In this morning's post I received a copy of the "Colour Guide to Hoverfly Larvae" by Graham E. Rotheray . It contains quite a few anatomical diagrams and says this about the "strange protrusions (referring to a diagram)":
"At first sight it might be difficult to see anything that could possibly work as a recognition feature i.e. a feature that all uniquely possess, the presence of which identifies them as belonging to the family Syrphidae. Fortunately, however, there is just such a character - the posterior breathing tubes. In hoverflies these two tubes are fused at the point they emerge from the body into a single, elongated, brown or black structure. ............Note, however, that the breathing tubes of first and second stages of all homopteran predators (.....) are separate, so be cautious in identifying all such larvae less than 6mm long.". Apparently all hoverfly larvae pass through three instars (bw).
The book also refers to the posterior breathing tubes as the "posterior respiratory process (prp)". As Ross said, the prp carries the posterior spiracles and can be an important factor in identification.
Bruce
