Nairobi Fly – Paederus melanurus

This week I thought I would be a bit informative and intellectual, to post info on something you may need to know about if you come to East Africa. No this isn’t malarial carrying anopheles mosquitoes, venomous snakes, or bilharzia carrying snails to prevent swimming in Lake Victoria. Today I wanted to write about the Nairobi Fly, Paederus melanurus. I was going to wait till the effects happened to me personally but thankfully I am yet to suffer and wanted to write about them!

Nairobi Fly
Nairobi Fly (from

The Nairobi fly is a small insect that is just coming into season again (following the rains) and is only around 1cm in length but can be one of the most painful experiences around. It’s black and red colouration suggests a warning to potential attackers that could cause some damage to them. We will often find them landing on the table while eating outside but they often make their way inside houses.

The piece of advice I would give is “sweep don’t squash a Nairobi fly”. However, as we all know the natural reaction when you feel something tickling your skin is to slap and try to get rid of whatever it is! Unfortunately, this is where the Nairobi fly is dangerous, as when squashed (or injured in some way) it releases a painful skin irritant, a substance called Pederin (a structurally unusual secondary amide with two tetrahydropyran rings that works by inhibiting mitosis in eukaryotes by blocking the synthesis of proteins at the ribosomes of eukaryotic cells) that causes burning and blistering. Below is a picture of a friend of ours who happened to squash one on her arm in her sleep – looks painful eh!? I’ve also heard people swatting them and squashing them on their necks and having long-lasting scars, ouch!

Nairobi Fly Blister
Nairobi Fly Blister

However, the question the scientist in me asks and one I discussed with an entomologist friend who used to live here is why do these tiny beetles possess quite such a potent amount of toxin which can do such damage to humans yet is of no selective advantage as the toxin is only released when the beetle is injured.

Well without going too deep, it seems is to do with defence as larvae. Studies have shown that pederin is accumulated in the eggs laid by females containing the toxin which are able to biosynthesise it. Paederus larvae are sufficiently agile to escape from insect predators by running away and studies have shown that the presence of Pederin in the larvae does not have any effect or deter these insects from eating them given the chance. However, unlike these insects, the wolf spider which is able to suddenly attack the Paederus larvae and capture them before they can get away is affected by the presence of Pederin. In these cases, when encountering a Nairobi fly larvae containing Pederin, the spider will leave it unharmed and go away and clean itself! This was tested in  laboratory experiments where wolf spiders were presented with larvae from females that had passed on pederin to their eggs and those that had not. Results showed that the spiders did not prey on the ones containing pederin but did prey on some of those without! What a great survival method against one of your main predators and something that makes me think of the variety that went into creation, with such a creative method to protect larvae! If you want to read more, the article below is free to download and is a good informative read where most of my info comes from! And remember, sweep don’t squash a Nairobi fly!

Differential efficacy of toxic pederin in deterring potential arthropod predators of Paederus (Coleoptera: Staphylinidae) offspring

Pederin – Wikipedia Article


About bousiesinmwanza

Moving to a new life in Mwanza, Tanzania.

Posted on 18 November 2012, in Uncategorized and tagged , , , , , , , , . Bookmark the permalink. 6 Comments.

  1. But I thought you were in Tanzania (Nairobi is in Kenya you Know)!!! Otherwise it all made sense, of course! Excuse our ignorance but is the photograph of the larvae, if not then where is the fly’s wings?

    Peter (your non-bioligical father-in-law)

  2. When I read that it was going to be science based musings I nearly didn’t read it but I actually found it really interesting but don’t ever want to come across one of those little critters! x

  3. I had to laugh at your description of Pederin and its effects – not the usual thing you’d find in a blog! But brilliant anyway. It’s interesting that the Pederin is only transferred to the eggs by some females, so not all larvae are protected. Should natural selection already have removed the unprotected line from the species or is there some other benfit from no protection. (Apologies to everybody else who doesn’t find this interesting!!!)

    • Hey Steve, not sure on the answer to be honest – I read another paper talking about the ability of females to biosynthesise pederin being key to passing it on to the eggs but not certain why all can’t – will find out!

    • Hey again Steve, I’m reliably informed that in the wild all females will transfer pederin, it was just for the lab experiments that these endosymbiants and henceforth capabilities were removed (by antibiotics).

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