29 April 2006

How a slug breathes

One essential step during the evolution of terrestrial gastropods (snails and slugs) from aquatic ancestors was the development of the ability to extract oxygen from the air. Gills that need to be surrounded with water to function efficiently are useless in air. Thus, in all terrestrial gastropods gills have disappeared and the inner surface of the mantle cavity has evolved into a lung. However, the lung surface must be kept wet at all times, because gas exchange always takes place across a film of water (even in human lungs).

In pulmonate gastropods, only a single hole, the pneumostome, connects the mantle cavity to the outside world. The large pneumostome of the slug Arion subfuscus is visible in the picture below. Unless the slug is abnormally sinistral, the pneumostome is always located on the right side of the mantle (the mantle is the flap of skin covering the back of the head).

If you watch a slug long enough, you will notice that its pneumostome rhythmically opens and closes, in other words, you will witness the slug breathing. The rate of breathing of a slug is probably determined by the metabolic rate and the state of hydration of the slug as well as the ambient temperature and humidity. The series of pictures below show the opening and closing of the pneumostome (arrow) of a Megapallifera during one complete cycle that took 23 seconds. Notice how the front of the mantle puffs up when the pneumostome is fully open. I am assuming that it is the air filling the mantle cavity that makes the latter to swell.

Part 2.

28 April 2006

Beaver's dam is no more

Back in February I posted pictures of a long beaver dam I had discovered (here and here). This afternoon I went back to the area. First, I was having difficulty recognizing some of my landmarks until I realized why: a section of the dam had fallen apart and the pond behind the dam was empty. So, the areas that had been previously flooded, including many of the canals, were now dry. The picture below shows what the dam looked like this afternoon.

Compare it with the picture taken on 31 March, the last time I saw the dam intact. The red arrow points at the gnawed tree that was back then in the pond.

Now, the beaver has definitely got some thinking to do.

27 April 2006

The thing from the meadow by the creek

This is a giant, virus-infected, disease-spreading, lymph-sucking mosquito.

Well, no, it isn't. It is only a crane fly. But, crane flies indeed resemble huge, parasite-infected, brain-inflaming, blood-sucking mosquitos, although in reality they don't even bite and are totally harmless, at least to photographers. They seem to be rather weak fliers and are quite easy to catch by hand.

Based on the venation of its wings, I have identified this individual as a Tipula sp.

Their larvae are mostly aquatic and feed on small children. Okay, maybe they don't do that either.

26 April 2006

You are what you digest

A recurring theme on this blog has been what terrestrial mollusks eat. Certain foods (e.g. indigestible fiber) notwithstanding, what an animal eats will be useful only if the animal has the means to extract nutrients from it. This is done by breaking down the food first mechanically and then enzymatically. In many cases, symbiotic gut bacteria contribute to the process. Terrestrial mollusks are no exceptions from these generalizations.

Yesterday, while searching thru the free online archives of the Journal of Experimental Biology, I found an old paper1 on the digestive enzymes of the land snail Helix pomatia. The authors determined the following enzymatic activities in the extracts of the gastrointestinal tract of H. pomatia.

Proteolytic enzymes (proteinases). All animals, including strict herbivores, need to have enzymes to hydrolyze proteins to release amino acids. Slugs would definitely need proteolytic enzymes to digest the earthworms they have eaten.

Lipases. These enzymes hydrolyze fats into fatty acids. Probably they are also present in all animals.

Carbohydrases. These enzymes hydrolyze anything from a disaccharide (e.g. sucrose) to a polysaccharide (e.g. starch) into smaller and simpler carbohydrates. Helix pomatia had enzymes that hydrolyzed numerous carbohydrate substrates, including sucrose, maltose, xylan, mannan, starch, snail glycogen, chitin and cellulose. Xylans are polymers of xylose and are constituents of plant cell walls. Mannans are plant polysaccharides that are polymers of mannose. The ability of snails to hydrolyze plant polysaccharides (xylan, mannan, starch and cellulose) as well as chitin, an animal and fungal polysaccharide, is probably an indication of their omnivorous diet.

1. FAY L. MYERS and D. H. NORTHCOTE. A Survey of the Enzymes from the Gastro-Intestinal Tract of Helix Pomatia. J. Exp. Biol. 1958 35: 639-648. pdf

24 April 2006

Tiny slug’s last meal


Saturday morning I saw several really small slugs under the wet rocks in my backyard. This one, barely 5 mm long, was one of them. It is definitely a juvenile, because there are no known species of slugs, at least in the eastern U.S., that don’t grow larger. I suspect this slug was either a Deroceras reticulatum or an Arion intermedius.

