I just returned from a week of vacation on beautiful Stony Lake, north of Peterborough, Ontario. A lot of time was spent sitting on docks (Note: the correct terminology should be Wharf instead of dock i.e. you ‘dock’ at a wharf; however, it is generally more commonplace to use the terminology Dock), at water’s edge. Where there are
docks wharfs, there are spiders. The most common species on the docks tends to be the (in)famous “dock spiders”. I am pretty sure that dock spiders are the largest spiders in Canada (if not, please correct me!). I receive many phone calls and e-mails about dock spiders, and I suspect an impressive amount of arachnophobia can be blamed on this hairy wonder of the Arachnid world.
Dock spiders belong to the family Pisauridae, which are closely related to wolf spiders (family Lycosidae). Both of these families of spiders show interesting behaviours towards their young (‘spiderlings’). Females lay eggs within a silken egg sac, and this sac is carried around by the female until it is time for the young to hatch. Wolf spiders attach their egg sacs to the end of their abdomen, and when the spiderlings hatch they are carried around on the mother’s abdomen before embarking on a solitary life. Pisaurid females, however, hold the egg sacs by their fangs, and it is carried underneath the female’s body – it looks like the females are carrying around a big wad of cotton by their mouths. Pisaurids are commonly known as nursery-web spiders, as females build a silken, tent-like ‘nursery’ for their spiderlings. Upon hatching, the young spiders live in a protected place, typically spun in and among grasses, low-growing vegetation, or between rocks around the margins of water.
Two species, Dolomedes tenebrosus and Dolomedes scriptus are the common ‘dock spiders’. Unfortunately it is difficult to tell these two species apart, without a microscope, forceps, and expertise. Both species are brownish-grey in colour, with black and light brown markings (‘chevrons’) on their abdomen. These spiders, especially the full-grown females, are the largest (native) spider species in Canada, and their body (including legs) can almost fill your palm – the body length (i.e., not including legs) of mature females can easily exceed 2 cm. But do not worry! These spiders do not bite people, and would rather eat land-dwelling and aquatic insects, and they are known to catch small minnows, which is the reason for their other common name, the fishing spider. The spider will wait with its front legs resting in the water, and when small tadpoles or fish come near, the vibrations alert the spider to its lunch. An invertebrate eating a vertebrate is not a common occurrence in the animal kingdom!
Dock spiders, as their name suggests, tend to be associated with the margins of lakes, ponds, swamps and rivers, where they typically sit motionless on tree trunks, rocks, boats, and docks. However, individuals are known to travel some distance from water, and are the reason for many alarmed people describing hairy monsters in their basements. This mainly occurs in the autumn months when the spiders are searching for a warm place to spend the winter (under stones, leaves, or bark, or inside buildings). After spending a winter as an immature spider, dock spiders typically mature and mate in the spring, with females carrying egg sacs for a few weeks, before the young hatch in the nursery. Females can then go on to produce a second, or sometimes a third egg sac before the end of cottage season. One egg sac can produce over 1,000 spiderlings.
Without a doubt, dock spiders are impressive animals and although not small and obscure, they are still worthy of study. They should be considered friends of cottagers, boaters and home-owners. I encourage you to watch them, observe their behaviours, and marvel at their size – it’s especially fun to do this when sitting down at the dock, having a glass of your favourite beverage, watching when the evening comes….