This species is a rather large one (23 – 32 mm) and differs from P. tabaniformis by the brown forewings with the clear presence of an outer transparent area (ETA) on the forewing that is missing in the completely black forewings of P. tabaniformis. The antennae are orange-brown and rather strongly pectinated. The proboscis is short and not functional while it is fully developed and functional with P. tabaniformis. There is a yellow V-shaped transverse line on the metathorax and the tegulae are also clearly bordered with yellow. With P. tabaniformis there are only some yellow spots near the base of the wings.
The abdomen is black dorsally with distinct yellow bands on segments 2 and 4–7 at the ♂ and segments 2 and 4–6 at the ♀. With P. tabaniformis, the yellow band on segment 5 is missing. The anal tuft is black with clear yellow hair-scales centrally and on both dorsal sides while it is completely black with the other species of this genus.
The males are attracted from early to late afternoon to the pheromone developed for this species. But also the pheromone originally developed for S. myopaeformis attracts P. insolitus fairly well.

The mine is very short and is just long enough to accommodate the caterpillar.

The larva lives in the branches of Quercus species.

The species is bound to warm, wind-sheltered biotopes in open woodlands or along forest edges with sun-drenched oak trees where it lives in the treetops. But also on lower branches of trees facing south and even relatively young trees are not despised.