June 2022

Eight-legged femme fatale

I recently spotted these cocoons in my backyard. What kind of insect created them?


The long silken egg sac of a Magnificent Spider (Ordgarius magnificus). The female spider is perched near the top, dwarfed by the huge egg sac. Close-up of a female Magnificent Spider (Ordgarius magnificus), showcasing its colourful body.Another species in the bolas spider group, the Bird-Dropping Spider (Celaenia excavata) has distinctive spherical egg sacs. Interestingly, this species no longer constructs a silken bolas for prey capture, but grabs male moths right out of the air!

The silken objects in your photo are not actually cocoons, but are the egg sacs of the wonderfully bizarre Magnificent Spider (Ordgarius magnificus). These spiders hide during the day and emerge at night to hunt, so their large and distinctive egg sacs are commonly the only evidence of them that we see. Each egg sac can contain hundreds of eggs.

Magnificent Spiders are one species within a larger group that are commonly called ‘bolas spiders’. For context, a bolas is a weapon consisting of a length of rope with a weight attached at one end. Bolas spiders don’t build circular orb-webs, but instead construct a silken tightrope. Perched on this tightrope they lower a strand of silk with a droplet of sticky fluid at the end (hence the resemblance to a bolas, and all that remains of the sticky orb-webs built by their ancestors). The spider then broadcasts pheromones that mimic those given out by particular species of female moths. Male moths detect the pheromones and approach to investigate. Once the moth is within range, the spider swings its bolas and captures the moth. Dinner! The spider can even produce different pheromones to target particular species; this ensures that it gets a moth of the right size for a meal.

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