Thursday, August 15, 2013

Making sense of smells

Today's Guardian Country Diary is about the close link between scent and memory, about the way in which a whiff of a natural odour can transport us back to personal memories of  our past.

Years ago, when I was learning to identify plants at school, a teacher made us crush and smell a piece of all the plants that we found, emphasising how important the smell of  plant can be in identification. There are many that can be named with your eyes closed, like..........

..... hogweed, for example. Just crush one of the seeds between finger and thumb and those oil glands - the brown stripes on the seed coat - release a pungent, slightly fruity aroma that for me is one of the smells associated with the drift from summer into autumn.

Our sense of smell is quite limited compared with that of many other animals, particularly insects. There is a whole scent landscape that we are unaware of that includes mating pheromones and scents that insects associate with food sources. Their scent perception is concentrated in their antennae and you can see how important these are just by watching a honeybee at work.

If you look closely at this one, exploring a day lily flower, you can see that it's touching the pollen with its antennae before feeding ....

...... and this one, visiting Astrantia flowers, is constantly touching them with its antennae while it feeds.

Scent is an important means of communication in bees. They label flowers that they've visited with secretions of their Nasonov glands and also use scents from the same gland to make other workers aware of food sources.

Scent production in flowers is a complex process too. When we poke our nose into a rose to appreciate its fragrance we are using a very crude instrument compared with an insect antennae. Chemical analysis of rose fragrances reveal subtle but distinct differences between the scent of petals and the scent of the pollen; the former is a general attractant to insects whilst the latter orientates them for the true purpose of the visit, to pollinate the flower, once they have landed.

Insect sensitivity to scent is also far greater than our own, with moths being able to detect the scent of nocturnal flowers like honeysuckle from long distances down-wind.

Our ways of describing subtle differences in scent in the English language are as crude as our sense of smell. When we describe an aroma we tend to use general terms like 'pungent' or 'acrid' or 'sweet' or 'foetid' and then qualify them by adding an analogy to another scent that's commonly experienced by fellow humans - like the scent of a herb, foodstuff or excreta.  Our language is seriously limited when it comes to describing, in absolute terms, subtle differences between the scents we can detect. Which is why, I guess, that wine tasters resort to such tortured terminology when they are describing the bouquet of wines and often lapse into incoherence.

 To see what I mean, watch this..........

But for something even more amazing in the world of insect olfactory senses, take a look at this post on Africa Gomez's BugBlog which reveals how butterflies detect scents with their legs and feet. 

Imagine what life would be like if we could do that .........


  1. Phil, I just posted a photo on my blog of a honeybee sticking her tongue between my fingers because she smelled the lavender oil on them. This is the second time it's happened to me. Scientists say scent cues are secondary to visual ones, but I don't look like a lavender flower at all! I also planted Cleome spinosa and see honey bees and sweat bees attracted to the pollen which is on the ends of stamen inches away from the nectary, I'm thinking it must smell good to them, although people don't bring these flowers inside because they have an unpleasant odor--to attract bats methinks.

    Loving your blog and radio podcasts.

  2. Hi Lori, Thanks for your message and for your kind words about this blog; I'm a regular follower of Beespeaker and find your blog fascinating.

    I'm sure you are right about the powerful attractions that scents exert on honeybees. The lure of lavender on your latest post is interesting - our lavender bushes have been humming with bees over the last few weeks. I think many butterflies have a strong response to scent too - I recall visiting a butterfly farm where visitors were advised to rub orange peel on their arms, so that the exotic butterflies would land on them.

    With all good wishes, Phil
    P.S. I wonder if the insect on the Angelica in the first picture in your current post is a digger wasp?

  3. Thanks Phil,
    You helped me identify the insect as a square-headed digger wasp.

    I am doing some inquiry into the chemical constituents of the Nasanov scent and their relationships to the flowers honeybees visit. I used some orange oil cleaner to remove labels off bottles when I was outside in our yard. Some of it must have sprayed onto the arm of my lawn chair and it really flummoxed a honeybee who wouldn't leave it alone.

    I am excited about your observations on how honeybees use their antennae. When I talk to people about bee safety I always tell them to be aware of the cosmetics they use and how sweaty they become because it directly affects the bees' behavior.

    And it's interesting that they say hummingbirds don't have a sense of smell, so they are looking for dramatic visual cues and can pick out tiny bits of red. I saw one examining some red glass beads I hung in a tree.

  4. Hi Lori, interesting that you should mention insects' attraction for sweat - I once had a scotch argus butterfly drink sweat from my arm. I wondered if it might be the salt that was the attractant - some tropical butterflies congregate around shallow pools of water to drink water containing dissolved minerals, that they need for egg laying.........

  5. Beautiful and informative post as usual, and thank you for the link to BugBlog!

  6. I love this blog! and I love your 'tortured terminology' description...i learn so much from you! Thanks.

  7. Hi Africa and Jan, Thanks for the kind sentiments...