Dare to look into a spider web and you may find some surprises. Even if a spider is present, it may not be the one that created the snare. A variety of insects, too, take advantage of spider webs to avoid their own predators, snack on the spider’s stored food, or eat the spider itself.
Flies Using Webs for Protection
Not all threads in a spider web are sticky. The foundation lines that support the body of a web are glue-free, and sometimes thicker than other strands. Certain flies take advantage of this and routinely roost on the lines, hanging from them like six-legged trapeze artists. Tiny, mosquito-like flies called gall midges, family Cecidomyiidae, frequently suspend themselves from spider webs when resting. If they even managed to notice the flies, predators would risk becoming tangled in the spider web should they attempt to catch one of the diminutive midges. This behavior is apparently widespread in the Cecidomyiidae, especially in the subfamilies Porricondylinae and Cecidomyiinae.
We have also observed crane flies in the genus Dolichopeza (family Tipulidae) doing the same kind of acrobatic maneuvers on spider webs inside of a tree hollow in western Massachusetts, USA.
Stealing Prey from Spiders
Not all visitors to spider webs mean the spider no harm. Scorpionflies, insects in the family Panorpidae, order Mecoptera, will steal prey tangled in spider webs. These insects are not tiny, so they must negotiate all the right threads to avoid becoming a spider meal themselves. We came across one instance of a female Panorpa species trying to pilfer a wrapped-up insect from a sheet-web weaver in the family Linyphiidae. The spider wanted no part of the intruder and chased it from the web immediately.
Even other spiders are not above snatching a meal from another spider. Dewdrop spiders in the genus Argyrodes (family Theridiidae) are very small spiders that live in the webs of very large orb-weaving spiders. We were fortunate enough to spy a male Argyrodes while photographing a big female Black and Yellow Garden Spider, Argiope aurantia, in southern Ohio during the summer. The enormous orb weaver took no notice of its tiny squatter.
Preying on Spiders
Pirate spiders of the family Mimetidae take invasion of spider webs a step further: they kill the owner of the web and take its place. Pirate spiders have venom designed to kill other spiders very quickly. They may also eat the egg sacs of other spiders. Pirate spiders can be confused with cobweb weavers, but they have extremely long spines on their legs.
Insects, too, can turn tables on spiders. Threadlegged assassin bugs in the family Reduviidae are superb spider stalkers. Skinny and lanky, they step carefully and patiently through a web before literally reaching out and grabbing the spider. Threadlegged assassin bugs have front legs like those of a praying mantis. These vise-like appendages are perfect for seizing and subduing prey. Emesaya brevipennis is a large (33-37 mm body length) but surprisingly cryptic species that is not uncommon over much of the United States. We barely spied the one above, imaged by Kris Light, at a research station in the Great Smoky Mountains of Tennessee.
Don’t be afraid to look closely at spider webs. Admire their amazing engineering and construction. Look for the maker of the web, but see if you can’t find a stalker, thief, or hanger-on lurking in the web as well. Happy hunting.