It was 5 years ago today

Time flies when you are procrastinating. I mentioned in this post back in December of last year that I was working on the material collected during a land snail survey we had done in Turkey early in the summer of 2004. I am still not finished, because after December I took a long break and worked on other stuff. I am now back at it and I intend to finish sorting all the specimens by the end of the year.

Imagine my pleasant surprise today when a bag of shells I picked randomly had the date of 4 July 2004. It seems like it was only, well, 5 years ago.

D51 was a pretty rich station with about 22 species. In the picture you can see a couple of long and narrow Bulgarica shells near the center and a couple of white Albinaria puella to the right. The tall white shells near the top are Zebrina cosensis, while the large, flat ones are Oxychilus samius. The tubes are holding the smaller shells.

One interesting non-snail specimen that was in the bag was this insect larva.


It appears to be a drilid larva (becaue it is hairy). The larvae of the beetles in the family Drilidae are predators of land snails. Surprisingly, I don't seem to have written about drilids before. But there is a summary of a paper I once wrote about them on this page. It was one of the first 2 papers I wrote on snails. One of these days I am going to scan it and turn it into pdf.

A belated celebration of the Evolution Day

This year is the 151st anniversary of the historic session of the the Linnean Society in London on 1 July 1858 when Charles Darwin’s and Alfred Russel Wallace’s independently developed ideas on evolution by natural selection were made public for the first time.

Darwin had been developing his ideas for 20 years, but before that day he had revealed them only to a few close friends and correspondents, including the American botanist Asa Gray. Wallace, on the other hand, had come up with his version of natural selection, very much similar to that of Darwin's, several months earlier while doing fieldwork in the Malay Archipelago and communicated it to Darwin in a now famous letter*.

The presentation at the Linnean Society was initiated with a letter of introduction by Darwin’s close friends Charles Lyell and Joseph D. Hooker, opening with the words:

My Dear Sir, -- The accompanying papers, which we have the honour of communicating to the Linnean Society, and which all relate to the same subject, viz. the Laws which affect the Production of Varieties, Races, and Species, contain the results of the investigations of two indefatigable naturalists, Mr. Charles Darwin and Mr. Alfred Wallace.

This was followed by the reading of extracts from an unpublished essay Darwin had written in 1844, part of his 1857 letter explaining his ideas to Gray and the manuscript Wallace had sent to Darwin.

Why not celebrate this great idea today and everyday? Read a book on evolution, teach someone about evolution, visit a natural history museum or take a hike in the woods or go to a sea shore to witness the products of evolution. And don’t forget to remember Darwin and Wallace, for, after all these years, their idea remains indefatigable.


Hooray to the bearded guys! Pictures of Darwin (left) and Wallace are from the Linnean Society.

*According to the Darwin Correspondence Project, Wallace's letter and unpublished manuscript are missing.

Helix aspersa from San Diego

A friend at work walked into my office today with a plastic water bottle containing a live snail. It was a gift* for me picked up by her husband yesterday in San Diego, California.

I am tentatively identifying it as Helix aspersa (Cantareus aspersus), an introduction from Europe. It was found with several others in a flower bed at a hotel, an unlikely place to find native local species. It is a juvenile with a soft, still not-reflected lip. First, I though it was a Helix aperta, a species I am not familiar with, but now I am leaning towards the former.

The snail still hasn't fully come out of its shell. It appears moribund, actually. Could it be suffering from jet lag? I hope it will recover and grow to become an adult so that I can be certain of its identity.


*It was the same couple who brought back Littorina littorea for me from Bar Harbor, Maine.

Purple poop

This purple stain surrounding what appears to be a deposit of bird ordure was on my deck yesterday.

What had this bird eaten?


My botanical knowledge, especially when it comes to identifying plants from their seeds, is pitifully poor. Any ideas?

Depth of field, f-numbers and diffraction (or how to take a rotten photograph)

In Handbook for Scientific Photography (1977), Alfred A. Blaker wrote:

As the substage diaphragm is closed, the depth of field increases (as when you close the diaphragm of your camera lens)...However, diffraction of light by the edge of the diaphragm increasingly impairs the image resolution until the image becomes "rotten."

He was explaining how to take pictures thru a microscope using a camera back and the substage diaphragm he was referring to is the one under the stage of a microscope. One encounters a similar diffraction effect when using a camera with a lens to take closeup* pictures. Blaker explained this in another book, Field Photography (1976):

...at significant magnifications the choice of f-number is...nearly always a matter for compromise between depth of field needs and the resolution of fine detail in the image. At very small apertures, diffraction of light at the diaphragm edge reduces resolution.

The bottom line is that the smallest aperture will give you the greatest depth of field, but not the best resolution†.

Here are some tests I did with my Olympus 35 mm Zuiko lens on Olympus E-500. In this case, the object was a flat wood surface and so the depth of field was not an issue. Notice the decrease in sharpness as I increased the f-stop (decreased the aperture diameter).


