Parasitic wasp larvae Bathyplectes anurus are perfectly hoppy inside cocoons hijacked from the weevil larvae they ate. Image credit: Yoriko Saeki
Tiny, cocooned parasitic wasp larvae hop their way to safety, jumping to get away from predators and to find cool, shady areas, a new study finds. These wee jumpers are adorable — though perhaps you might find them less so, once you learn that their cocoon shells originally held alfalfa weevil larvae, which the wasp larvae consumed after hatching.
Females of the parasitic wasp species Bathyplectes anurus lay their eggs in alfalfa weevil (Hypera postica) larvae. As the parasitized weevils spin cocoons during their final development stage, or instar, they're essentially sealing themselves into a tomb with their murderer. Once the wasp larva hatches, it eats the host, spins its own cocoon inside the host cocoon, which measures about 0.1 inch (3.5 millimeters) in length, and settles in, double-sealed for safety, waiting to pupate.
But even within their double-layered cocoons, the wasp larvae are responsive to their surroundings, the scientists found. Researchers observed the larvae moving their cocoons by using a "whipping movement," to shift away from danger or environmental stresses. [Googly Eyes: Photos of Striking Wasp Faces]
Don't worry, be hoppy
The scientists wanted to identify what could motivate the larvae to jump, an action that likely requires a lot of valuable energy.
In laboratory and field experiments, they examined the effects of light, temperature, humidity and predator proximity to 100 cocoons, and observed how much more — or less — the larvae jumped in response, and whether there were indicators that jumping came at a price.
Their results showed that the larvae jumped responsively when they were exposed to unfavorable changes in light, temperature and humidity, seeking shady, cooler areas with higher levels of moisture. More dramatic changes in conditions seemed to stimulate more jumping, the researchers also found. And the survival rates of the cocoons were higher in shady zones, suggesting that the larvae's jumps indicated they were searching for more favorable conditions, the researchers said.
Hoppiness is the truth
In response to predator exposure, jumping frequency increased approximately 83 percent when the scientists introduced predatory ants to the cocoon's environments, though the study did not detect a trigger that warned the larvae when a predator was near.
And jumping did appear to exact a toll on the larvae. The researchers reported reduced body mass in individuals that jumped more, which suggests that the greater energy expenditure of increased jumping affects the larvae's development. They concluded in the study that there is still much to be learned about the energy costs of jumping in insects, and that future studies could help explain how insects evolved their pursuit of hoppiness.
The findings were published online Dec. 21 in the journal The Science of Nature.