Two days ago one of the juvenile Mexican red knee tarantulas (Brachypelma smithi) molted.
During a molt there is typically some leakage of the secreted exuvial fluids that separate the hard, older exoskeleton from the underlying, somewhat soft, wrinkled, new exoskeleton.
The sideways positioning of the spider during the molt (see image below) caused a rush of fluid out of the old exoskeleton as the newly emerged spider pushed away from it.
The substrate immediately darkened as the fluid came into contact with the coconut fiber substrate.
Moments later the darkened area below the tarantula began to vibrate. A wormlike larval beetle head emerged from the substrate and the larva began to feed on the exuding liquids. It rapidly transferred its feeding activity from the substrate to the old exoskeleton of the emerging tarantula. This was clearly a Dune sandworm moment.
Knowing how vulnerable spiders are when molting, I poked the emerging beetle larva’s head with a pencil, causing it to draw back into the bedding. A number of smaller wormlike beetle larvae immediately moved upward onto the top of the coconut fiber bedding at the periphery of the dark, moistened area.
I had no idea that these larvae were in the inch-deep layer of substrate on the bottom of the tarantula container.
Where did the larvae come from?
In retrospect, I introduced several mealworms (Tenebrio molitor) into the tarantula’s cage in December. I assumed the mealworms had been eaten, but the occurrence of two beetles in March made clear that I was wrong. I removed the beetles from the enclosure, but obviously not before adults had opportunity to mate and lay eggs.
Lesson learned, fresh substrate every other molt from now on, even if the tarantulas do have to rebuild their tunnels.