Naturespeak A naturalist's view of the world

July 12, 2011

Wet Little Moths

Filed under: Uncategorized — wykes @ 9:17 pm

I’ve raised a whole lot of caterpillars in my time. Early on in my captive (not necessarily captivating) caterpillar career I killed more than a few of them out of ignorance. The ignorance was mostly on my part and not the caterpillar’s, by the way. ‘Pillars require fresh green food and it was always a major task, often a major pain, to cut fresh greenery for them on a daily basis.  I tried keeping their leafy food green by putting the twig ends in an open jar of water – you know, like cut flowers. That way, my captives could munch happily away on the freshest possible of salads.  The best part? I only had to collect leaves on a weekly basis rather than a daily one.

There was nothing wrong in my thinking, only in my methodology. I soon learned that the top of the water container needed to be sealed off. On several occasions I discovered that my caterpillars (Monarch and Polyphemus to be specific) would come to an untimely end in such an open water situation. They would commit unintentional suicide by crawling down the stem into the water and then drown. The poor little dead beasts would remain attached to the stem in their original life position.

You might ask at this point why the creatures didn’t just back up? It’s not that they can’t physically back up, it’s just that they don’t feel a need to in this situation. Before they realize it, their laterally located breathing pores (spiracles) are immersed and they suffocate. It was, and is, that simple. It was, and still is, an amazing thing to see. I can only think of the legendary stupidity of domestic turkeys that are said to drown in rainstorms when they forget to close their bills or lemmings blindly jumping to their death off sheer cliffs. O.K. the latter incident was artificially created by Walt Disney, but it is a fact that Caterpillars and water don’t get usually along. Today, I use fresh cut leaves to raise my caterpillars.

Oddly enough, not all caterpillars avoid water. In fact, there are quite a few that actually live (and breathe) in it. In Michigan alone, there are some 34 species of aquatic or semi-aquatic moths. In all cases, it is the larval stage that is actually aquatic. My attention to this fact was recently focused by the sight of a piece of Spatterdock leaf crawling across a very wet water lily pad on Dollar Lake (see beginning photo). I reached down to pick up the oval shaped piece and discovered a fleshy larva semi-encased within. It was the caterpillar of the China Mark Moth.

The China Mark spends its entire larval life in the water. Because they are succulent bits of desirable fish food, the caterpillars protect themselves by covering their bodies with an artificial shell. This structure, much like a turtle shell, consists of a large upper “carapace” made out of a single large oval segment and a lower “plastron” of smaller leaf segments.  The whole thing is held together with a silken bag.  As the larvae grow they cut new leaves to cover their larger selves.

Totting their mobile home about in the manner of a caddisfly, they crawl with their set of pro legs, eat with their head extended out, and retreat into the confines of their home when threatened. More often than not, they will hang out on the completely submerged underside of the leaf. On occasion, they will cut loose and float around until making contact with fresh water lily. So, not only do these fellows swim, they also navigate on tiny ships. A sailing worm – who’d figure?

In the world of aquatic caterpillars, there are those who have external gills and those who do not. The China Mark larva does not. They look pretty much like your average caterpillar, only wetter and livelier. I did notice that the individual I encountered (and shown here) had an air bubble entrapped within his casing. He was actually positioned directly over a stream of bubbles issuing out of a severed section of water lily stem when found. I can not be sure whether this was an intentional thing or not (see below).

Unfortunately, there is not a whole lot of easily accessible information out there regarding China Mark Moths other than references in aquatic gardening sites which view it as a pest species.  Obscure lepidopteran references only contain descriptive terminology and little life history. I therefore have no further tid-bits of wisdom to share on this subject without starting to make something up, so I will stop. I do know that the common name refers to the marks found on the adult moth (a dry winged critter that can walk on water but not immerse itself in it).  In common jargon, however, the encased aquatic caterpillars are often referred to as “sandwich men.” I guess that name could be a double meaning if you consider that sandwich to be a Submarine sandwich?


  1. I love this blog. Thank you Gerry! I have learned so much reading this and look forward to each post.

    Comment by Robin Bloomberg — July 13, 2011 @ 6:47 am

  2. Ooo – interesting! Now I have one more thing to keep an eye out for as I investigate Michigan!

    Comment by Ellen — July 13, 2011 @ 5:05 pm

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  4. This is an excellent series of images of an insect that is rarely photographed.

    And I am writing to ask if there may be some arrangement we could make where I could include a couple of these in a book I am trying to complete.

    The book is a revision of Garden Insects of North America (Princeton University Press), which was first published in 2004. I am trying to do a top-down revision and am looking for the best images to illustrate this.

    If you would like to discuss this further please contact me.

    Thank you for considering this.

    Whitney Cranshaw
    Professor/Extension Specialist, Entomology
    Colorado State University

    Comment by Whitney Cranshaw — February 4, 2016 @ 9:12 am

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