dark colour of the bee and jackal flies caught my eye, and only on closer inspection did I see the crab spider. I watched in fascination as the flies seemed to be heckling the spider despite their small size, and wondered if they were trying to prey on the bee. Now I know they are called kleptoparasites - my new favourite word of the week - thanks iNat :)

While the spider’s coloration matched the flower’s quite well to the Richard’s eye, scientists have discovered there’s a lot more going on when you look at the UV spectrum - which is the way many pollinators see things. Flowers, such as Potentilla anserina, have markings visible under UV light (check out these photos), and it is believed they attract and guide pollinators to the nectar and pollen. Under UV light, crab spiders often mimic these UV guide markings, making them quite attractive to bees and other insects, while their visible light coloration protects them from predators. 

And as Richard noted, the jackal flies swarming the crab and its prey are kleptoparasites, meaning they steal or feed off of another animal’s prey item - in this case, the bee. It is believed jackal flies (also called freeloader flies) are especially attracted to the smells given off by dying hemiptera and hymenoptera, and it is usually the females who are kleptoparasitic. And some in the genus Desmometopa, 

[appear to act] as cleaners of the spiders, with the spiders spreading their wet and sticky chelicerae thus allowing the flies to feed actively all over the bases, fangs and mouth. McMillan also observed the flies to feed at the anal opening when the spiders defaecated. This behaviour appears to be more a case of commensalism, which is beneficial to both parties, than of kleptoparasitism.

Richard (above) had been using groups on Facebook as a place to upload his photos, but found that 

the information isn't all searchable and gets lost over time. So iNat has been awesome; it is great to be building a decent searchable database of my various observations, with quality IDs, and a network of like-minded people, while knowing my observation may be useful in future. I am loving it!

- by Tony Iwane

- The adult Epicadus heterogaster crab spider sits on leaves and uses UV reflection and its flower-like body to attract prey - no real flowers needed!

- Watch this video of jackal flies dining on a crab spider's prey.

Posted on September 23, 2019 05:38 PM by tiwane