What to Expect When You’re Expecting: Egg Care – July 2013

By Erin Lane

What to Expect

In the May issue of the Reptile Times we discussed maternal care of gravid bearded dragons, from conception to laying.  With any luck, your breeding endeavors have so far been fruitful, and you are preparing for a clutch of eggs.  While incubation requires the least amount of action on your part, it can in some ways be the most nail-biting aspect of breeding reptiles.  In this issue, we’ll try and relieve some of those worries by going over some tips and techniques that will help those of you first time dragon breeders.  Luckily, incubating beardie eggs is about as easy as using an Easy Bake Oven.  If you follow a few simple instructions, you should be on your way to whipping up a batch of dragons.

Egg Deposition

Cupping the clutch

A simple, yet important, aspect of incubation is what to put the eggs in.  A lot of it depends on the size of the clutch and the size of the incubator.  If you only have one dragon’s eggs to worry about, a smaller incubator and smaller containers are probably the easiest.  Deli cups—those with pre-punched holes—are readily available, and easy to use.  The 8 oz cups that come in the incubator specials (described later) are perfect for dragon eggs.  I have found that you can easily fit 5-6 into each deli cup.

Larger deli cups or other ventilated containers can also be used if you prefer to put all of your eggs in one container.  This is sometimes helpful if you have more than one female.  You can then put each clutch in its own container without having to worry about keeping track of multiple smaller cups.  Regardless of which type of receptacle you choose, remember that proper labeling is important.  It’s easy to forget the lay date even when you only have one dragon’s eggs to keep track of.  I always label the container (either with a makeshift sticky tag, or directly on the lid) as soon as I get done cupping the eggs. 

Happy medium

Last time we talked about preferred medium for egg laying.  If your female is getting ready to drop eggs, having the lay box set up ahead of time is a good idea.  Anything from vermiculite to organic potting soil can be used as a substrate for egg deposition.  However, what you put the eggs in after they are laid is a little more important.  Luckily, there are a few good and easy to use options.  If you are looking for a ‘no brainer’, Hatchrite is a great option.  This incubation bedding looks a lot like perlite, but has the advantage of being ready to go right out of the bag.  Unlike other egg incubation media, Hatchrite does not require you to add water, taking out a sometimes tricky step for a new breeder.  Simply add a couple of inches of Hatchrite to your egg container, place your eggs, and leave alone until they hatch.  I have had good results with this product, and would recommend it to anyone who is a little daunted by figuring out just how much water to add to a traditional egg substrate. 

 

Perlite will clump, but not drip, when water to media ratio is correct.  Make finger sized impressions in the medium.

Although Hatchrite is easy to use and reasonably priced, many breeders opt for more traditional media, such as vermiculite, perlite, or a mix of the two.  I have used both, and have found that they both work well.  Let’s start with vermiculite, as it is the established go to.  I have found that it works well when the water to vermiculite ratio is done right.  The usual advice is to combine 1 part water to 2 parts vermiculite.  However, I don’t think that this is always helpful, as a lot depends on the moisture content of your particular bag of vermiculite, as well as the size of the granules.  A good way to do it is to start by adding a small amount of water, mixing it in, and testing the result by seeing how well it sticks together.  Keep adding water in small quantities and mixing until you don’t have any dry sections that won’t clump.  You can then take a handful of the moist mix and squeeze it.  If more than a few drop come out, then it is probably too wet.  If you can’t squeeze any water out, you can probably add just a little bit more. 

The problem that I have run into with vermiculite is that the size of the granules can vary from batch to batch.  I have found that the more coarse, or larger, grains work better.  The finer grains tend to either get too wet or too dry.  A good egg substrate will hold onto moisture for a long period of time without being ‘wet.’  Too much water can ‘drown’ eggs and encourage mold growth, whereas too little water can lead to your eggs desiccating.  The larger grain vermiculite seems to absorb water better, and can then provide moisture for a longer period of time without being too wet. 

This year I went with perlite, and that has seemed to work well so far.  Yes, you still have to add water, but it seems to keep the humidity at a consistent level throughout incubation.  Mix it the same way you would vermiculite, keeping in mind that though it may not feel very wet or release much excess moisture when squeezed, it probably holds onto to more than the vermiculite does.  You can buy perlite at any plant nursery or garden center. 

