S. formicaeformis is a rather small species (14–24 mm). The bright red wingtips, the large transparent areas on the forewings and the single red band on the abdomen make this species easily recognised. The proboscis, as in all species of this genus, is well developed and functional.
The males are readily attracted to the pheromone developed for this species and are best lured in early afternoon, although the optimal time span is very weather dependent.

The egg is elliptical and dark brown. The surface is covered with a delicate mesh-like structure.

The larva is off-white and has a brownish black head. As in all Sesiidae, the prolegs have a double row of crochets.

In most cases only short galleries are constructed.

The cocoon is constructed from silk and small gnawed plant-particles. The pupa itself is yellow-brown to red-brown and has an upwardly oriented bicuspid outgrowth on the head. This horn is more pronounced than in S. flaviventris and S. spuleri, both species also living in Salix* spp.

The eggs are sometimes deposited on a twig of the hostplant, but in most cases they are laid near damaged parts of the trunk or twigs. The females appear to prefer cankerous swellings, both on trunks and twigs, for ovipositing. These often very irregularly formed swellings are in caused mainly by Agrobacterium tumefasciens, a soil bacterium. Here, the larvae live in rather short galleries between dead and living parts of this so called 'crown gall'. Occasionally, when there is no living tissue available anymore in smaller galls on branches or twigs, the larvae leave the gall and make longer galleries in the twig. Often the galls show traces of pecking caused by birds, by various tit species or sometimes woodpeckers.
After the hibernation within the gallery, the caterpillars construct an exit gallery to the surface of the bark, leaving a thin cap as a separation with the outside world. Parasitism by flies (mostly Tachinidae) or wasps (mostly Ichneumonidae) can be considerable.
Pupation takes place in a cocoon made from gnawed plant particles spun together. On hatching, the pupa works its way out through the cocoon and the remaining thin layer of bark . At the relative low altitudes in Belgium, development is always completed in one year.

The larva develops inside the trunks, stems, branches and twigs of various Salix species.

S. formicaeformis occurs in most places, especially near water, where Salix spp. occur.