Spider of the Year 2025
The Snake-back spider Segestria senoculata (Linnaeus, 1758)
The snake-back spider, Segestria senoculata (Linnaeus, 1758), belongs to the family of tube-dwelling spiders (Segestriidae). This spider family consists of five genera with 181 species worldwide. The genus Segestria includes 22 species, eight of which also occur in Europe.
The snake-back spider has a Palearctic distribution and occurs throughout Europe. The altitude distribution ranges from the planar to hilly elevations (up to 800 m above sea level) up to the tree line at approx. 2200 m altitude (in the Central Alps). Segestria senoculata lives in forests and in log and rubble heaps. Due to its abundance and the high availability of suitable habitats, it is classified as not endangered in the Red Lists.
Description
The body length of Segestria senoculata is 7-10 mm for females, while males are slightly smaller at 6-9 mm. Both sexes are similar in colouration and markings. The cephalothorax is shiny brown, the oval abdomen is pale beige/grey, dorsally with a dark brown, lobed and laterally bookended median band. This band resembles the dark pattern of an adder and is thus the source of the English common name: snake-back spider. The legs are light brown and have dark annulations. Most species of spiders have eight eyes, but tube-dwelling spiders only have six, which are arranged in three groups of two eyes each and form a rough "H" shape when viewed from above.
Way of life
Like all tube-dwelling spiders, the snake-back spider is predominantly nocturnal and builds a characteristic web to catch its prey. This consists of a mesh-like tube built into cracks and crevices (under tree bark, between stones, etc.) from which individual silk threads radiate out from the front entrance of the living tube. These strands act as tripwires for passing prey (especially insects) and transmit corresponding vibrations.
After dark, Segestria senoculata moves up to the entrance of the living tube, where it lies in wait for prey. It shows a leg position that is characteristic of all fishing web spiders: the first three pairs of legs are directed forward together and the fourth pair of legs are directed backwards. No other spiders show such a leg position!
During the mating season, males leave their living tubes to search for females. Sexually mature individuals of the snake-back spider can be found all year round. When a male which is ready to mate encounters a female's web, it shakes her signal threads. If the female does not react aggressively, mating occurs during which the male clutches the female's abdomen with his jaws spread wide apart.
After mating, the eggs are fertilized by the female in the living tube and encased in an oval cocoon; this contains between 60 and 180 eggs. The snake-back spider, like its two Central European sister species, needs around two years to reach sexual maturity.
Similar species
Two other Segestria species occur in Central Europe: the Bavarian tube-dwelling spider, Segestria bavarica, which is slightly larger (up to 14 mm) and whose dark spots on the abdomen are not continuous, and the green-fanged tube web-dwelling spider, Segestria florentina, which can grow up to 22 mm long and has a completely black body.
Text: Christoph Hörweg
