S. spheciformis is one of the larger clearwing species in Belgium (23–32 mm). It is easily recognisable by the almost completely black colouration and only one yellow band on the abdomen. Another striking feature is the pale yellow colouration of the distal part of the black antennae.
Males of this species are reasonably well attracted by the pheromone originally developed for S. tipuliformis.

The eggs are dark brown to black and covered with an fine hexagonal mesh-like structure.

The larva is off-white with a brown neck-shield.

The caterpillars construct galleries between wood and bark if they live in larger trees (often the remaining stump of a cut tree). If they live in smaller stems (the most common situation) a gallery, going downwards into the root-system, is constructed in the centre of the stem.

This species does not construct a cocoon, so the pupa can move freely up and down in the gallery.

The female lays the eggs on the underside of leaves of the foodplants. The young larva works its way towards the base of the stem and lives on the sap flow between wood and bark. After the first wintering, the larvae start to make mines in the wood as well, and also construct a gallery in the central part of a stem. Before the second hibernation the caterpillar enlarges that shaft and, at the end of it, bends it outwards towards the bark to construct the future exit hole, leaving just a thin layer of bark.
In Alnus, which has a smooth and thin bark, this "membrane" discolours reddish to brown or even almost black. When living in Betula, with a more rough bark, the future exit hole is invisible from the outside. Next springtime, the larva pupates without constructing a cocoon. The pupa is very mobile in its gallery, giving it the ability to respond to weather conditions or external disturbances. When hatching, which mostly takes place in late morning, the pupa breaks through the bark membrane and slides out halfway. The exit hole is always quite low in the stem and is rarely found higher than 25 cm above ground level.

The larva mainly bores into the stem and root of Alnus but Betula is also often used as hostplant.

The preferred habitats are moist and even marshy places where the main hostplant Alnus is common. In drier areas, (young) Betula trees are occupied as well as Alnus. This is often the case in heath landscapes where there are often a lot of young Betula trees together with young Alnus trees in the vicinity of fens.
In managed areas S. spheciformis can be really abundant in the new shoots formed by regularly cut Alnus trees at the borders of fish ponds.