Later, while looking at the pictures on the monitor, I noticed the patches of green visible thru the slug’s body (arrows in the picture below). Slugs don’t have green organs. So, I believe what we are seeing is what the slug had eaten, probably some algae growing on the rocks.


Compare the positions of the green patches with the positions of the stomach and the upper part of the rectum in the drawing below. I think it is safe to assume that the lower arrow in the slug picture is pointing at the stomach and the upper arrow is to the upper rectum.

Generalized representation of the digestive system of a pulmonate slug. Figure is from Barker, G.M. 1999. Naturalised terrestrial Stylommatophora. Fauna of New Zealand. No. 38.

22 April 2006

Saturday's beer review: Franziskaner weiss beer from Germany


I truly enjoyed this beer. It has a mild, almost fruity aroma and a slightly sweet taste with no bitterness. The golden color is darkened by the abundant turbidity. In fact, the label gives instructions for pouring the beer properly to get most of the precipitate into the glass. The cloudiness results from the top-fermenting yeast used to make the beer.

According to the company's website Franziskaner (Franciscan) weiss beers are prepared with a particularly high proportion of wheat malt in addition to barley malt. Presumably, the high wheat content is what gives this beer its distinct aroma.

Incidentally, their website was the most informative one among all the brewery websites I have so far looked at.


Index to the Snail's Tales' Beer Reviews

21 April 2006

Some like it really hot

A paper in last week's Science1 was about the temperature tolerance of the polychete worm Paralvinella sulfincola that lives near deep-sea hydrothermal vents. The study demonstrated that when presented with a range of temperatures from 20º to 61ºC in their aquaria, P. sulfincola moved to regions between 40º to 50ºC. The temperature they prefer to be at is only about 10ºC below 55ºC at which temperature they begin to suffer physiological stress. They die at 60ºC within minutes.

The authors also mention an ostracod and an ant that can survive exposure to temperatures close to 50ºC.

Some aestivating land snails can also survive body temperatures near 50ºC. This was demonstrated in a classic study by Schmidt-Nielsen et al.2 with the snail Sphincterochila zonata (=S. boissieri) that lives in the Negev desert in Israel. These snails aestivate on the surface fully exposed to the sun. Lab experiments demonstrated that 100% of 20 dormant snails survived at 50ºC for up to 8 hours, 30% survived at 55ºC for 2 hours, but none survived at 60ºC for 30 minutes. The diagram below from the paper shows the temperature distribution in and around a snail's shell exposed to the sun on the desert surface.

The body temperature of one dormant snail in the desert was measured to be 50.3ºC. The snails avoid reaching the lethal temperature of about 55ºC by withdrawing deep within their shells. This creates an insulating air space between the snail's body and the very hot surface below the shell. The snails are able to withdraw deep into their shells, because they have shells larger than their bodies as I discussed before here, here and here.

The Science paper mentions that the upper temperature limit for active animals appears to be about 55ºC.

1. Peter R. Girguis and Raymond W. Lee. 2006. Thermal Preference and Tolerance of Alvinellids. Science 312:231.
2. Schmidt-Nielsen et al. 1971. Desert snails: problems of heat, water and food.
J. Exp. Biol. 55:385-398.

19 April 2006

A comma in the afternoon sun


I have discovered a nearby wooded hill where these butterflies, Eastern Comma (Polygonia comma), are common in the afternoons before sunset. They frequently land on trees and let me approach them to take close-up pictures. Now is the best time of the year to photograph them in natural light, because in another week, all the trees will be covered with leaves and it will be difficult to take pictures without resorting to a flash.

comma2I have noticed that these butterflies always land on trees with their heads facing either down or sideways, but never up. This results from the way they approach a tree: they always come from above even when they are landing high above the ground.

I can think of one survival advantage in their preferred position. Since a predatory bird is most likely to come from above, a butterfly facing down or sideways will take off away from the bird, whereas a butterfly facing up will have no choice but to fly up and will be more likely to get caught.

Next time I am on that hill, I have to pay closer attention to the flight directions of these butterflies when they are taking off from trees.

18 April 2006

How to write without vowels and still be understd

Over at Abnrmal Intersts, Duane, who is probably smarter than me, because he can look at chicken scratches on ancient clay tablets and come up with stories about people with unpronounceable names, has an informative post on ancient Ugaritic written without vowels.