 

Eggs can quickly go bad if kept too moist.

Regardless of the type of medium you use, remember that checking the substrate about once a week or every two weeks is probably a good idea, especially with your first clutch of the year.  Eggs can and do dry out, so too little moisture can be just as big of a problem as too much.  To avoid the too-wet-or-too-dry issue, I mix my substrate a little on the dry side, and then add small amounts of water to the substrate a few times throughout incubation.  I determine whether or not to add water by sticking my finger down into the corner of the substrate. 

If it feels bone dry, I gently add water with a pressure sprayer to the corners (if in a larger container) or around the edge and in between the eggs.  Avoid spraying the eggs directly, as you really don’t want them to be wet, but don’t stress if a little water does get on them.  You can always gently wipe it off with the corner of a rag or a paper towel.  Remember, it is easier to add water than it is to take it out.

Eggs that have gone bad can and will attract bugs quickly.

Placement parameters

The number of eggs you fit into your container is obviously dependent on the size of the cup or box.  As stated previously, you can decide what will work best for you.  However, how to actually go about placing the eggs in the medium can be a little confusing; different sources will tell you different things.  In my experience, whether you cover your eggs completely, or you rest them on top, they will probably all come out okay.  The easiest way, I have found, is to make an indentation of about ¾” with your index finger or thumb into the egg substrate, about ¾” apart from each other.  Place the eggs on their sides into each indentation, and leave them alone until they hatch.  Don’t worry about covering them up; just keep them about ½ way buried.  As the media dries and the eggs enlarge, they will often seem to unbury themselves.  You can go with this, or make new indentations when you add a little more water to the container. 

Many people will tell you that you need to be careful about how you place your eggs.  Many sources will say that you must place them in the same orientation that the mom laid them, and to not turn them over or you will kill the embryo—or it will drown, or die, or break it’s eventual yolk stalk.  From my experience with this species, this is relatively unimportant when moving newly laid eggs.  Eggs can be moved about and placed with little care as to up or down early on.  Because I now candle all of my eggs after being laid, I usually try and place each egg with the ‘pink spot’ up.  This pink or red spot is seen as a faint pink ring around a red dot, usually found on one of the long sides of the egg.  If you can’t see the pink circle through the shell, it can easily be seen when candled (as described below).  My advice would be to not worry so much about which side is up when they are first laid, but to be more gentle with them as they start to develop. 

After one week of incubation you can see veins beginning to develop.

Easy bake

There are a variety of incubators available, from the simple Hovabator to the advanced Exo Terra Reptile Incubator.  The incubator you choose should be dependent on a few things.  The first is size.  How many eggs are you expecting?  Do you have one lizard, or 5?  The smaller Hovabator incubator is fine for holding a few clutches at the same time, but if you are planning on more than that, it would be worth it to get a bigger incubator.  One advantage to the Hovabator is that you can order it as part of the incubator special, which gives you 5 deli cups and a bag of Hatchrite for a great price, meaning you don’t have to look around for what you need—it all comes together in one package. 

After three weeks, you can see the embryo and the network of veins in the egg.

The second consideration is the ambient temperature in your house.  Where you put your incubator becomes important here. Most incubators will only heat, not cool, meaning that your eggs can overheat more easily if kept in a room that gets hot.  If you put your incubator in the garage where the temperature can soar in the summer months, then you should probably go for the Exo Terra Reptile Incubator, which can both heat and cool to maintain the desired temp.

The third thing to think about when purchasing an incubator is ease of operation.  How much monitoring do you want to do to maintain the proper temperature?  If you plan on placing your incubator in a room where the temperature doesn’t fluctuate much, and it stays in the 70’s most of the time, then you won’t have much to worry about regardless of which one you choose.  However, if you don’t have that luxury, a higher end incubator is probably going to make the process a little easier.  While the Hovabators are effective and easy to use, you have to monitor the temperature and adjust the thermostat accordingly.  The Zoo Med ReptiBator Digital Incubator and the Exo Terra Reptile Incubator are both programmable, meaning that you set the temperature, and they will adjust to keep it stable, even when the room temperature drops or rises.  The Zoo Med ReptiBator is a good middle ground for ease of use.  While it doesn’t have a cooling mechanism to keep things from getting too warm, it otherwise gives more temperature control and also comes equipped with a humidity gauge. 

