Triangle Fat Spider

Article

January 19, 2022

The triangular fat spider or heat-loving orb spider (Steatoda triangulosa) is a spider of the crested web spider family (Theridiidae). The species is also known as the brown widow, but should not be confused with the brown widow (L. geometricus) of the same name, which belongs to the true widow genus (Latrodectus) within the same family. Originally only distributed in the Palaearctic, the triangle fat spider was introduced to America, the island of St. Helena and the Canary Islands, and currently appears to be spreading to Europe as well.

Features

The female of the triangle fat spider reaches a body length of 3.5 to 8.6 and the male of 3.5 to 5 millimeters. The basic physique of the species corresponds to that of other fat spiders (Steatoda). The prosoma (front body) appears wrinkled overall and is 1.8 to 2 millimeters long in females and 1.6 to 2 millimeters in males. It is red to black-brown in color and also very shiny. The prosoma has no patterns. The chelicerae (jaw claws), which are also shiny, are brown. The legs are pale brown in the female and orange in the male. The femora (thighs) and tibiae (splints) are faintly darkly ringed. The legs may also be darkened distally (away from the center of the body). The opisthosoma (abdomen) is strongly bulged. Its coloring ranges from reddish to almost black. Most often it is colored dark brown. In contrast to the prosoma, the opisthosoma is strikingly patterned. The markings are composed of lateral (side) bands and a median (middle) longitudinal row of diamond-shaped spots. Fine, net-like and dark lines run through all the drawing elements. These stand in stark contrast to the otherwise rather inconspicuous coloring of the spider and thus give it a characteristic appearance. However, since the expression of the character elements varies greatly, individuals of the triangle fat spider can appear both very light and very dark. Detail views of a female

Genital morphological characteristics

The conductor (head) of a single bulb (male sex organ) is long compared to that of other fat spiders (Steatoda) and is also curved. Moreover, in the male, the tibia of each pedipalp (converted extremity in the head area) is longer than the cymbium (frontmost sclerite or hard part of the globe). The epigyne (female sex organ) is characterized within the genus by its small and rounded appearance.

Differentiation from other fat spiders

There is a possibility of confusing the triangular fat spider with some other species of fat spider (Steatoda) occurring in Europe, such as the similarly large common fat spider (S. bipunctata). In this case, however, the opisthosoma is less curved, especially since the diamond pattern found in the triangular fat spider is missing. With a body length of 10 millimeters in the female, the large fat spider (S. grossa) can be distinguished from the triangular fat spider, above all by the longer legs, apart from the different markings on the opisthosoma here. The false black widow (S. paykulliana) reaches a much higher body length as a female with 13 millimeters and otherwise differs from the triangular fat spider by the dark brown to black coloring together with the clearly different character elements on the opisthosoma. In North America it can also the triangular fat spider may be confused with the species S. borealis that occurs there, which also remotely resembles the triangular fat spider. However, the opisthosoma of this species is darker in color and

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