People seemed to like the idea of a marsupial land snail, so today I thought I'd go one step further, and introduce you to land snails that give birth to live young. 

I was lucky enough to spend a little time in Vanuatu a while ago, and, although I was really there to relax and see in a new year, I couldn't travel that far and not spend a little of my time looking for snails. As it turns out the island on which we stayed  is heavily modified, and there is not much natural habitat left for native land snail species. In fact, the only really interesting snails I found were living on the side of our host's house. I collected a few of those snails, transported them to the fridge in our lab and forgot about them for the best part of year.

More recently it dawned on me that these snails would be useful for a project I am working on, so I grabbed them from the fridge, set them up under the microscope ready to dissect away a tissue sample for genetic work and saw this:

 

Embryos developing inside the shell of their mother. 

We sometimes think of live-bearing as being a trait that sets the mammalian branch of the tree of life apart from other animals, but that's wrong. Most of the major groups of animals have some species that give birth to live young - there are live-bearing frogs, snakes, lizards, insects, fish, crustaceans and star fish. In fact, the only large group without live-bearing species that I can think of is birds (and, it seems, dinosaurs, a group that contains birds). Most land snails lay a clutch of many eggs, each containing a single-celled zygote which is left to develop on its own. A few species, like theses ones, have evolved a different reproductive strategy: producing fewer eggs than their relatives, but retaining those eggs within their shell before giving birth to much more developed young.

This behaviour seems to be particular common in snails that live in rocky outcrops, and those that live in the tropics, especially the Pacific. I'm not sure about what species the snail depicted above fall into - but they are from the sub-family Microcystinae, which is one of the dominant groups of land snails in the Pacific and is made up entirely of live-bearing species. The large evolutionary radiations that used to live in Hawai'i and the Society Islands were also all live-bearers.

So why give birth to live young? It is easy to see why live-bearing is an advantage to snails living in rocky habitats with few places to deposit eggs. It's less clear why the Pacific is full of live-bearers. It has been suggested that tropical weather can lead to unpredictable patterns of boom and bust - with snails that can hold on to and grow their offspring in the bad times and release them "ready to go" when conditions are better having an advantage over egg-layers. As far as I know no one has ever come up with a way of testing that idea, so the reasons for the prevalence of live-bearers in the Pacific remains an open question.

You might be wondering how you can given the name "fly" to a flightless creature. Taxonomic groups reflect the evolutionary history of life. So for instance the great ape family contains orangutans, gorillas, humans and chimps because all these species descend from a shared common ancestor. When entomologists talk about "flies" they are referring to the taxonomic order Diptera - a group of insects that share a common ancestor that lived approximately 250 million years ago. Of course, organisms don't come with little tags telling us where they came from, it's the job of taxonomists to find characters that can be used to reconstruct that evolutionary history. The character that most sets flies apart from other branches of the insect tree of life is the presence of "halteres" - the stubby yellow structures  this predatory robber fly is modelling (more on that fly here).

Diptera means "two winged" and the name refers to the fact that most flies get about with one pair of wings, having reduced their second set into the flight-satablising halteres. This little innovation has allowed dipterans to become precision fliers, and one of the most successfully groups of organisms on earth with about 150 000 species (maybe 10% of all species are flies).

The true flies in order Diptera all descend from an ancestor which flew with two wings, but many of the species that descend from that ancestor have given up their wings. Because taxonomic groups are reflections of evolutionary history even wingless species are still flies - you can call them the apterous members of Diptera (wingless two-winged insects).

Flightless hippoboscids

Bat flies, Bat flies, Bat flies

Bats seem to attract flies like... well they seem attract flies. At least three different lineages of flies have evolved a close relationship with bats. The largest group are, unsurpsingly, called "bat flies" (the taxonomy of this group is uncertain, but it includes the family Nycteribiidae). I have a soft-spot for most bugs, but even I have to admit bat flies are pretty gruesome looking creatures:

The bat flies have lost their eyes as well as their wings, by have made up for those loses in other body parts. The massive spider-like legs end in tiny claws that let the flies grip on the bat's fur and move about. Once stuck on a bat these flies drink blood.

The fauna of New Zealand is very keen of flightlessness. Charasmatic mega-fauna like moa, kiwi and kakapo are the obvious examples, but we are also one of only two places on earth to have flightless perching birds (our three wrens are now extinct, as is the Canary Island Bunting). There are no truly flightless bats, but our short tailed bat is probably the closest thing - spending most of its time crawling around the forest floor. Our terrestrial bat hosts its own flightless flies. Apparenlty Mystacinobia zelandica descends from a blowfly but its association with the short talied bat has changed its body so greatly that is was not originally considered to be a fly at all, and thought to represent a unique order of insects. I couldn't find any photos of Mystacinobia avaliable with a permissive license, but check out the New Zealand Geographic story and Te Ara's article, which even has a video.

Mystacinobia has been placed in a family that is endemic to New Zealand, but there is one fly family with an even more restricted range. As far as we know, the family Mormotomyiidae is represented by a single species (Mormotomyia hirsuta) which is only known from a particular site in on one mountain in Kenya. As you may have guessed the site is a bat roost, and the animal, despite being only distantly related to the other bat flies discussed above, has taken on the spidery form that is associated with flies that spend their lives with bats.