I have noticed that the letter-writers of the 19th century frequently shortened English words by omitting certain vowels and even consonants. For example, here is a sentence from a letter by Charles Darwin to Joseph D. Hooker1:

Secondly will you tell me, at a guess, how long an immersion in sea-water you shd. imagine wd. kill the more susceptible seeds?

Another sentence from another letter from Darwin to Hooker:
But I shd. like to hear whether you are prepared on reflexion to uphold this doctrine of the commonest being least widely disseminated on outlying islds.

Here are some examples from across the Atlantic. In 1817, the American naturalist Thomas Say wrote to his friend John F. Melsheimer2:

...the last set which has been recd from England was imported by a friend of mine...

With the highest respect
I remain your most Obdt Servt
Thomas Say

And this is from another letter from Say to Melsheimer:

Philada April 26th 1823

My dear Sir!
I duly received yr excellent letter & more recently the valuable box of insects...

Interestingly, the practice of omitting the vowels appears to be common to several Middle Eastern languages, including the Ugaritic and Egyptian Hieroglyphics as well as the modern Hebrew and Arabic. I wonder if this practice started in conjunction with the invention of what seems to me labor intensive cuneiform writing. I suppose writing on soft clay wouldn’t be too difficult, but what about carving a long text on solid rock? Imagine yourself being forced to carve into hard rock some Hammurabi guy's bright ideas while sitting in the blazing sun with dust in your mouth and rock chips in your eyes. Wouldn't you be thinking hard to figure out a way to lessen your suffering? Obviously, the more letters a scribe could have omitted without becoming totally unintelligible the more time and energy he would have saved.

1. Burkhardt, F. (editor) 1996. Charles Darwin's Letters. A Selection 1825-1859. Cambridge University Press.
2. Weiss, H.B. & Ziegler, G.M. 1931. Thomas Say: early American naturalist. Charles C. Thomas.


Last week I wrote about how my son and I had chanced upon slugs eating a dead earthworm in our backyard. The following nite we decided to reproduce our observations by actually offering earthworms to slugs. During the afternoon we went out and collected dead earthworms from the sidewalks—casualties left from the morning’s rain. A brief soak in a solution of trisodium phosphate, followed by a water rinse, restored, not their lives, but their plumpness.

Later in the backyard around 9 pm, the first slug we saw was a large Arion subfuscus. I placed one intact “reconstituted” worm a few centimeters in front of it. A minute later, the slug had already contacted the worm and was starting to eat it.

Soon the slug had literally chopped the worm into smaller pieces and was consuming each one slowly.

I don’t know how often these slugs get a chance to eat dead earthworms and other animal matter in the wild. But I have just thought of a way this question can be answered easily. In a previous post, I mentioned how I had determined the diet of the land snail Oxyloma retusa from the microscopic examinations of its feces. The feces of slugs should also reveal what they normally eat. Earthworm flesh, once it has gone thru a slug’s stomach and intestine, would probably be difficult to identify. But, I am almost certain that the tiny hairs (setae) that cover an earthworm's outer skin will survive intact slugs’ digestive processes and can be identified in their feces.

That will be the next weekend’s project.

17 April 2006

Flight of the carpenter bee – Part 2

Photographing hovering carpenter bees turned into an obsession over the weekend. A friend who read the previous post suggested using a flash to create more static images of their rapidly beating wings and also switching to manual focus.

While implementing his suggestions, I took about 80 pictures of flying bees throughout yesterday. The pictures below are two of the best. Using a flash was indeed a more effective way to freeze the motion of the bees' wings.

Also, focusing manually wasn't as difficult as I had initially assumed it would be. Once I got used to the bees' routines, I could follow them thru the viewfinder, while changing the focus quickly.

For the technically oriented: Pictures were taken with an Olympus E-500 camera with a Zuiko 35 mm macro lens. The camera was hand held at all times. To take the previous set of photos at high shutter speeds, I had to set the sensitivity at ISO 500 or higher on a sunny morning. This, however, increased the noise. Furthermore, despite the high sensitivity, the pictures were still mostly underexposed. Another advantage of using a flash was that I could lower the sensitivity to ISO 250 and thereby decreasing the noise. The resulting photos were also better exposed.

