Diario de antonva

Genus very easy to identify by its pointy rostrum. Two species are present:

-Rhingia campestris
-Rhingia rostrata

They can be identified by the color of their thorax (almost black in R. campestris and grey in R. rostrata) and the abdomen, which in R. campestris has a median black line and a black border on the sides.

https://en.wikipedia.org/wiki/Rhingia_rostrata
https://www.flickr.com/photos/63075200@N07/collections/72157629958990061/

Very difficult genus of hoverflies quite similar to Scaeva (althouth the tergite marks are less curved). In Galicia, there are 4 recorded species, but there are more that have been found nearby.
These are sure:

-E. corollae
-E. latifasciatus
-E. lucasi
-E. luniger

But these (and maybe more) may be present:

-E. bucculatus
-E. flaviceps
-E. nielseni
-E. nuba

Identifying them in situ or by pictures is often not possible, especially for the males, for which genitalia might be necessary. Descriptions for some British species can be found here:
https://www.flickr.com/photos/63075200@N07/collections/72157629460588728/
This is a general guide in Dutch:
https://waarneming.nl/download/fotogids_Syrphidae.pdf
And this post helps identifying females of the most common species:
https://www.biodiversidadvirtual.org/insectarium/Eupeodes-cf.-luniger-(Meigen-1822)-img311682.html

Other than genitalia, other useful traits are the color of the face in females, and whether the yellow marks reach the edges of the tergites

There are 6 species, all of which can be found in Galicia.

-V. bombylas
-V. elegans
-V inanis
-V. inflata
-V. pellucens
-V. zonaria

They are easily identifyable with this helpful guide

Helophilus is a genus of big, yellow colored hoverlies, with two species present in Galicia.

-H. pendulus
-H. trivittatus

While H. pendulus has a black vertical line on the face, and the basal 2/3rd of the hind tibiae orange, H. trivittatus has a orange line on the face, and only about 1/3 of the hind tibia yellow. The abdomen can also be somewhat different, with H. trivittatus having a lighter shade of yellow.

Parhelophilus is a similar genus with only one species in Galicia, P. versicolor, but they are much smaller, less brightly colored, and have orange antennae rather than black.

Sources:
https://www.flickr.com/photos/63075200@N07/collections/72157629445217282/
https://www.flickr.com/photos/63075200@N07/collections/72157630263217876/
https://www.biodiversidadvirtual.org/taxofoto/taxofoto/2041

There are 4 species of Eristalinus, a genus easy to identify thanks to their spotted/banded eyes.

-E. aeneus
-E. megacephalus
-E. sepulchralis
-E. taeniops

There is this very useful identification guide, but to point out the main differences:
E. taeniops is the easiest to identify because it's the only one with banded eyes, while the rest have spotted eyes.
E. megacephalus has a much lighter abdomen, with either white bands or big orange blotches.
E. aeneus and E. sepulchralis are the most similar ones, but while E. aeneus has a uniformly metalic abdomen, in the sides of segments 2 and 3 E. sepulchralis has pollinose areas. The eyes are also different, in E. sepulchralis they have more hair, and in the male they don't touch as in most hoverflies.

Big, striking yellow and black hoverflies. Somewhat similar to Chrysotoxum, but these have long protruding antennae.
There are three species:
-X. citrofasciatum
-X. dives
-X. pedissequum

X. citrofasciatum is very easy to distinguish becuase it has orange legs, but the other two species are really similar. Here is a guide on how to tell them apart: https://www.inaturalist.org/posts/38160-xanthogramma-species-in-europe

Two Xylota species have been reported in Galicia, and two more nearby. For sure:
-X. segnis
-X. xanthocnema

Nearby:
-X. ignava
-X. sylvarum

The all look similar (black slender flies, with enlarged femurs for males), but they can be divided in two groups. X. xanthocnema and X. sylvarum are black with the last segments of the abdomen yellow, but the first has all yellow hind tibiae, and X. sylvarum partially black.
X. segnis and X. ignava have most of the abdomen orange, with black tip, but X. ignava has hind tibiae with yellow tips and yellow hind tarsi. Also, deeper red abdomen

Further ID help for X. ignava:
https://www.inaturalist.org/observations/180417146

Three species have been reported in Galicia, and one more has been reported nearby:

-S. dignota
-S. pyrastri
-S. selenitica
-S. albimaculata (nearby)

S. pyrastri is easy to identify: the markings in the abdomen are almost white and not very curved, while S. albimaculata has squarish spots, and the other two species have heavily curved yellow markings.

S selenitica and S. dignota are difficult to distinguish: males are possible to ID by the angle that the eyes make when they meet in the front of the head: 90-106 degrees for S. dignota and 120-136 in S. selenitica. Females are tricky, and multiple traits should be used. They are described in this article: https://www.researchgate.net/publication/262917841_Scaeva_dignota_RONDANI_1857_Diptera_Syrphidae_-_first_records_in_Poland

There probably are 2 species of Microdon in Galicia:

-M. analis
-M. mutabilis
-M. myrmicae

M. analis is easy to identify because it's the only one for which the hind margin of the scutellum is black. The other two, on the other hand, are cryptic species, as shown here: https://biblio.naturalsciences.be/associated_publications/societe-royale-belge-dentomologie-koninklijke-belgische-vereniging-voor-entomologie-1/srbe-145-2009/van-de-meuter-et-al-bulletin-145-vii-xii-116-120-2009.pdf

There are only three species:

-S. ribesii
-S. torvus
-S. vitridipennis

S. torvus can be IDed because in both sexes it has hair on its eyes. Under direct sunlight, this makes white reflections, but seems difficult to see otherwise.
As for S. ribesii and S. vitridipennis, the males are practically identical (other than genitalia and micropilosity of the wings), but the females are different because in S. vitridipennis 3/4 of the hind femora are black, and in S. ribesii yellow.

Sources:
https://en.wikipedia.org/wiki/Syrphus_ribesii
https://www.mapress.com/zt/article/view/zootaxa.4216.5.1