Rugathodes sexpunctatus is a minute species of spider in the family Theridiidae

Theridiidae

Theridiidae is a large family of spiders, also known as the tangle-web spiders, cobweb spiders and comb-footed spiders. The diverse family includes over 2200 species in over 100 genera) of three-dimensional space-web-builders found throughout the world...

, the cobweb or tangle-web spiders. This family includes the medically important Latrodectus genus--the widow spiders. The species in the Rugathodes genus are too small to be dangerous to humans. There is very little known about most species in this genus.

Description

Total length (of body, excluding legs) is 1.5-2.5 mm. Typically, the cephalothorax

Cephalothorax

The cephalothorax is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. The word cephalothorax is derived from the Greek words for head and thorax...

 is pale yellow-brown with a dark/blackish median band, usually extending the full length of the carapace, widest at the posterior eye row, surrounding the eyes. The cephalothorax

Cephalothorax

The cephalothorax is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. The word cephalothorax is derived from the Greek words for head and thorax...

 margins are dusky. The abdomen

Abdomen

In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

 is pale yellow-brown with a variable pattern of three pairs of black and white spots, giving the species its name (from the Latin sex meaning "six" and punctum meaning "spot"). There is often a broken white band bordering the rows of spots. The sides of the abdomen are dusky and the ventral surface is pale. There is considerable variation within the species, with east coast specimens tending to have distinct spots, while west coast specimens can have a totally black abdominal dorsum. Alaska specimens tend to be intermediate in coloration, but are variable, with overall coloration from the typical pale yellow-brown to dark brown or greenish-gray, and occasionally pinkish areas on the dorsum between the spots. Darker individuals can be very difficult to diferentiate from the closely related Rugathodes aurantius.
Female distinguishing characteristics: The paired openings of the epigynum (epigyne

Epigyne

The epigyne or epigynum is the external genital structure of female spiders. As the epigyne varies greatly in form in different species, even in closely related ones, it often provides the most distinctive characteristic for recognizing species...

) are nearly merged into a single oval opening, with a variable median divider extending in from the anterior side, giving the impression of a rounded ‘B’ laying on its back.
Male distinguishing characteristics: The chelicerae

Chelicerae

The chelicerae are mouthparts of the Chelicerata, an arthropod subphylum that includes arachnids, Merostomata , and Pycnogonida . Chelicerae are pointed appendages which are used to grasp food, and are found in place of the chewing mandibles most other arthropods have...

 (fangs) are enlarged with a very distinctive shape, as shown in the photograph.

Distribution

This species is widespread across the United States, southern Canada and Russia. In North America, it has been reported from the far Aleutians (Kanaga Island) to Newfoundland, south to central California, northern Arizona and North Carolina. It is usually found in coniferous forests, both in the trees and in the understory vegetation. They have been observed overwintering in leaf litter. It is considered a boreal species that is found at higher elevations in the southern parts of its range.

Life History

R. sexpunctatus has roughly an annual life cycle. Both males and females usually overwinter in the pentultimate (1 molt prior to adult) stage. They mature and breed in late spring, with the males probably dying soon after mating since they are rarely collected past early summer. Adult females have been collected through early fall. Egg sacs have been observed through September, with later egg sacs containing fewer eggs/spiderlings. It is not known whether the later egg sacs are repeat layings by the same female. Females have been observed moving their egg sac around their web, carrying it under their abdomens, especially when disturbed. Spiderlings molt once in the egg sac prior to imerging as second instars. The fifth instar is the adult stage.