15 April 2006

Flight of the carpenter bee

(Alternate title: Man wastes morning trying to photograph flying bees)

When I went out to the backyard this morning, I saw several large bees flying around the apple tree, which is blossoming. I noticed that they had an occasional tendency to hover. I figured that would be a challenging subject to photograph. So I took my camera and spent a while to get a good picture.

It was indeed challenging. The main difficulty was getting a sharp picture. Even when they were hovering, the bees were staying at one spot for only a second or two. That was hardly long enough for me to locate the bee in the viewfinder, bring the auto focus target on the bee and press the shutter release. The other problem was that the shutter speeds I was using were not short enough to freeze their wings. Both the picture above and the one below were taken at 1/640 s. You can imagine how fast they beat their wings.

In the picture below it looks as if the bee had landed on the ground. Actually, it was hovering above the ground. That one was taken at 1/500 s.

I first thought they were bumble bees. Then one of them landed on the underside of the horizontal bar of the fence slightly above the ground and disappeared. When I looked under the bar, I saw an almost circular hole (see below). That's when I realized they were carpenter bees.

They are damaging the fence, but at the same time providing a service by pollinating my apple tree. It's fair enough.

Part 2 with better pictures is here.

14 April 2006

Feast in the backyard

Last nite my son and I did slug watching in our backyard. Even though it was dry, there were many slugs out. Among those we saw were a Limax maximus and a Deroceras reticulatum (and several ants) eating a dead earthworm. When we first arrived at the scene, a large Arion subfuscus was also approaching. After I moved some grass blades out of the way, L. maximus and D. reticulatum got disturbed and left. But A. subfuscus stayed and started eating the worm ignoring the ants and us. Actually, the ants didn't seem to be bothering the slug.

A. subfuscus, Limax maximus and a Deroceras reticulatum are all European slugs introduced into the U.S. Although they are generally known to be plant-eaters, they won't pass an opportunity to eat dead worms, insects and even larger animals.

13 April 2006

Paper read today: frogs wrapped in cocoons

"We use only the finest baby frogs, dew-picked and flown from Iraq, cleansed in the finest quality spring water, lightly killed, and then sealed in a succulent Swiss quintuple smooth treble cream milk chocolate envelope, and lovingly frosted with glucose."
Monty Python's Flying Circus Crunchy Frog

The physiological and morphological adaptations invertebrates and smaller vertebrates have evolved to survive the harsh physical conditions of terrestrial life have always fascinated me. Today’s paper review is along those lines and is about how certain Australian frogs survive dry conditions.

PC Withers & GG Thompson. Cocoon formation and metabolic depression by the aestivating hylid frogs Cyclorana australis and Cyclorana cultripes (Amphibia: Hylidae). Journal of the Royal Society of Western Australia, 83:39-40, 2000. (pdf)

Several species of Australian burrowing frogs ( Cyclorana spp.) cover their bodies with a cocoon when aestivating during dry periods. The multi-layered cocoons form from outer epidermal cells that the frogs seem to shed in layers; the longer they aestivate, the more layers they add. The cocoons made by the 2 species of frogs that were the subjects of this study, Cyclorana australis and Cyclorana cultripes, during 46 and 51 days of aestivation in the laboratory, had 34 and 32-33 layers (erroneously given as 51 in the abstract, see Table 1), respectively. The cocoons cover their entire bodies, including the eyes, mouth and cloaca, except for the openings of their noses.

During aestivation in the lab, the oxygen consumption rate of Cyclorana cultripes fell significantly by 70%. In both species, evaporative water loss decreased from about 15 to about 1.1 mg/g x hour. My only criticism of the paper is that the aestivation conditions in the lab were not clearly described, especially the ambient humidity was not reported.

According to the paper, some species of these frogs may aestivate in the wild for more than a year waiting for the rains. Do they keep adding layers to their cocoons during the entire aestivation period? The shedding of so much skin must be costly. I wonder if they eat their cocoons after they revive.

12 April 2006

Anatolian toponymy 1: Nevşehir

Note: To properly view the Turkish characters, set the encoding of your browser to Unicode, UTF-8. If you are using another browser, read the comments to this post.

Nevşehir is a city in central Turkey. Its original name was Neapoli, "new city" in Greek. However, by the mid-19th century (if not earlier) the town was known by its "Turkish" name Nevşehir, which is actually Persian for "new city". The picture below is a detail from John Arrowsmith's 1844 map of Turkey (downloaded from the David Rumsey Historical Map Collection). The name of the town (arrow) is given as Neb Shehr (the Turkish letter "ş" has the sound of "sh").