Egg in the final days before hatching

Tools of the trade

Even when using a higher end incubator, it never hurts to have a second thermometer on board.  I use the Zoo Med Digital Thermometer (with probe) in my incubator to watch the temperature.  This is especially useful if you have an Hovabator, so that you don’t have to open the lid to check the temp.  You just insert the probe into one of the ventilation holes, and keep the unit resting on top of the incubator.

Another consideration is to use an external thermostat in your incubator.  If you already have an Hovabator or equivalent, which lacks the more precise temperature control of the higher end models, you can always set up an external thermostat to control the temp.  This is really more of a safe guard as the Hovabators usually work well as is, assuming that you keep them in a room with a moderately stable temperature.  This isn’t necessarily more beneficial from a cost perspective, but simply another way to go about regulating the temperature for your eggs.

Egg just beginning to dimple before hatching

Temps and times

Dragon eggs are fairly easy to incubate in that they can withstand a fair amount of range when it comes to temperature.  I always shoot for 84 F throughout incubation, but slightly lower or higher temps have resulted in perfectly healthy hatchlings.  There is some anecdotal evidence that eggs incubated at room temperature and those incubated hot (let’s say around 90 F and above) result in lower hatch rates and, sometimes, weak babies.  I have always incubated mine in the low to mid 80s with success, so have not strayed from that recipe yet. 

Clutch of eggs hatchinh

Just like when you are baking a cake, the temperature of your oven will determine how long you need to keep it in.  If your incubator is set to 85 F, your eggs will likely hatch faster than those set at 81 F.  A few degrees can make a difference of a week or more in some cases.  If you are incubating in the mid 80s, you can probably expect your eggs to hatch between 60 and 70 days, plus or minus a week.  Last year, at a relatively steady 84 F, my clutches went an average of 72 days before hatching.  A good idea is to start checking for hatchlings every day starting at around day 50 or 55.  While babies can stay in the incubator (and is often a good practice) for a day or two, you will want to be on top of it, and keep track of when they hatch.

Proof in the pudding

All of this information is helpful only if you have a healthy clutch of fertilized eggs.  It is possible for unmated females to lay unfertilized eggs, just as it is possible for seemingly healthy mated females to lay bad ones.  You never know until they are laid, and even then, you might have to wait and see.  I have incubated fresh, seemingly good eggs only to have them go bad sometime during the incubation process.  It can be heartbreaking, but these things do unfortunately happen.  Even when you have done everything right, you can still wind up with eggs that don’t make it.  Remember, in the wild, hatch rates are likely significantly lower than in captivity—not all eggs (or hatchlings) are destined to make it.  That’s why females lay multiple eggs, and usually multiple clutches. 

There are a few things that you can do to determine if your eggs are good, and even worth incubating.  Let me start by saying that I am incredibly optimistic when it comes to bad eggs.  Even when I suspect an egg will not make it, I will give it a chance until it is extremely evident that it’s no good.  This is especially true for newly laid eggs.  You will often times have one or two that aren’t plump and seem to be lacking filling.  Bearded dragon eggs, like those of manyreptiles, have a soft shell, which is designed to swell as the egg absorbs moisture from its surroundings, and as the embryo grows.  A ‘squishy’ egg will sometimes plump up after a day or two in moist incubation bedding, so I would always give it a chance—you might be surprised.  

Egg that has just pipped

Shell texture can also tell you a lot about whether or not an egg is good.  A good egg will typically not only feel plump between your fingers, but will have a relatively smooth dry feel to it (a reasonable time after being laid, that is).  If the egg feels slimy or slick more than an hour after being laid, chances are it’s not good.  The exact reason for this is unclear to me, but it probably has to do with the calcification process.  Females often expel underdeveloped eggs when conditions aren’t right (e.g. they are not fertilized, the female has an underlying infection, or she is young).  It is sometimes unclear what causes this to happen, but when it does, it never hurts to give the eggs a chance. 

