---

I think it is safe to say that number of dipteran species is in the 1oo.ooos. More than 1oo.ooo species have been described if I am not mistaken (though I could not give you a reference)

Among Arthropods the beetles are the largest group and the second place goes to....

well it is one of the following: Diptera, Lepidoptera, Hymenoptera.

I think currently the second place are held by Lepidoptera species, followed by Diptera and than Hymenoptera as far as the described number of species go. But since the various groups are studied and described with various intensity, my gut feeling is that in the long run Hymenoptera will become nr.2, Diptera nr.3 and Lepidoptera will fall back to the 4th position.