… The fly can employ its labella …
This is from Insects and Flowers: The Biology of a Partnership by Friedrich G. Barth (1991):
… In the middle is the tongue (glossa). This is covered with long hairs and has a spoon-shaped end. With the tongue the bee reaches the nectar and licks it up, very like a cat licking milk.
[line break added] This is what happens: the nectar rises in the narrow spaces between the hairs of the extended tongue by capillary action, a passive process. Then the bee pulls its tongue back into the actual suction tube formed by the other four elements of the proboscis, which are grouped around the tongue.
… This suction tube is joined by an airtight connection to a pump in the head, which allows the fluid that has risen passively between the tongue hairs and between the tongue and the tube wall to be actively pumped higher.
… The mosquitoes and flies present us with [another] variant of proboscis construction…. There are no less than three tubes, one within the next. The innermost is the actual food-sucking tube. Outside it is the saliva tube through which saliva flows out to dissolve food such as the coffee drinker’s sugar cube or to prevent coagulation of the host’s blood where the mosquito has bitten.
[line break added] These two tubes are enclosed by a third. The comparative anatomy of the mouthparts has revealed that the feeding tube is a modification of the labrum (an “upper lip” above the mandibles), whereas the saliva tube is an elongation of the pharynx, and the third tube is formed by the labium.
… The fly labellum is covered with sense organs which provide information about the food — its mechanical consistency and in particular its chemical composition. About two hundred and fifty long hairs are especially conspicuous. All of them are innervated; they tell the fly, for example, how concentrated is the nectar into which it has just dipped its proboscis.
On the inner surfaces of the labella, around the mouth opening, are little teeth. These are used to scrape pollen out of a flower and by grating movements they can break down large food particles. And that is not all. The fly can employ its labella in quite different ways. Depending on how they are spread apart, they can filter fluid food, soak it up with the tube system described above, or assist direct sucking through the mouth opening.
My most recent previous post from Barth’s book is here.
-Julie
Coloring 