Egg color can also be a giveaway that something isn’t quite right.  Eggs that appear very yellow usually aren’t good.  This can also be a sign that they have dried out.  Mold can also  be a sign that the egg is bad, though not in all cases.  Although I am loathe to throw out an otherwise good looking egg just because of a little mildew spot, an egg that is covered in it probably has something wrong with it.  I would recommend not throwing the egg out unless it starts to collapse.  While eggs will start to dimple just prior to hatching, or collapse when too dry, a bad egg will often collapse when others around it look fine.  Mold is usually the first sign that there is a problem with the egg, but it may also be that you are keeping the substrate too wet.  If this happens, try drying things out a bit, and see if it clears up.  If it doesn’t, but it otherwise looks okay, leave it alone—it might come out just fine.  That being said, the shell of a healthy egg should be mostly white.  You can sometimes see a pink spot or circle where the egg is beginning to vascularize, and the embryo is developing.  Healthy eggs will also usually have a soft pink glow when a light is placed next to them. 

The same egg a few minutes later – you can see the slit where the baby will emerge! 

If you have given a bad egg a shot, or a good egg has gone bad, it is best to remove it from the incubator sooner rather than later.  They can go from a little ugly to really bad in a hurry, which will attract insects or provide an opportunity for mold to grow.  Although a bad egg will usually not impact the healthy eggs around it, it is better not to let it go.  If you have an egg that you suspect isn’t going to make it, check on it every couple of days, or move it to its own container.

Hold a candle up

I have found that candling the eggs is a fun and fascinating way to pass the time until they hatch.  While I don’t recommend doing this every day, candling an egg or two from the clutch once a week doesn’t seem to cause any harm.  Again, you will read that doing so can kill the embryo, and to not candle any egg that is within a few weeks of hatching.  I disagree with this, though I will say to be gentle and proceed with caution in the later stages of incubation. 

The baby beardie emerging.

When candling, any small flashlight should do.  LED lights might be a better option since they put out bright light without much heat.  Gently hold the egg by its ends, and hold over the flashlight.  Early in development you will see the egg begin to vascularize, and the tiny embryo begin to grow.  As the embryo develops it will be harder to see what is going on in there since its body will obscure much of the light.  A few weeks out from hatching, you can often see the shadow of a tail along the side if the egg, and notice small movements.  When handling eggs this far along, I would here say that placing it back in the same position may be more important.  Will it kill the embryo to place it upside down? Probably not, but sometimes it’s best to be a little cautious. 

Time’s up

When your eggs finally get ready to hatch, you may notice a few things start to change in their appearance and turgidity.  Eggs will often start to dimple when hatching is imminent, so don’t be too concerned if this happens.  However, they do not always dimple.  I have found that when kept a little more humid, dragon eggs will often not dimple at all.  You may also notice that the eggs start to ‘sweat.’  If this is happening before you are expecting your first hatchlings, then you may need to dry things out a little by keeping the lid off of the egg container for a day.  However, if you notice this on late-term eggs, you can probably expect them to pip within a day.

Eggs that are about to hatch will also get a little softer feeling, almost as if they are full of water.  Again, be gentle with eggs that are about to hatch.  While beardies are pretty sturdy, it is probably better to be careful.  Once the egg has pipped, the egg will look deflated.  If you wait a few minutes, you might even see a little snout poke out of the end!  Once they start to hatch, leave them alone, and let them do the work.  You aren’t doing the hatchling any favors by pulling it out of the egg the rest of the way.  It will come out when it’s ready.  Neonates can stay partially in the egg for up to 24 hours as they finish absorbing the last bit of yolk.  Leave them alone, and only take them out of the incubator when they are moving around on their own. 

The next phase

Breeding reptiles can be fun, though there are often unforeseeable challenges.  Bearded dragons are an extremely rewarding pet, and have the added advantage of being easy to breed in captivity.  Whether you planned on eggs, or had a surprise, incubation can be a simple process with the right tools and a little patience.  In many ways, it is like baking a cake.  When you start with healthy, fertilized eggs, and follow the basic recipe, you will usually end up with a rewarding end product.  In other words, when set up properly, there is every reason to look forward to a good hatch rate and lots of mouths to feed in the near future.  Next month we will go over tricks and tips to taking care of those little mouths, from feeding and watering to lighting and housing. 

Reptile Vision: Nocturnal Geckos – November 2013

By Jennifer Greene

Geckos make up an extremely large group of species, the majority of which are nocturnal or at least crepuscular – that is, active at dawn and dusk.  It is worth noting that geckos evolved from diurnal lizards, and initially had the full set of rods and cones that we discussed in last month’s issue of the Times.  However, as time went on and these diurnal lizards were active only during bright daylight hours, their rod cells began to disappear, and eventually the ancestors of geckos lost their rod cells completely.  When these lizards evolved into geckos, they began moving back into nocturnal niches in the environment, and needed to develop better nighttime vision once again.  “In response to the demands of nocturnal vision without rods, the cones of nocturnal geckos have become much larger and more light-sensitive than those of their diurnal relatives” (Roth 2009).

