Saturday, August 11, 2012

Harvestman

Click to enlarge
I enjoy watching a Granddaddy Longlegs hanging on the outside walls of our creek house any time, day or night.  Usually there are more in the vicinity, moving cautiously along the wall, reaching out with their incredibly spindly legs to touch one another as if to say, "Is that you? This is me."

They are one of the first insect-like creatures I learned to name.  "Granddaddy Longlegs" are harvestmen, arachnids in the order Opiliones.  The eight-legged Arachnida class incudes spiders, ticks and scorpions.  Occasionally spiders with exceptionally long legs are called granddaddys as well.

The harvestman's most striking feature are those long legs.  If we had the same leg to body ratio, our legs would be over 40 feet long!  You can tell them from spiders by their bodies.  Harvestmen have their cephalothorax (head and chest) fused with their abdomens, creating a single oval body.  Spiders such as the one below right, have a separate cephalothorax (tan in this case) and abdomen (colorful part below.)
Spider- abdomen & cephalothorax separate

Harvestman- fused abdomen












They differ from spiders in other ways as well.  For one, unlike spiders, harvestmen have no venom or silk glands.  While spiders suck up their calories as liquid, harvestmen have mouth parts that allow them to take in chunks of solid food.  This allows them an omnivorous diet, eating small insects, plant material and fungus, and in some species even dead organisms and bird dung.  They hunt prey primarily by ambush but some species stalk as well.

They can be seen, standing on six legs, swinging around their longer second pair of legs in the air.  These serve the function of antennae, sensing their surroundings.  Their front legs have three sections, ending in pincers used to groom, fight rivals and tear up food to deliver into their mouths.

Note red spots on leg
Frequently you will see harvestmen with 5-7 legs.  As a survival strategy, their legs can easily be detached by a predator.  (Notice the missing leg in the closeup picture above.)   Once detached, it continues to twitch for up to an hour, entertaining the predator while the harvestman moves on.  This occurs because of a clever pacemaker located in the femur (the segment nearest the body) which continues to send contraction signals intermittently down the leg.

They also have stink glands under the first pair of legs.  They signal that "You don't want to eat me- I taste terrible."  Another set of glands can lay a secretion for their colleagues to follow.

A female harvestman (how is that for an oxymoron?) has eggs fertilized by copulation via the male's penis.  She may deposit the eggs soon after or carry them for months.  The eggs then hatch anywhere from 20 to 180 days later!  The young go through 4-8 molts before reaching maturity.

Erythraeidae mites- Click to enlarge
One of my two specimens of Leiobunum vittatum  pictured also has another feature- mites!  It turns out that there are specific species of Erythraeidae mites whose larvae parasitize harvestmen.  The larvae bite a hole in the tough outer layer and insert a tubular stylostome through which they suck out body fluids.  You can see them dangling from the legs by their stylostome.  And I thought chiggers were bad!



MoBugs.com has a more comprehensive 2013 blog.
More information is at this texas site.











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