Seasonal activity of boreal forest-floor spiders (Araneae)

We studied the seasonal occurrence of forest-floor spiders by collecting samples with 100 pitfall traps operated throughout the growing season of 1985 in a mature coniferous stand in southern Finland. Samples were collected at five-day intervals in May-August and at longer intervals in September-November. The entire sample from the 26 trapping periods consisted of 6753 adult spiders of 100 species. The overall abundance and species richness was highest in the early season, May and June. Seasonal spider catch was not correlated with fluctuations in temperature. but was negatively correlated with rainfall. Nine of the ten abundant species ( gtoreq 2% of the sample each) belonged to the family Linyphiidae (sensu lato) and one to the family Agelenidae. Macrargus rufus (Wider) was the most abundant species comprising approximately 1/3 of the sample followed by Lepthyphantes alacris (Blackwall) (17% of the sample). The activity peaks of the ten abundant species were usually short, only a few weeks, and occurred in the early and mid-season, May-July. Only L. alacris was collected every trapping period. Although numbers of males clearly exceeded that of females in most species, the seasonal occurrence of the sexes coincided. Our results differ somewhat from earlier observations about spider phenology in Finland. It is possible that this discrepancy depends to a great extent on different sampling methods used.