How Do Flies Eat?

Fly Proboscis observed with a Motic B1 Upright Microscope

Flies have a special mouth part called a proboscis which, much like a straw, is used to drink liquids. The proboscis of the fly is a fascinating microscope object and in fact has been used on microscopy studies for a long time. The main interest is the tracheal ring structures which look spiral under low power but they are not.
These rings do not surround the whole tube but are terminated in a set of arches that pass from one to the other. These are called pseudotrachae and are the suctional organs. Between these are many rows of minute hairs which allow delicate food surfaces to be scrapped to create crumbs. The fly uses the proboscis to suck up liquid food and to pass digestive juices onto solid food. After a few seconds the juices break down the solid food and the fly sucks it back up. These details make the object useful as a test of microscope low and medium power objectives. 

Microscope – Motic B1
Objective – 4X Achromatic
Light – transmitted
Camera – EVIL with direct projection of objective onto sensor
Specimen – Prepared by C M Topping circa 1850