Refresh your memory on the color spectrum and what wavelengths match which colors.

What does that mean?  That means that geckos see at night, but they see in color.  When we see at night, we are seeing in shades of grey, as rod cells simply pick up whether or not light is present, regardless of the color of that light.  Geckos can see color at light levels that equate to dim moonlight – where we would hardly be able to see at all, much less determine color!  There are studies, for example, that show that helmeted geckos can differentiate between the color blue and the color grey at extremely low light levels.  Scientists were able to test this by dusting crickets in powder dyed either blue or grey.  Crickets dyed blue were “tasty”, or had nothing extra added, while crickets dusted in grey powder were “distasteful”, and extra salt was added to the dust.  Very, very quickly, the geckos learned the difference, and chose the blue crickets over grey crickets nearly every time.  (Roth and Kelber, 2004)

In the diagram above, you can see the test used in the study.The crickets were offered to the geckos on forceps, and the ones coated in grey were always salted.  The geckos almost always refused these crickets in favor of the tastier, non-salted blue crickets.

They made the choice of blue crickets independent of the intensity of the grey coloration on the crickets.  These tests were performed at extremely low light levels, comparable to that of a night with no moonlight, and demonstrate that the geckos were capable of color vision.

An interesting point made in the study was that the scientists varied the shades of blue and grey to match in a black and white view (so if the geckos were not using color vision, the crickets would look identical), as well as grey colors that were brighter and darker to cover UV reflection.  Why would they be concerned about UV reflection, you ask?  Another study looking at crepuscular and nocturnal illumination in regards to a particular moth found that there is enough UV reflection at night for nocturnal animals to have UV sensitive vision.  (Johnson, Kelber, et al 2005)   Geckos in particular have eyes sensitive to blue and green, which makes sense when you consider that in most habitats, the wavelengths of light being reflected most fall into that color range.  Most geckos have minimal red light sensing cones, which is what leads to the use of red light bulbs for heating nocturnal reptiles – they can, at best, see minimally when red light is used to illuminate their cage.

Instead of red, the cone cells in gecko eyes see into the UV range – UVA at least, if not into the UVB range.  When testing spectral irradiance, or the radiation of various wavelengths of light off of surfaces, UV was found to be a substantial portion of light being reflected at night.   This is due to the lower amount of visible light making it through our atmosphere, allowing for more UV radiation and non-visible light to make it through, relatively speaking.  While UV is still being reflected, it is in much lower quantities (relative to overall light being reflected) during the day.  I know, I know, it sounds confusing!  During the day, because there is so much light coming through our atmosphere, it filters out most wavelengths, and what ends up making it through is mostly the visible spectrum, with smaller quantities of other wavelengths.  At night, the light being reflected from the moon, as well as starlight, is less intense in visible light.  This allows for a wider range of other wavelengths which may reflect better to make it through our atmosphere, so while there is a smaller amount of light being reflected, a larger portion of that is not visible light, but instead ranging into the infrared and ultraviolet (UV) range.

Comparing Honey Bees (Apis mellifera), Hawkmoths (M. sctellatarum, D. elpenor, H. lineata, H. gallii) and nocturnal geckos (Tarentola chazaliae).

So, back to our friends, the nocturnal geckos.  Aside from the study on helmeted geckos and their ability to differentiate between grey and blue colored crickets, there really isn’t much in the way of studies on their sight.  They are capable of multifocality, or the ability to have multiple focal zones, while interestingly, the day gecko (top row) had only one focal zone.  The varying colors in the diagram below show how much light was passing through different parts of the pupil.  The study noted large variation between individual animals in sight, which raises an interesting question for keepers – do different geckos have varying ability to see?  Do some geckos need glasses?  That’s rhetorical, of course, but it’s an interesting thought that not all geckos see as well as others. See the study in works cited for more details on this particular study on gecko vision. (Roth, Lundstrom, et al 2009)

The limited other studies on nocturnal vision and non-mammalian animals active at night show that with a full moon, the available light spectrum is nearly identical to that of daytime colors.  Naturally, it is not as bright or as intense as midday light levels, but the range of light is similar.  However, on moonless nights, the color range shifts towards the red or infrared end of the spectrum, meaning that things visible during moonless nights would seem to be redder in tone than they would during daylight or a full moon.  But geckos can’t see red, right?  So are they blind on moonless nights?  Not quite – there are other sources of light, such as star light, as well as other reflective surfaces bouncing light off of each other, leaving enough light for the blue and green seeing geckos to still be active.

