Sunday, 28 September 2008

Harpagoxenus sublaevis

My parents collected some ants from Austria in June. I was pleased to find that the first tube I looked in contained this little beast.

Harpagoxenus sublaevis is interesting in that, despite its relatively small size, it is a 'slave-maker'. In this instance it was apparently using Leptothorax acervorum as slaves, though it also enslaves L. muscorum and L. gredleri.

Describing ants as slave-makers is generally a misuse of the term, as it's only possible to enslave members of the same species. Instead, myrmecologists have given this form of social parasitism its own name: dulosis.

H. sublaevis conducts raids on Leptothorax colonies, carrying larvae and pupae away to rear as slaves. This can be quite drawn out process, as H. sublaevis recruits through tandem running, so it can take quite a while until there are enough H. sublaevis workers to conduct the raid. During the raid, H. sublaevis produces a propaganda pheromone, which causes the Leptothorax workers to attack one another, rather than the intruders. Nevertheless, H. sublaevis is heavily built, with antennal scrobes into which antennae can be drawn and straight edged mandibles that can be used for shearing through the appendages of attacking Leptothorax.

The evolutionary origins of dulosis in ants still has a lot of questions that need answering. There are three main hypotheses for the origin of slave-making:
  • Predation: This was the only possible origin for slave-making suggested by Darwin in the Origin of Species. In essence, he suggested that slave-making emerged when one colony raids another colony and steals brood to be used as food. If any of the brood were allowed to survive then these would be adopted as slaves within the host colony. There is currently no evidence for this as the prime mover leading to slavery.
  • Territoriality: This hypothesis suggests that territorial aggression between neighbouring colonies, and the stealing of rival brood for food, led to some captured brood to be allowed to survive to adulthood and be accepted as nestmates. There is some evidence in support of this hypothesis.
  • Transport: This was originally suggested by Buschinger. He suggested that slave-making may originate from transporting brood between nests in polydomous (multi-nest) colonies. If this was extended to other colonies then it could lead to an early version of slave-raiding. Again, there is little evidence in support of this theory.
None of these hypotheses are fully convincing. For example, none of them explain the aggressive usurpation of host workers and queens. However, they are not mutually exclusive, so it may be a combination of these and other factors that led to the evolution of slave-making.

Tuesday, 23 September 2008

31 Today

This seems appropriate today. I wonder why.

Tuesday, 16 September 2008

That's one important ant!

I don't normally just link to ant related news without having something to say myself. However, I think that Martialis heureka warrants making an exception.

Myrmecos does a good summary of the story, with some nice photos. Other good coverage is at ScienceDaily, Nature News and Discover. The paper is available from PNAS, at a cost, though the abstract is available for free.

Sunday, 7 September 2008

Separating Lasius niger and Lasius platythorax workers

By popular request! Well, one person requested it anyway.

Lasius niger and Lasius platythorax are undoubtedly hard to separate. This becomes particularly difficult when you find specimens that seem to be somewhere in between. However, it is always possible to assign them to one species or another, with a bit of work.

First of all, L. niger and L. platythorax are small dark brown Lasius with outstanding hairs on their antennal scapes. In the UK there are no other species that have these characteristics, though in Europe it gets a little more complicated1.

Some people seem to base identification of the workers of these species on one or two characteristics. However, experience has taught me that it is usually more reliable to use all of the following:
  • Density of pubescence on the clypeus
  • Shape of propodeum in profile
  • Setae length on the mesosoma
  • Shape of clypeus in profile
These are all shown in the photomontages. I have, on occasion, also felt the need to use the length of the setae on the underside of the head, but I can usually identify these species reliably without this and the setae are difficult to measure.

Probably the most useful character is the clypeal pubescence. Quite often this can be visually assessed, with practice, though on occasions an entire nest series will have moderate clypeal pubescence and have to be assessed morphometrically. To do this you use a measuring line, as shown below. Measure the length of this line in µm (l) and then count the number of hairs that intersect it (n). Seifert (2007) gives the average distance between the hairs (l/n) for L. niger as <16µm and for L. platythorax as >19µm.

Clypeal pubescence in Lasius niger. Measuring line shown in red.

Clypeal pubescence in Lasius platythorax. Measuring line shown in red.

Whilst you're looking at the clypeus it's worth noting its shape in profile. Typically, L. niger has a rather curved clypeus, whilst in L. platythorax it is comparatively straight. The idea that they differ has been floating around in BWARS for a couple years now, though I must admit I'm one of the last to accept that this is probably a good character. I feel like I've now seen enough and have not been able to dispute it - plus Barry Bolton is apparently using it, so it must be good!

Check the shape of the propodeum in profile. In L. niger the propodeum tends to be a smoothly rounded dome, whilst in L. platythorax it tends to be more conical in shape.

Finally, look at the relative length of the setae on the mesosoma, especially the pronotum. In L. platythorax these setae tend to be distinctly longer. Seifert (2007) gives figures for the length of the longest setae on the pronotum divided by the length of the head as 0.119±0.009 in L. niger and 0.159±0.010 in L. platythorax.

The two species have distinct ecological preferences. L. niger prefers drier habitats and has synanthropic tendencies - this is the species that sometimes invades houses. L. platythorax prefers wetter conditions, and is usually the species found in woodland and wet grasslands.

I've also started noticing that the larvae in L. platythorax seem to be more slender than in L. niger. Of course, this is based wholly on my subjective observations, but might be a good indicative field character.

NB: The photomontages are of British specimens2 that show generally typical characteristics. However, both specimens show setae on the propodeum that curve forward, which is not typical. In the Scottish L. platythorax, which were collected in atypical habitat at their most northern known location in the UK, the entire series had curved forward propodeal setae.

1 American L. niger are most likely a different species again, as the images I have seen do not have the dense clypeal pubescence of L. niger, but do not appear to be L. platythorax either.
2 To be honest, I don't think that them being British matters - if anything I find the characteristics of these species to overlap more in the UK than in mainland Europe. However, I know some British myrmecologists who get funny about such things!

New title bar

It only took two hours of fiddling to get it right.

For those who care, the species used for the mugshots are, from left to right:

Saturday, 6 September 2008

Focus stacking ant images

I wish I had the kind of technology that the guys at AntWeb have for taking photographs. They use Automontage to focus stack the images, ensuring that everything is in focus. The principle of focus stacking is that you take photographs with different parts of the subject in focus and then use the software to combine the in-focus bits of all the images to create a new image.

The big issue for me is the cost, as the technology involved is pricey. The photographs that I take at the moment are done in a very low-tech way with what is now a rather dated camera, but it seems to work without costing the earth.

I recently read an article on tardigrades in British Wildlife, in which the author discussed briefly the technology he uses for photographing these tiny beasts. It turns out that you can focus stack images on the cheap, with two software packages freely available: Helicon Focus (for Macs) and CombineZP (for Windows).

I've had a bash at producing some stacked ant images in CombineZP. It's fair to say that the process involved is incredibly fiddly, time consuming and computer memory hungry, with the images produced rather variable. However, it does work fairly well, once you've learnt to keep the camera completely still whilst taking the images.

I know some people dislike focus stacking, because it makes images appear flat, so I thought I'd put CombineZP to the test.

The following photographs are of British Lasius niger (it's worth clicking on the images to view larger versions). Image 1 is the best of the original images used to create the composites below:

CombineZP produced two images (from nine original images), using different stacking techniques, that were of reasonable quality. Image 2 is perhaps the closest to the original and was produced using Pyramid Maximum Contrast methodology:

Image 3 uses Pyramid Weighted Average methodology:

I have my own thoughts on which I prefer, but I thought I'd put it to the vote:

Which of the above images is best? free polls