Monday, November 11, 2013

Notes on a small caterpillar

Notes about the frangipani flower
and the caterpillar (c) Mary Adam
On an afternoon a few months ago, I brought a sprig of frangipani inside to look at it more closely. I used a loupe placed over it which gave me approx 3-4X magnification. That's when I saw a little caterpillar which had been carried in on the stem of the flower. It was no more than 1 cm long. It was walking along the stem towards the flower. I looked at it through the loupe -- it was still just a little caterpillar except the loupe made it bigger and easier to see. Suddenly the stem shook slightly. The little caterpillar drew back in alarm, startled, just as I might jump at an unexpected sound. There was something extraordinary about that movement. It could have been any animal. In some ways it was uncannily like a human reaction. Maybe it was the fact of witnessing it through the loupe that made it so vivid and human-like. Just a little 1 cm caterpillar. A while later it munched through the yellow pollen of the frangipani flower and after that I put it back in the garden. 

I have tried to estimate the approximate relative sizes of a small caterpillar and a human being. I can't guarantee the following calculation is correct as maths is not my best subject so please let me know if there's a mistake:


Caterpillar weighs  0.3 gm

Human being weighs 60 kg = 60,000 gm

60,000 divided by 0.3 = 200,000

Therefore a human being is about 200,000 times bigger than a caterpillar.

What would be 200,000 times the size of a human being?

60 Kg x 200,000

= 12,000,000 Kg -- twelve million Kg = 12,000 tons -- a large liner?

(10,000 tons = 10 million kg)

Therefore, 1 ton = 1000 kg

A ton is 2000 lbs (1000 kg).

A car weighs a ton, also an elephant.

12000 tons = 12000 cars.

Or 12000 elephants.

It's difficult to imagine that size difference.

Caterpillars have about 4,000 muscles (the human being has only 629). They move through contraction of the muscles in the rear segments, pushing the blood forward into the front segments elongating the torso. The average caterpillar has 248 muscles in the head segment alone.
Caterpillars do not have good vision. They have a series of six tiny eyelets or 'stemmata' on each side of the lower portion of their head. These can probably form well focused, but poorly resolved images (Scoble 1995). They move their heads from side to side probably as a means of judging distance of objects, particularly plants. They rely on their short antennae to help them locate food.
Some caterpillars are able to detect vibrations, usually at a highly specific frequency. Caterpillars of the common hook-tip moth, Drepana arcuata (Drepanoidea), produce sounds to defend their silk nests from members of their own species, by scraping against the leaf in a ritualized acoustic duel (Yack et al. 2001). They detect the vibrations conducted by the plant and not air-borne sounds. (New World Encyclopedia)
I should have mentioned that the Frangipani is the host plant for the Frangipani Horn Moth. I don't think the small caterpillar was one of those because the Horn Moth caterpillars are orange and black with yellow stripes and are very fierce looking. I have a photo somewhere...

Caterpillar of Pseudosphinx tetrio, the Frangipani Hornworm Moth,
my not-very-good photo 

There are better photos of both caterpillar and moth in the Wikipedia article --

1 comment:

Claudine Fear said...

Dear Mary, You're so right about keeping up the sketch book. Such an important source of inspiration. However, in this particular case,
what a pity you didn't also take a photo with a macro lens. It would have been so interesting to be able to identify the caterpillar. I try and arm myself with a camera at all times just in case I get to see something interesting that I can't identify on the spot. My phone now serves as my camera of choice when I'm out and about. I also take pictures of most work I'm making to see progress or, more usually, where I've gone wrong!! Keep up the good work. All the best Claudine