Mormotomyia doesn't have the hooks that other species use to cling on to bats, so it may be a little more free-living than than the species discussed above.

Phorids

The family Phoridae might be the strangest group of flies . Hell, one genus is made up entirely of flies that burrow into the heads of ants, decapitate them, and use the severed head as a cocoon.

I'm going to end this brief survey of here, but I want to emphasise that these are just a few of the flies that go against their name. I could have talked about "snow fleas", or "bee lice",  in fact the flylogeny project records about 40 families with at least some flightless members. When the Daily Mail ran a story on the rediscovery of the Kenyan bat fly Mormotomyia hirsuta one of that newspaper giant page-view creating algorithm's readers felt they needed to add this opinion to the ensuing discussion:

That looks nothing like a fly. How come this evolved into a 'walk' ? It makes no sense. If I could fly I wouldn't evolve into something flightless. Evolution is flawed

 

The exponential function can tell us how big a shell gets, but of course, shells don't simply grow, they also spiral at the same time. If we want to model both the growth and the spiral pattern of a snail's shell we need to leave our familiar "x,y" system of placing points (called the Cartesian coordinate system) and think in "polar coordinates".

Just as any point in a two-dimensional space can be identified by its distance from another point along horizontal and vertical axes (x and y), it can also be identified by its angle and distance from another point. Think about a point at x=3 and y=2, you can just as easily, and just as uniquely, identify that point with polar-coordinates:

Using polar coordinates it's very easy to write an equation that describes the growth of a snail shell:

r = e ^k.θ 

Here "θ" (theta) is an angle relative to the starting point, "r" is the amount of growth the spiral has made by the time is swings around to that angle and k determines the "tightness" of the spiral the shell forms. I was playing around with Wolfram Alpha in preparation for this post, drawing spirals with different values of k, when I came across this spiral at k = -0.2:

I know that shape, that's a Wainuia shell!



 With a little bit of tweaking you can make a paua (k = -0.6) or something close to a tightly-turning charopid (k = -0.1).




Just changing one parameter in a pretty simple equation is enough to produce spirals that fit most snails' shells. In fact, spirals like these ones, which are called logarithmic spirals, pop up in nature all the time - from the arms of galaxies to the nerves in your eyes. Logarithmic spirals have some pretty cool properties, the most interesting of which is that not matter how large they grow they ever change shape. A snail that grows according to these equation will be the same shape from the day it's born to the day that it dies.

If you know a bit more maths you can extend these models into a third dimension and, with one more parameter, create flat disc-like shells or tall conical ones. I think it's truly amazing that you can get a good approximation of snail shells using so few parameters - but it's worth remembering mathmatical constructs are just models we use to examine reality. David M. Raup got a bit carried away with the mathematical description of shells in the 1960s, and created what he called the "museum of all shells" by exploring the three dimensional shapes you could make by tweaking just three parameters in a model of shell-growth. But Raup's virtual musuem doesn't include all the shells that snails can grow. Biology is weird, and any "law" that a biologist might claim to have discovered will have an exception. None of the shells above quite fit the spiral I've super-imposed on them, and some snails grow shells that radically deviate from logarithmic growth . My favourite example of such a radical departure are the "worm snails", marine snails that cement the apex of their shell to a rock then grow an almost un-coiled tube of a shell.

Worm snails grow in way that is radically different from most of their close relatives, but more subtle deviations from logarthmic spiralling are just as interesting. Remember these guys?

 Libera fartercula are one of a great deal of snails that change shape as they age. Very young shells have a very broad opening (an umbilicus) on the underside:

As the shell get's bigger, the opening to the umbilicus gets smaller...

...and smaller.

Of course, the original "wide" umbilicus is still part of the older shells. In effect, this pattern of growth creates a cavity within the shell which has lots of space at the top, but a very narrow opening. Amazingly, L. fratercula is a marsupial snail. Over the course of its growth this species creates a pouch within its shell, which it then lays its eggs in, protecting them from would-be predators who can't get inside the narrow opening.

Land snails usually don't do much for their young. A few snails lay extra large clutches, so that the first of their offspring to emerge will have eggs to eat before they set off on their lives. Others hang on to their eggs, either within their shells or withing their body. Libera fratercula takes parental investment to a much greater level. Here's an older shell:

Most of the larger shells from this species show this sort of damage. When you zoom in on the damage you can see a slighltly irregular pattern.

These holes are creatued by immature snails emmerging within the brood pouch and eating their way out of their parent's shell. Such damage doesn't seem to kill the snails - they effectively wall-off the first few whorls of the shell once they are large enough, so there is no animal within the part of the shell that is broken.

I can tell you  a lot more about these snails. Allen Solem described the "brood pouch" and a little of their ecology in 1968, but he worked from old shells and no one has studied their behaviour in situ to be able to measure the impact of this strange adjustment to snail-life has on parents.

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The shell photographs onto which I've super-imposed the sprials are all Creative Commons Licensed courtesy of Te Papa 1,2,3.