A friend found this orb spider in the driveway last week. She noticed something wasn’t quite right and called me over.
At first glance, the spider looked like it was wearing a white belt. A closer look and I could make out that it wasn’t an arachnid fashion accessory but rather something much more sinister – a grub, most likely a parasitic grub of some sort.
I have seen a parasitic grub on a spider once before but it was much smaller and was on top of the spider’s abdomen instead of hugging the spider’s “waist” as in this case. After taking a few pictures I released the very active spider on the railing of the deck.
I went in and searched the web for parasitic grubs on spiders and quickly found information which made me believe this is a wasp grub. One reference said the grub clings to the juncture of the spider’s two body parts as this is a place that the spider cannot reach. After reading this I decided to keep a watch on the spider (it remained near the deck railing overnight) for a few days and see what happened.
The next day when I checked on the spider I was amazed to see the grub dangling in a loose web, but no spider. The grub had grown significantly overnight. I went in for the camera and came back out to photograph the grub (Canon 7D, 100mm Macro with extension tube, and Canon MT-24EX Macro Twin Lite flash). It was wriggling and as I watched I could see it was beginning to spin a cocoon in the loose webbing. Then I happened to look down and I noticed the remains of the spider.
The grub had essentially completely drained the spider of its body fluids before discarding the dried remains. This explained the sudden increase in size of the grub overnight. It had been feeding on the spider all along, but the final meal killed the spider and caused the grub to almost double in size. Another search of the web added some more pieces to the puzzle in the form of a description of a program with David Attenborough from Life in the Undergrowth. The documentary reveals a strange twist in the final scene between the spider and the grub. The grub apparently secretes a hormone into the spider which alters the spiders behavior causing it to forego the usual neat orb web that it spins every day and instead causes it to spin a loose web that the grub then uses as the stage for spinning its own cocoon.
About five hours after I first saw the grub wriggling in the loose web (it was 10 p.m. at that point), much of its cocoon was complete.
The next morning, I could see the grub still moving inside a neat sac cocoon made of gold-colored silk. Now I just had to wait and hope I saw the wasp when it emerged.
That same day I happened to notice another spider on the other screen door on the porch with a similar parasitic grub. Each day since I have checked the cocoons as I walk by.
Two days later, the cocoon looked complete and a little more opaque.
Eight days after I first saw the grub wriggling in the web I took a closer look and backlit the cocoon by moving one of twin lights behind it. I could see what appeared to be a developing wasp inside with the dark spot at the top being the head or eyes.
Today (on day 9 since the spider died) I stopped to look and saw the cocoon was empty. You can see the exit hole at the top where the adult wasp chewed its way out. I went to the other cocoon and it was also empty. But then I noticed something else…
A few inches above the cocoon was something wrapped in silk. It was a wasp caught by a different orb spider who happened to have a web just above the loose tangle made by the orb weaver that the wasp in its youthful grub stage had killed. Now the wasp had fallen prey to another spider just as it emerged from the cocoon. The complexities and interrelationships of nature just beyond (literally) our doorstep never ceases to amaze me. Take time to see what you can find outside – you may be astounded.