I don’t think I’ve ever photographed webspinners before, so this represents another first from my most recent trip to Costa Rica. That’s notable to me because they have an order all to themselves, Embioptera.
As I walked the coastal trail leaving Manzanillo, I noticed many trees with webbing on their trunks. I stopped to look at a few, but I didn’t see any movement. I suspected webspinners were responsible, but it was only later when I decided to probe one of the webs that the webspinner above emerged. It didn’t seem very happy with the situation, and moved quickly to try and take refuge within the web again. Eventually it chewed a hole through which it disappeared.
While reviewing the photos, I noticed that I managed to also photograph a smaller, presumably less mature webspinner beneath the web.
A long streamlined body befits the narrow passageways they create within their silken lairs. Forced to navigate tight spaces, they often must move quickly in reverse. Their cerci, those appendanges at the tip of the abdomen, are highly sensitive analogs to antennae to aid in quick backwards movement.
Only males develop wings, so the large one that I photographed here must be male. All the photos I’ve seen of adult males show the wings covering the abdomen. I therefore assume this is an immature male, despite its relatively large size.
Webspinners create their webs using the large tips of their forelegs. The thickened portion of the leg contains many silk-producing glands.
Here’s a few views of the galleries they create. Despite staring at them in the field, I couldn’t spot anything beneath the web. Occasionally I’d see some movement, but once it stopped, I lost track of the source. Now, having the advantage of looking at my high resolution shots, I can see at least three webspinners hidden in the photo on the left. Click on the image to see a larger version and see if you can find any.
One thing I’m left wondering is how vulnerable webspinners might be to parasites. Their web is strewn with frass, which must be a beacon for something like a parasitic wasp. They must indeed move quickly to avoid such things. I did also notice that they are covered with fine hairs. Maybe they use these to detect movement that’s transmitted through the web.
by Vincent H. Resh and Ring T. Carde