Posted by: Rob Viens | June 2, 2012

“Love Darts” and the Courtship of Slugs

June 2nd found Darwin working near home – collecting samples and recording the weather:

“Collected in the neighbourhead of the house: I trust there is a change in the weather: the Hygrometer showed the air to be twice as dry in the middle of the day as in the morning. — There was a good example of what Humboldt says of “the thin vapour, which without changing the transparency of the air, renders its tints more harmonious, softens the effects” &c &c. In one of these days when there is such a profusion of light, the consequent dark shadows are well opposed to the general brightness of the view.” (June 2)

You do have to love Darwin’s collection of gadgets – he really stocked up before leaving to assure that he could make every measurement possible. A hygrometer, for example, is used for measuring the relative humidity of the air.  Even today, they require a lot of calibration due to the fact that changes in air temperature play a part in how much moisture the air can hold. It is likely that Darwin’s was not all that accurate.

Bear with me for a moment while I explain the train of through that led to the remainder of today’s post.  I wondered what Darwin might be collecting today so I went to his Zoological Notes (which are not all that well dated but allow you to approximate what day Darwin was talking about). In early June I found a description of Vaginulus – a genus of land slug belonging to the Stylommatophora (which  are the land / air-breathing snails and slugs of the Class Gastropoda). In reading about Stylommatophora I found that some of these snails and slugs (including Vaginulus) use “love darts” during mating.  I think, “how can I not write about love darts?”  So here it is…

Darwin provides a detailed description of his little slug, noting that it is a “rather pale honey yellow”, and that it “feeds on leaves of a tree”. So, although this is not the same species, it may have looked something like this Vaginulus photo from The Train of the Snail:

Vaginulus sp.

Land slugs are related to sea slugs and have several similar characteristics. (See some of Darwin’s encounters with sea slugs in Darwin and the Sea Bunnies and Purple Snails and Blue Slugs.) They are all basically snails without  shells, though the land slugs have the ability to breath air rather than drawing oxygen from the water like their saltwater cousins.

Some of the Stylommatophora, including Vaginulus, have an interesting adaptation called a “love dart”. Seriously – a “love dart”.  This is not a sex organ in the sense that it is not involved in the exchange of sperm, but it does play a role in the “pirate-like” courtship of these slugs.

These darts are typically made of calcium or chitin and come in a lot of different shapes and sizes (depending on the species that creates it).  A few examples (SEM images from Wikipedia Commons) are shown below (the bottom images of each pair shows the darts in cross-section):

Love darts

During their courtship phase (which can last hours), our two slug “lovers” fire one or more of these darts at their potential mate (skipping the small talk altogether and going right to the stabbing).  There are no concerns about being caught in the crossfire though – the darts are not actually flung through the air, they are released on contact after the slugs have completed any preliminary “dancing around one another. The dart can be quite large, and in the case of some snails, it has been known to completely penetrate the body and protrude out the other side of the snail recipient!

So what is all this stabbing and harpooning all about?  Apparently it increases the likelihood of successful mating, hence making it more likely to be passed on to offspring (as Darwin would later tell is). According to some sources, the dart is coated in a mucus that contains a hormone-like substance that increases likelihood that the sperm will survive and make it to the egg. Slugs are hermaphrodites, so it may be a way of letting the system know that the sperm is coming from another snail. I’m not sure, but it sure makes for a good story.

I certainly have a little more respect for slugs since starting this project – though not enough to remove the beer traps from my garden.  I have to draw the line somewhere. (RJV)

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Responses

  1. Love to read your posts,it’s great to have the opportunity to learn more about Darwin, Beagle journey and nature-thank you!But-are snails in your garden stupid enough for beer traps-evolution in my garden went further-nothing helps against snails in my hostas ;(

    • Good point – my snails never fall for it either, only the slugs. It seemed like the only way I could get rid of the snails was to go out at night (following the munching sounds) and hand-remove them. Though my “Snail Relocation Program” never really seemed to work.

      There numbers went way down though a couple years ago, when (sadly) a very large rosemary bush (at about 12-ft diameter it was more of a tree really) located at the edge of our small garden died. I think it was giving them a place to hide during the day.

      • Ok ,I do use the same “relocation program” but hate to do it as then I always feel guilty, at the end I decided to plant what is not interesting to our garden visitors (snails, deer, water vole);)

  2. For your amusement: Leopard Slugs in flagrante!


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