This question came back to the fore for me recently when I had to attempt to identify a number of spider-hawks for work. With no recent key available for Australian pompilids, I had to try and piece together clues. As it turns out, a large part of the difficulty in identifying spider-hawks is that they are, overall, a conservative bunch. Though coming in a range of sizes and colours, they tend to be structurally uniform. This makes it difficult to find reliably key-able characters, and means that evolutionarily quite distinct species can look superficially quite similar. Take, for example, one of the most 'familiar' of the Australian pompilids, the black-and-orange Cryptocheilus bicolor. Recently, Wahis (2008) established that this was not a true species of Cryptocheilus, but belonged to a distinct (albeit related) genus as Heterodontonyx bicolor. The two genera can be distinguished by the shape of the marginal cell in the forewing, which is distally pointed in Heterodontonyx but rounded in Cryptocheilus. The thing is, many of the photos one may find online labelled as 'Cryptocheilus bicolor' are true Cryptocheilus, not Heterodontonyx. Those on Wikipedia may be correctly identified, but these here are not. Not every large orange-and-black spider-hawk in Australia is Heterodontonyx bicolor.
So what of Telostegus, the genus that I was complaining about being unable to find the diagnostic characters for in my earlier post? Evans (1972) describes it as having bifid tarsal claws, and a vena spuria in the forewing. A vena spuria ('spurious vein') is a fold in the wing that might be mistaken at first glance for a wing vein. In the images above, it can be seen as a dark line along the middle of the wing in the dorsal view. Evans (1972) separated two genera of spider-hawks, Telostegus and Elaphrosyron, on the basis of the number of submarginal cells in the forewing (two in Telostegus, three in Elaphrosyron) but more recent authors have not regarded this distinction as valid.
Evans, H. E. 1972. A review of the Australian species of Elaphrosyron and Telostegus, with notes on other genera (Hymenoptera: Pompilidae). Breviora 386: 1–18.
Wahis, R. 2008. Contribution à la connaissance des Pompilides d’Australie (Hymenoptera : Pompilidae). 2. Sur quelques spécimens récoltés par G. Else (Natural History Museum, London) avec descriptions de deux espèces nouvelles des genres Auplopus et Ctenostegus. Faunistic Entomology 61 (1–2): 23–31.