Before the 1924 population exchange between Turkey and Greece that followed the Turkish-Greek War of 1919-1922, Nevşehir boasted a large Greek population. The Greeks of Nevşehir spoke Karamanlıca, a dialect of Turkish, and wrote it using Greek characters. The postcard below, probably from the early 20th century, spells Nevşehir with Greek characters. The phrase in parentheses transliterates as "kısm-ı-sani", which is in Ottoman Turkish (actually Arabic), meaning “upper part”.


A conversion table giving the Turkish equivalents of the Greek characters of Karamanlıca is available here.

Many thanks to Yorgi Sangiouloglou for the postcard.

11 April 2006

Nothing but crap

A short little paper of mine just got published in Triton No. 13 (pdf). It is about the diet of Oxyloma retusa, one of my favorite land snails. In the spring and summer I find them abundantly crawling on the mud and on cattails along the shores of a small local lake.

About 4 years ago I decided to figure out what these snails were eating. At different times of the year, I collected a bunch of snails and kept them in clean wet containers without food for several hours. Then I collected the feces they had deposited inside the containers and examined them under a microscope. It was a very simple study, but it told me a lot about their diet.

Here is a bunch of samples of Oxyloma feces preserved in glycerol.


Once broken apart and magnified, even crap can become interesting, believe it or not. For example, this snail ate some pollen (cattail?) and some other dead plant material, whose stomata are easily recognized.


The picture on the left below shows the feces of a snail that ate zucchini in the lab. The green particles are probably chloroplasts. Similar green particles are visible in the picture of the feces of a snail that had eaten green plants in the wild (right).


The results indicated that the snails were eating both dead and live plants.

10 April 2006

Weekend remembered

Slug (Megapallifera sp.) crawling on beech trunk.mgpllfr

Rainbow down our street.rainbo

Temi relaxing in the backyard.

08 April 2006

Dermestid beetle taking off


Among the many arthropods we share our home with are dermestid, or carpet, beetles (Coleoptera: Dermestidae). These beetles and their larvae feed on fur, horns, silk and are considered pests, but we haven't noticed any damage that we could attribute to them.

Dermestids are quite common in our basement. The adults can fly, but I have never seen them actually flying around, perhaps because they only fly short distances. They tend to appear out of nowhere, as did this particular individual last nite on my computer desk. It was crawling on a stack of papers while opening and closing its wings. I noticed that an upside down dermestid beetle rights itself up by opening its wings and pushing itself up.


As you can tell from the picture above, this individual was about 2 mm long. If I am not mistaken, it is Anthrenus flavipes, the furniture carpet beetle (correct me if I am wrong).


Here is how you can use dermestid beetles to clean the skeletons in your closet!

Correction added 9 April: I was mistaken. The pictured beetle is Anthrenus verbasci. See the comments by Andreas Herrmann below.

07 April 2006

Paper read this week: Land snails of Beringia

About a week ago I reviewed a book on Beringia, the land connection that existed between Alaska and the northeastern Asia during several geological periods in the past and most recently during the last ice age. This week's paper review is about the land snails of the same area. By coincidence, a few days ago Pascal at Research at a snail's pace mentioned the same paper.

Bernard Lauriol, Yannick Cabana, Jacques Cinq-Mars, Marie-Anne Geurts, F. Wayne Grimm. 2002. Cliff-top eolian deposits and associated molluscan assemblages as indicators of Late Pleistocene and Holocene environments in Beringia. Quaternary International 87:59-79.
The study area was along the Porcupine River about 100 km north of the Arctic Circle and just east of the U.S. (Alaska)-Canada border. The purpose of the study was to examine the usefulness of cliff-top eolian deposits* as indicators of past climate and environments. Samples were taken from several sites, radiocarbon dates of the deposits were determined, pollen analyses were carried out and snail shells were extracted and identified.

The radiocarbon dates obtained for the deposits range from about 130 to 15900 years before present (BP). Twenty-one species of land snails were found in the samples from eolian deposits. In comparison, one sample indicated to be "Modern" yielded only 6 species of land snails. But the paper does not clearly state if those snails actually lived at the site or if their shells were also wind deposits.

The majority of the fossil species belong to Vertigo, a genus of tiny (~2 mm or less) snails. One such species, V. ovata, was represented by one shell out of 640 shells in one sample of 25 kg of sediment. The author who identified the snails, Wayne Grimm, considered this single record "significant northern extensions [sic]" of the range of V. ovata. I would hesitate to make such a broad generalization based on one shell from a wind deposit. Furthermore, this record doesn't extend the present range of the species, because the sample the shell came from was dated to 8200 BP or earlier.

