A way to find out what’s good before you put it in your mouth
Have you ever thought how practical it would be if we could taste with our fingers? We would not have to bite into each apple or chocolate candy to locate the most delicious one – we’d simply run our fingers along and let them do the tasting. Sounds a bit like science fiction? Well, numerous insects and crustaceans actually taste this way. Taste and smell are so-called chemoreceptors, but smell operates over distances, taste requires physical contact (disregarding the remote taste receptors of snakes known as Jacobson’s or vomero-nasal organ, which will be the topic of a separate blog some day).
I used to keep rock lobsters in an aquarium at the Waterman Bay Marine Lab in Perth and when I dropped a mussel, which the rock lobsters love to crack open and eat, into the tank, it did not take long and the antennules of the rock lobster were quivering: the animal had smelled that food was present. It would then crawl around and, searching for the food, stop the moment that one of its feet would brush against the mussel. This was the signal that feeding could commence.
Rock lobsters, crayfish and many species of insects possess taste receptors on their feet and if you observed a fly carefully, you’d see something interesting. When it happens to run across anything of its liking, be that a grain of sugar, a drop of blood perhaps, out comes its proboscis to lap up the food. Usually tidily tucked away under the head, the fly’s proboscis is extended whenever food needs to be ingested and that food is initially tasted with the feet.
Each tarsal receptor (that’s the proper name of the group of 5 cells on the fly’s feet involved in taste reception) contains one water-tasting cell, two salt- and one sweet receptor plus one mechanoreceptive cell, i.e., a touch receptor. How do we know all this? Well, the proboscis extension is a simple reflex and, thus, comparable to the mouth-watering sensation we humans get at the smell or sight of our favourite food. We cannot control that response; it’s automatic. And so a fly cannot help sticking out its tongue, so-to-speak, the moment it steps onto something tasty. Consequently, tickling the feet of a tethered fly with a variety of stimuli, ranging from cotton buds soaked in plain water or different concentrations of a sugar solution to bits of cheese or fruit, and then observing the fly’s reaction (i.e., whether the proboscis comes out or not), provides us with an answer as to what the fly can taste and likes. Flies, perhaps unsurprisingly, are more sensitive than humans and taste much smaller concentrations of sugar. Although the fly’s overall taste spectrum is similar to that of humans, the fly can distinguish artificial from natural sugar and finds saccharin detestable. It will, however, accept a dilute solution of saccharin as long as at least one of the fly’s feet stands in a drop of proper sugar solution. On the other hand, if only a single leg happens to be in touch with something salty, a fly will not be prepared to stick out its proboscis even with 5 legs out of its 6 in contact with something sweet. Therefore, while we are said to have “a sweet tooth”, a fly has definitely a “sweet foot”.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2017.
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