Friday, October 30, 2015

A caterpillar green around the gills, part one

This Halloween, instead of featuring a seasonable insect or spider, I will share a spooky story of a "possessed" caterpillar.

Parasitoids may be among the smaller enemies that a caterpillar has, but they are also among the most insidious.  When a female parasitoid (generally a small wasp or fly) finds a suitable caterpillar, it lays its eggs on or even inside the caterpillar.  The parasitoid larvae then emerge from the eggs and proceed to feed on the caterpillar.  What becomes of the caterpillar once the parasitoid invasion is complete?  It may be mummified, vanish from inside a fuzzy shroud, or perhaps more strangely -- be left apparently unharmed.

A copper underwing (Amphipyra pyramidoides) caterpillar and recently emerged parasitoid wasp larvae.
The light green caterpillar shown above and below is surrounded by the darker green larvae of a parasitoid wasp.  The wasp larvae had recently finished feeding inside the caterpillar and had broken through the sides of the caterpillar to finish their development outside.

The slits that the wasp larvae emerged through are visible behind them.
Meanwhile, the caterpillar just sat there as if unperturbed by the litter of parasitoids.

However, the caterpillar was hardly passive.  When the parasitoid larvae were threatened by an approaching object, the caterpillar became surprisingly aggressive.

The parasitoids had left the caterpillar alive, but not at liberty.  Though the caterpillar was no longer needed as food, it could still be manipulated into serving as the parasitoids' protector.

To be continued... but in the meantime, you can explore some more: Parasitoid Increases Survival of Its Pupae by Inducing Hosts to Fight Predators

Monday, September 7, 2015

Synchronized twitching

The fall webworms' silk nest forms a formidable defense; however, the caterpillars do not always rely on the protective layer alone.

A dense group of fall webworms (Hyphantria cunea) inside a silk nest.
In order to construct and extend the sheets of silk, some of the caterpillars spend time exposed on the outside of the nest. There, they take advantage of their numbers to defend themselves in another way -- by making a coordinated, threatening display. 

The synchronized action of the caterpillars is thought to discourage predators and parasites.  Though the twitching sometimes appears to begin spontaneously, it can also be triggered when something approaches the caterpillars.  For example, in the video above, the last bout of twitching appears to be in response to the arrival of a small fly or wasp on the other side of the nest (~1:44).

Monday, August 3, 2015

On the safe side

When feeding in a large group, caterpillars may be more noticeable to predators -- but they may also be better able to defend themselves.  Groups of tent caterpillars and webworms can consist of hundreds of individuals, which makes them easily spotted (and probably smelled) even from a distance. To thwart their many enemies, these caterpillars surround themselves and their feeding zones with a protective netting of silk.

A nest of fall webworms (Hyphantria cunea) on a mulberry branch.
The caterpillars then proceed to devour nearly everything within their tightly woven nest.  As they move along a branch, all that they leave behind are leaf skeletons wrapped in silk.

The larvae can completely skeletonize large leaves.
Meanwhile, any predators that locate the caterpillars are likely to depart without obtaining a meal. Watch a wasp get foiled by the silk enclosure in the video below: