Modest Sphinx Moth Caterpillar at Oxtongue Lake

How would you like to come face to face with this fierce-looking caterpillar if you were about knee high to a grasshopper?  It was about 4 inches long (10 cm), around 1/2 inch in diameter (1 cm), and surprisingly quick at navigating through the tangle of grass.  This is the larva of a Modest Sphinx Moth (Pachysphinx modesta) that we saw in the community of Oxtongue Lake, Ontario, near the end of August.

As luck has it, Bob and I were in my home community for an evening event that featured…Bob!  He was giving a talk, as part of the community’s once-a-month summer Speaker Series, about the making of our documentary film Painting the Wilderness of the Oxtongue that premiered at the end of June.  The moody atmosphere captured in Bob’s early morning photo of Oxtongue Lake on the first day of this stay almost perfectly represented the essence of our documentary, the connection of Tom Thomson and the Group of Seven to this small community.

To help pass the time one rainy afternoon, Bob and I struck out on a walk along the road that runs within sight of the lake.  We had barely gone a hundred feet or so before I noticed this large, pale green caterpillar crawling across the pavement.

A number of homes and businesses are built along that stretch of the road, but on the lake side, various sections of natural habitat still remain where dozens of poplar trees, birch trees, and thick masses of understory bushes and saplings thrive.  The Modest Sphinx Moth Caterpillar appeared to be making its way over to a grassy area removed from the trees.

I was worried that it would soon be run over by a passing car so thought it best to move it towards its destination on the other side of the tarmac.  Not sure if the oils on my fingers would harm the little creature, I picked up a fallen leaf and attempted to slide it underneath the plump body of the caterpillar.  It did not like that!

Immediately, it constricted its body into a U-shape that caused the caterpillar to swiftly roll over and away from me.  Several times it did this, and because I was tentative, not wanting to cause any harm to the larva, it managed to avoid being captured.

I finally had to nudge the Modest Sphinx Moth Caterpiller with my fingertip into place on the leaf so I could pick it up and remove it to safety at the side of the road.  Bob made a hasty retreat to the house for our other camera used to take micro and macro shots while I kept a close eye on the caterpillar to make sure we didn’t lose sight of it.

Once placed back on the ground, the Modest Sphinx Moth Caterpillar played dead while still keeping its body curled and taut, a combination of characteristics that probably would make the larva more difficult for a bird to snatch up.  For several minutes, the caterpillar remained motionless.  Only when I moved off to a further distance did the larva stretch out and begin to crawl again.

Once Bob returned with the other camera, I plunked myself down on the ground near the caterpillar for some closeup shots.  The larva totally ignored me by this point…

and manipulated itself over the fine grains of sand,

between damp blades of grass,

and under little twigs.  By this time, we were taking note of the finer details of its anatomy…the granular white spots that are tiny bumps arranged in rings around the circumference of the larva’s body, and the prominent oblique white stripe that extends up each side of the caterpillar from the bottom of its body to meet at the tail spine near the rear of the larva’s abdomen.  The spots and stripe seemed to glow in the dim light of the overcast day.

The Modest Sphinx Moth Caterpillar began life earlier in the summer, near the end of June.  The pupae of this species winter underground in a shallow burrow.  About mid-June, fully developed adult Moths emerge from their chrysalids and promptly mate.  A female subsequently deposits her eggs on several of the larvae’s host plants and then dies.  The adults’ one flight period/year occurs over the course of about 7 days.  The eggs hatch about 9 days after being laid, and then the larvae eat and grow through four instar stages until the fall.  Judging by the pale green colour and size of the caterpillar we found, this larva is in its 4th instar stage and is ready to pupate.

It was with great interest that we observed the Modest Sphinx Moth Caterpillar begin to burrow beneath a tangle of dead needles and brown grass.  It was excavating a burrow where it will pupate and then overwinter until next June.  Bob and I left the caterpillar to the task, and when we returned a half hour later, all that remained of the larva was a small hole where the caterpillar had tunneled into the soft sand.

When examining the above photo of the Modest Sphinx Moth caterpillar, it is possible to discern a couple of the oval-shaped, reddish spiracles that are spaced in a straight horizontal line low on the abdomen along the length of the larva’s body.

I was fascinated to learn that these spiracles are the external respiratory openings of the larva!  Who knew?  We are learning here along with so many of you.  And the pale green colour is a sign that this larva has progressed through at least 3 earlier instar stages, when the caterpillars were a darker green, to the final stage before it pupates.  Another change that has occurred as the caterpillar grew and transformed through the instar stages is the length of the tail spine or anal horn.  Modest Sphinx Moth larvae are hornworms, and by the time they are ready to pupate, the horn has grown considerably shorter.  It is barely visible in the above photo.

There is good reason that the Modest Sphinx Moths laid there eggs where they did.  Loads of Poplar Trees grow around Oxtongue Lake, and the leaves of these trees are one type  upon which these hornworms feed.  Helping to ensure their survival, early instars are the colour of the tops of poplar leaves whereas the final instar larvae are the colour of the bottoms of the leaves.

When this caterpillar emerges next June as a Modest Sphinx Moth, it will be neither modest in size or appearance.  Modest Sphinx Moths are considered to be our largest and most common sphinx moths, and the unremarkable coloring of their forewings belies the stunning grey, crimson and blue pattern on the underwings.  We plan to be at Oxtongue Lake next June and attempt to spot one of these beauties!  Time will tell if we are successful.

Related:

Atlantis Fritillary Butterflies in Algonquin Park

Frame To Frame – Bob and Jean