The diagram above illustrates the relative levels of different wavelengths of light at different times of day – showing that while there are lower amounts of light, the wavelengths available are still similar to that of daytime illumination.  Note the impact that light pollution has on the colors of available light – interesting to consider what our captive geckos may be experiencing with the ranges of light available to them indoors, entirely surrounded by artificial light sources.

In addition, geckos seek out light to thermoregulate, which seems counter-intuitive to what many keepers have observed with their own animals.  Yes, we can keep geckos without visible light – but one study performed on Tokay Geckos demonstrated that using visible light in addition to heat enabled them to more precisely control their body temperatures both during the day as well as night.  (Sievert and Hutchinson, 1988)  The conclusion the researchers came to was that “it appears that G. gecko is using the position of the light source as well as time of day in establishing diel  (24 hour period of time) cycles of temperature selection.”  So while geckos may not actively bask out in the open under bright, white lights, they do utilize the light source as a reference point for seeking out basking areas to reach their preferred body temperature.

Nothing here is intended to drastically change established husbandry practices of reptiles we have been keeping in captivity and breeding successfully for many years.  I do, however, hope that it encourages some thought for naturalistic enclosures, or helps those with difficult species try new things to help their geckos become established.  I feel it also highlights how little we still know about these incredible animals and their natural habitat, especially when compared to other species commonly kept in captivity. When setting up naturalistic displays, I hope you find the information here helpful in setting up basking areas, full spectrum lighting, or even whether you feel those things are needed.  There is still a lot to learn, and next month, we will be examining diurnal basking lizards.

Watch the video here! 

Works Cited/ References

Lina S.V. Roth, Linda Lunstrom, Almut Kelber, Ronald H.H. Kroger, Peter Unsbo (March 30th, 2009).  The pupils and optical systems of gecko eyes.  Journal of VisionVol. 9 no. 3, article 27 .
Retrieved from: http://www-mtl.journalofvision.org/content/9/3/27.full

Almut Kelber and Lina S.V. Roth (March 1st, 2006).  Nocturnal colour vision – not as rare as we might think, The Journal of Experimental Biology, Vol. 209 
Retrieved from: http://jeb.biologists.org/content/209/5/781.full

Beate Roll (July 2001), Gecko vision – retinal organization, foveae, and implications for binocular vision, Vision Research, Volume 41 Issue 16
Retrieved from: http://www.sciencedirect.com/science/article/pii/S0042698901000931

Lynnette M. Sievert, Victor H. Hutchinson (Sept. 1988.  Light versus Heat: Thermoregulatory Behavior in a Nocturnal Gecko Lizard (Gekko gecko), Herpetologica, Vol 44 No. 3
Retrieved from: http://www.jstor.org/stable/3892340

Lina S.V. Roth, Almut Kelber (December 2004). Nocturnal color vision in geckos, Proceedings of the Royal Society of Biological Sciences, Volume 271
Retrieved from: http://rspb.royalsocietypublishing.org/content/271/Suppl_6/S485.full.pdf+html

Carrie C. Veilleux, Molly E. Cummings (July 30th, 2012).  Nocturnal light environments and species ecology: implications for nocturnal color vision in forests, The Journal of Experimental Biology, Volume 215 
Retrieved from: http://jeb.biologists.org/content/215/23/4085.full

Sonke Johnsen, Almut Kelber, Eric Warrant, Alison M. Sweeney, Edith A. Widder, Raymond L. Lee Jr., Javier Hernandez-Andres (December 20th, 2005).  Crepuscular and nocturnal illumination and its effects on color perception by the nocturnal hawkmoth Deilephila elpenorThe Journal of Experimental Biology, Volume 209
Retrieved from: http://jeb.biologists.org/content/209/5/789.full