Wednesday, September 10, 2014

Army of Caterpillars - Datana major

Caterpillars eat.  Their life is one of consuming and processing food until they move on to the next step of development or are themselves eaten.  Towards the end of summer, when the nights begin to get cool, caterpillars seem to go about their task with an increased sense of urgency.  Most will survive winter as a pupa and they must reach that stage before temperatures get low enough to bring their metabolic processes to a halt.

I imagine there are caterpillars in every tree or shrub you see.  Add to those the multitude of species that feed on grasses and forbs and I doubt that you are ever very far from a caterpillar when in the field.  The trick is in finding them.  This Deerberry shrub is showing signs of the recent dry weather.  Dead leaves are piled beneath the plant and live leaves all show signs of discoloration.  I wouldn’t pick this as the most likely place to find caterpillars, but I always give the Deerberries a check when I’m out.

The view from three feet away reveals what was totally obscured at ten feet.  The shrub is full of caterpillars.  These are larva of the Major Datana Moth, Datana major.  Early instars of this caterpillar are gregarious and feed as a group.  When they reach their final instar, they tend to disperse and feed individually.

It’s hard to imagine such a brightly colored creature blending into its surroundings.  The pattern however, does harmonize with the mosaic of greens, reds and browns in the leaves and the grays and blacks of the stems.  As you come into range, the materialization of the caterpillar image is similar to the moment your eyes reach the proper degree of focus to reveal the subject of one of those 3D Magic Picture prints.  It’s a definite “Ahhh” moment, not to be confused with the more cerebral “Ah-hah”.

The appearance of the caterpillar changes as it moves from one instar to the next.  The individuals with black bodies and broken white stripes represent the final instar prior to pupation.  The brown and white striped caterpillars will soon be proceeding to that final instar form. 

The profusion of cast off skins indicates that the transformation to final instar caterpillars has been a recent event.  These fragile skins are easily dislodged by wind or rain.  In fact, several fell from the branch as I maneuvered it for better viewing.  We haven’t had rain, but the open field in which the Deer Berry grows has experienced an abundance of wind.

Leaves are rapidly disappearing from the  Deerberry.  Dispersal of these larger caterpillars is necessary in order for each individual to feed without disturbance.  The caterpillars have moved out through half of the shrub and a few individuals have arrived on neighboring shrubs.  They will soon make their way to the ground where they will create a chamber in the earth in which to pupate.  Adult moths will emerge next spring to begin the process anew.

The red legs become quite visible when the caterpillars are positioned below the branches.  It’s interesting that so many colorful caterpillars eventually emerge as drab adults.  For some reason, the coloration of these caterpillars makes me think of 1950’s era vinyl patterns.

The hairs on this caterpillar serve no defensive purpose.  When threatened, the caterpillar raises its head and tail with thoracic legs thrust upward like horns.  In addition to the threatening appearance, a bit of ingested material is regurgitated at the mouth and a droplet of liquid is released from the anus.  I’m assuming that both of these substances are unappealing in some way to predators.  The anal droplet is visible in the photo as a honey colored sphere on the end of the upraised tail.  Quite an interesting little creature.

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