Click on the image to view a bigger version. These are unretouched images. Some image quality was lost because I had to compress the composite picture before I could post it here.
The decrease in sharpness going from f5.6 to f11 is almost not noticeable. Even f14 would be acceptable for some purposes. But ordinarily, I avoid the apertures above f14.

Here is another example; shots of a flower bud of trumpet vine at f5.6 and f22, again with the 35 mm lens. There is a definite loss of resolution at f22.


But now let's compare the overall images.

At f5.6 with diffused sunnlight:


And at f22 with flash light:


The one at f22 has a much wider depth of field and the overall loss of resolution due to diffraction is not noticeable at this magnification. Therefore, we follow Blaker's advice and compromise and chose between depth of field and resolution. If we want a wide depth of field despite the loss of overall sharpness, we decrease the aperture; if we want a sharp focal point amidst blurry surroundings, we increase the aperture. How you take a picture depends on what you want the picture to look like and what you will do with it. In many cases, there is no right or wrong photograph as long as the image is not too rotten.


*Blaker's definitions of closeup photography as photography at image magnifications of actual size or less and photomacrography as photography at image magnifications greater than actual size are arbitrary and pointless.
†Don't confuse this with the pixel resolution of a digital camera.

Snail Poster Museum Official Hiking Laboratory

Or, official hiking shoe museum garden path snail poster. Or, cat fortune house laboratory hiking shoe.

Today’s temperate silliness has been inspired by a webpage by Charles H. Bennett. As Bennett notes, English speakers can create arbitarily long and yet meaningful chains of nouns even when starting from just a handful of appropriate words. Bennett gives an example of 2 noun loops of 5-4 words joined in a figure 8. Here is my example of 3 joined noun loops of 5-2-5 words.

You can start at any word and go in either direction to obtain a meaningful sentence of any number of words. Admittedly, some sentences may be a bit more meaningful than others, but none is grammatically wrong. Note that the word official, both a noun and an adjective, adds more variety to the endless possibilities.

Cat hiking shoe museum poster snail.

Official hiking laboratory house fortune cat hiking shoe museum garden path snail poster museum official hiking laboratory house fortune cat hiking official museum poster snail path garden...

Grazing slugs and the evidence they leave behind

The northwest wall of the house receives sunlight filtered by trees and bushes and thus remains wet longer than the more southerly sides; it is a perfect habitat for cyanobacteria (blue-green algae). If it weren't for the resident slugs, a thick jungle of microscopic proportions would long have developed on the siding.

Slugs love the green stuff and on warm, humid days come out to graze on it. Here is an Arion subfuscus with its feeding track behind it.

There are a variety of tracks on the wall. Some are more elaborate than others. I suspect different species are involved.


This one reveals the marks of the individual teeth of the radula that does the scraping.


I am curious to know if I can tell the species apart from their feeding tracks. Updates will be posted.

Weight lifting snails

Anyone who has tried to pick up a snail crawling on a smooth surface like a sidewalk or glass may have noticed that snails can have quite a tenacious grip. One way to measure how strongly a snail can hold onto a surface is to measure how much weight the snail can lift.

To collect some relevant data, I put together a crude apparatus consisting of a small plastic bottle taped to 2 glass microscope slides. I can increase the weight of the apparatus by adding objects, usually coins, into the bottle. The snail is placed on the upper slide. After it attaches its foot on the glass, I hold its shell and lift it up, at the same time starting a stopwatch. I selected 5 seconds as an arbitrary minimum time necessary for a lift to count as successful.

This Cepaea nemoralis carried, in addition to the apparatus itself, 3 quarters and 1 dime, or a total of 41.3 grams for 39 seconds.

That weight will be more meaningful once I express it in relation to the surface area of the snail's foot. I haven't had a chance to do that.

Keep in mind that a snail does not not actually hold onto a surface using muscle power; its grip results mainly from the functioning of the sole of its foot like a sucker. Parker* wrote in 1911:

As means of attachment snails secrete a bed of mucus, and use the foot as a sucker. Both methods are commonly employed by the same species, but in a given form one method is usually developed much in excess of the other. For instance, in Helix pomatia, Limax maximus, and other allied species, the moist surface of the expanded foot will stick with some tenacity to glass. But if such an animal be allowed to creep its length over a glass surface and thus spread a bed of mucus on which it can rest, it will be found to have multiplied the strength of its attachment many times. The mucus adheres to the glass and the surface of the foot to the mucus very much more powerfully than the foot alone can adhere to the glass.

I have noticed it is somewhat difficult to obtain reproducible results. There may be some habituation involved. If the snails are picked up too frequently, they appear to start letting go off the surface more easily.


*G. H. Parker. 1911. The mechanism of locomotion in gastropods. Journal of Morphology, 22:155-170. pdf