Friday, March 31, 2017

Leggy Bugs--Inchworms

Inchworms are caterpillars in the Geometridae family of Lepidopteran larvae, which I wrote about in “Leggy Bugs—Caterpillars (Lepidopteran Larvae, which Become Butterflies and Moths)”. They have front and back leggy sections and long middles—caterpillars' version of canines' dachshund.

Wisegeek’s “What Are Inchworms?” has an overview and an image for introduction for these critters. They're leggier than adult insects, but seemingly shortchanged compared to caterpillars in my previous article. (I used the picture as a basis for my blog image.)

For a more extensive introduction to inchworm and animal categorization, Bug Guide's website for “Family Geometridae - Geometrid Moths” is a good place to start.

Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Hexapoda (Hexapods)
Class Insecta (Insects)
Order Lepidoptera (Butterflies and Moths)
Superfamily Geometroidea (Geometrid, Swallowtail Moths)
Family Geometridae (Geometrid Moths)

Although inchworms are caterpillars, their locomotion is so odd that they deserve an article all their own. Their family, Geometridae, pertains to “earth” and “measuring”. Their physiology is more specialized than the caterpillars I wrote about previously. The Bug Guide explains:
the lack of prolegs in the middle of the body necessitates the peculiar method of locomotion, drawing the hind end up to the thoracic legs to form a loop, and then extending the body forward.
An excellent video of inchworm locomotion on a flat surface is “Inchworm Walking”. The caterpillar thrusts its front section forward, pulls the rear section up to the front legs, forming a loop, then repeats the motion. Its movement makes me think of a pelvis-shape strong enough to thrust out a very long torso and head. And yet, when fully extended, the head and front “arms” are strong enough to hoist the body and legs back to immediately behind the front end. The movement also seems like something a small, self-propelled slinky might make.

From Encyclopedia.com’s "Inchworm" description of inchworm movement:
inchworms lack appendages in the middle portion of their body, causing them to have a characteristic looping gait. They have three pairs of true legs at the front end, like other caterpillars, but only two or three pairs of prolegs (larval abdominal appendages), located at the rear end. An inchworm moves by drawing its hind end forward while holding on with the front legs, then advancing its front section while holding on with the prolegs.
The Bug Guide’s “Superfamily Geometroidea - Geometrid and Swallowtail Moths” provides most explanations and descriptions at the subfamily level.

Inchworms, unlike general caterpillars, metamorphose into only moths. Sciencing’s “Inchworm Life Cycle” explains:
The thousands of moth species in the family Geometridae are often referred to as inchworms when in the caterpillar stage. … This group of moths has a complete metamorphosis: They go through four stages during their life cycle.
More website resources:
Additional videos featuring inchworms and their movements, some with interesting sways:
  • "Inchworms"
    Ability to lift their front of the body at an angle as though bending from hips.
  • INCREDIBLE INCHWORM
    Acrobatic inchworm with hoisting its body almost totally vertical on horizontal surface and other seemingly gravity-defying surface grippings.
  • Inch worm Highway
    Inchworm walking the front legs forward, then dragging the rest of the body and rear legs forward. The body forms a loop as the rear section stops just behind the front legs.
  • Inchworm
    Another great example of locomotion, this inchworm being brown-patterned.
  • The Happy Inchworm
    Animation that shows motion as push from behind, pull from front, and no separate leg movement. This video is more for entertainment, although some commenters object to the violent outcome.

Leggy Bugs articles:

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