To explain the presence of so many species of snails in wind deposits, the authors suggest that the fauna they recovered could "represent a dynamic combination of endemic species and of new species that were constantly introduced by migratory birds coming from the unglaciated southern regions of the continent, or from Asia". There is quite a bit of evidence indicating that birds indeed disperse mollusks, which I have discussed here and here.

However, when the authors further speculate that "cliff-top eolian deposits, with their rich molluscan colonies, could well have been very attractive sites for the migratory birds particularly around 10,000 BP. The birds would have been able to feed on molluscs, preferably on shells, since live snails may contains [sic] toxic compounds", they seem to be forgetting that the majority of the species they found had shell lengths of about 6.5 mm or less. Birds do eat snails, but I am not aware of any records of birds eating such small snails, although the 2 succineids grow larger and may have been eaten.

Pupilla hebes, one of the tiny fossil species recovered from cliff-top eolian deposits. According to Pilsbry (1948), most records of this species were from high altitude mountains in the U.S.

Considering that most fossil species they found are rather small snails, it is also quite likely that most shells could have been brought by winds from distant locations. Nevertheless, direct evidence for long distance wind transport of live snails or their shells is lacking, as far as I know.

One land snail species whose distribution has been puzzling me is Zoogenetes harpa. It is possible that this snail, whose range extends from northern America, thru northern Asia, all the way to northern Europe, may have used Beringia to migrate between Asia and America. Interestingly, however, no specimens of Z. harpa were found during this study.

*An eolian deposit is soil deposited by the wind.

06 April 2006

Land snails of Turkey: Papillifera papillaris


The clausiliid snail Papillifera papillaris is not native to Turkey. Its homeland is believed to be the Italian Peninsula and the nearby islands, including Corsica and Sicily. The species has been inadvertently introduced to many other locations by humans possibly on marble and marble objects. In the AMG Newsletter No. 9, I have a note on the occurrence of P. papillaris in Istanbul.

These snails naturally live on calcerous rocks. Hence their propensity to be transported on marble and limestone. The species gets its name from the more or less regularly spaced white protuberances along its sutures. The shell in the picture, from Yedikule, a Byzantine-Ottoman fort in Istanbul, was 12.9 mm long.

Update posted 21 August 2008: See this post about the discovery of this species in the UK.

05 April 2006

Archaeo+Malacology Group Newsletter No. 9

The AMG Newsletter No. 9 (and the previous 4 issues) is available here. This issue contains articles on the use of dog whelks (Nucella lapillus) to produce purple dye, neolithic ornaments made from shells of the spiny oyster Spondylus gaederopus, the clausiliid snail Papillifera papillaris (by yours truly), an obituary for Nicholas Shackleton (1937-2006) and other useful bits of information on mollusks and archaeology.

03 April 2006

Tracks in mud

I have posted pictures of white-tailed deer (Odocoileus virginianus) and racoon (Procyon lotor) tracks before (here and here). Here is a deer print in soft mud that shows the marks of the dewclaws behind the cloven hoof. The dewclaws are small toes farther up on the deer's foot. They don't normally touch the ground except in deep mud or snow.


Here is another set of prints photographed at the same location. Next to the deer print is a pair of raccoon prints.


02 April 2006

How to tell railroad tracks from roads in aerial images

In previous posts (here and here), STUFIT* presented aerial photos of trains. Today, we show you how to distinguish railroads from ordinary roads in aerial photos.

This picture shows a divided road with 2 lanes on each side.


And this is what 2 sets of parallel tracks look like.


Any questions?

For one thing, there are usually cars on roads. One normally doesn't see cars on railroads, although I have occasionally seen special trucks with train wheels on railroads. Also, roads intersect with other roads and make sharp turns. Near the lower lefthand corner of the picture above, you can see one side street intersecting at right angles with the main road. Two railroads never intersect with each other like that.

The main distinguishing feature of railroads visible in aerial photos with high enough resolution is the parallel tracks running very close to each other. This is something one wouldn't see on roads. In the picture below, compare the appearance of the tracks on the right with that of the road on the left.


Photos are from Terra Server.

*Snail’s Tales’ Useless Frivolous Information Team

01 April 2006

Arboreal wisdom