How to find Philadelphia's purse-web spider
Look for the characteristic vertical webs at the base of trees and shrubs in neighborhoods, parks and woodlands. You can do this without disturbing (or even seeing) the spider!
OR LOOK FOR THEM ONLINE! -- Remarkably, Dr. Andy Deans, Curator of Entomology at Penn State University, used Google Street View to spot some A. snetsingeri* purse-webs on a neighborhood fence at a site where he'd seen them in Upper Darby! Amazing.
When to Look
Purse-Webs can be found at any time of the year, and the durable webs may persist in the same location for many years.
Places to Visit
Purse-webs have been found in Delaware County and Philadelphia, and adjacent areas of Chester and Montgomery Counties (see the current map here). The spiders are fairly common across eastern Delware County, West Philadelphia and the Wissahickon Valley.
The kinds of places where they've been found in our area include public parks and arboreta, forests and woodlots, nature preserves, and wooded corridors along streams and old rail lines, anywhere with wild-ish plants and soil and bugs or worms to eat. Webs can be very common in neighborhoods that have old, mature trees and dense hedgerows (for example, Lansdowne).
The webs are NOT found where the ground is frequently wet or flooded, or overly flat or manicured, but they may be uphill from those areas.
Curious citizens with legal access to private and commercial landscapes in the region are especially encouraged to participate in the project by reporting what they find there!
What to Look For
Check for the vertically-oriented webs, pencil-thick and up to 6" tall, at the base of trees, shrubs, hedges and large rocks, or along cement walls. You may also find them on fence posts or old stone benches and foundations. In hilly terrain the webs are found on the downhill side of trees. Purse-webs found in grass, thatch and ivy are built horizontally near the ground or reach out diagonally through complex vegetation, but those forms are much harder to see.
Purse-webs are camouflaged with bits of dirt, leaves and moss to blend in and match their suroundings. They do that very well indeed! But with a little experience you can learn to spot webs at the base of trees from 30 ft away and amaze your friends.
How to tell if something is a purse-web or not
If you are not sure if something might be a purse-web, gently touch it with a blade of grass or twig to see if it 'gives' - the purse-web is soft and collapsible, not rigid like a stem or root. Purse-webs are silken tubes that have no other webbing around them. There are other spiders that live in the same habitats, but their webs are not simple, closed tubes.
Detect different species?
At this time there is no way to distinguish between the purse-webs of different Atypid species. When you report the presence of any purse-web you are helping us toward that goal. Once we find out where the webs are, we can investigate to determine species present and their life histories. The current mapping effort suggests that Atypus snetsingeri* has a fairly small range that is ~35 miles wide; new sites visited within the range tend to have the webs, and sites vigorously searched at and beyond the borders of that range did not reveal any webs.
Look for gossamer in the spring!
Well, this isn't actually searching for purse-webs, but it works. In mid-March into early April if you see lots of silk spread over vegetation and also spot spiderlings, check to see if they look like little tarantulas. If so, look around nearby for purse-webs.
The transient silk from a mass of dispersing spiderlings can be easier to see than a hidden purse-web. At Tyler Arboretum in Media, PA, the annual emergence event begins sometime in March or April every year, depending on weather, and can last for a few weeks of uneven spiderling activity, mostly in the morning hours. The visual gossamer effect they produce is sometimes dramatic and in other years the silk is hardly visible, and in either case it disappears over a few days. It's worth taking a closer look if you happen to notice silk tents or sheets on vegetation in the spring.
* Note: Recent DNA evidence indicates that Atypus snetsingeri is actually an introduced naturalized population of Atypus karschi from east Asia. The name 'snetsingeri' will still be used here until the official synonomy is declared in the World Spider Catalog.