This is a most majestic butterfly, huge and floating effortlessly. It is not, as its English name implies, scarce, and in my experience it is much more common than the Swallowtail (Papilio machaon). I believe the English name originates from the early English lepidopterists and refers to its scarcity as a rare vagrant to the UK in the 1800s or thereabouts.

The previously-considered subspecies feisthamelii (English name: Iberian Scarce Swallowtail or Spanish Swallowtail) is now classified as a separate species. It is principally an Iberian species with distribution "spilling" into the eastern Pyrénées of France (http://diatheo.weebly.com/iphiclides-feisthamelii.html). It differs from podalirius in that it is noticeably paler, white in the male, and pale yellow in the female.

A paper (in French) published by Tristan Lafranchis: http://diatheo.weebly.com/uploads/2/8/2/3/28235851

/feisthamelii_podalirius_lafranchis_2015__2_.pdf

This paper includes a very useful graphic comparison of the features of podalirius and feisthamelii. One of the identifying features quoted is the shorter black mark attached to the upf costa as shown in the comparison below. It says that for podalirius this mark is pointed and for feisthamelii it is flat-ended. This is certainly and clearly true for the two specimens shown below. However, for 36839 it is very clearly flat-ended but this cannot be feisthamelii as it was seen in Var, a very considerable distance from feisthamelii territory. 31820 is also closer to flat-ended than pointed, not as convincingly as 36839, and this was also seen in Var. It may be the case that if this mark is pointed, it is podalirius, but if flat-ended, then the location becomes the critical factor (among other factors).

Podalirius is not scarce in France, as mentioned earlier, where it outnumbers machaon maybe 5:1 and can sometimes be found in numbers, which does not seem to be the case for machaon. It is also a rather lowland species, rarely in my experience over 1000m altitude, whereas machaon is often found at 2000m and above.

Podalirius is a large species in terms of wing area and quite unmistakeable in flight. I recall first seeing it in 1996 in Vendée on the Atlantic coast. Transfixed, I followed for nearly a kilometre.

In 2006 I saw at least 15 very fresh podalirius "puddling" (taking salts and moisture from wet earth), and when two machaon arrived to puddle at the same place they were driven away by the podalirius. 

A superb video of the life-cycle of podalirius has been produced by Filming VarWild and can be viewed on YouTube here: https://www.youtube.com/watch?v=PFHBSUb60hA

ref

sex

observations

alt. m

7939

M

a male, taking salts. This shows the width of the enormous podalirius wings.

900

a male, taking salts.

a female, I believe.

another quickly taken shot to catch the underside of this male.

a male underside. A quick snap as the wings are constantly on the move.

an ovum, on a leaf of Blackthorn (Prunus spinosa).