Monthly Archives: September 2015

Many arthropods are masters of crypsis, either being camouflaged so well that they’re almost impossible to see, or being such good mimics of other organisms that they are very difficult to tell apart.

Today, I want to share some neat info about a moth larva that does a bit of both.

In specific, slightly more than four years ago, I walked out onto our deck to discover this large “inchworm,” i.e., the larva of a geometrid moth (also commonly called spanworms, geometers, cankerworms, or loopers) on the gate:

Now, I don’t have any idea what species this is.  I didn’t kill the larva, so I really didn’t (don’t) have any way to determine it, so we’ll just be happy calling it a geometrid (that means it’s in the family Geometridae).

Anyway, pretty clearly it’s camouflaged really well to look like a twig; you can imagine that if this caterpillar had been holding onto a branch with its prolegs (the fleshy, false “legs” on the rear segments of most caterpillars) instead of a red-stained gate, I probably wouldn’t have seen it.

It’s hanging belly up so at the right end you can see where its six true (jointed) legs pointed are tucked in and pointed skyward.

So score a very strong point for camouflage.

But this beastie isn’t done–it turned out also to be mimicking a small snake.

If we look at this critter head-on in this upside down shot you can see the faint Y mark of the head’s frontal sutures.  This would be its “face”…if caterpillars had faces.

As caterpillars are larval insects, they have six true legs, one pair on each segment of the thorax. The way this caterpillar is hanging, you can see four of them partially: just to the right of the head is the very tip of the leg on its 1st thoracic segment, behind that a bit more of the leg on the 2nd thoracic segment, and then much more clearly both legs on the 3rd thoracic segment, which–perhaps significantly–resemble the forcipules (“poison claws”)  of centipedes. (Click on photo to enlarge)

 

You can also see on the large, 2nd thoracic segment an obvious pair of eye spots.  Remember, the head is that little thing with the Y.  Those dark spots that look to be eyes are primarily on the 1st and 2rd thoracic segments, far from the head.  Let’s turn that photo over and mark it up:

 

All of a sudden, we have the face of a snake. (Click on photo to enlarge)

 

 

 

 

 

But coolest of all, this “snake” defends itself by striking!  Check out this video; watch what happens when I touch the prolegs.  

Despite the fact that this caterpillar is absolutely, completely, utterly, totally harmless, when it first “struck” at me, I reflexively pulled my hand away! (Sorry for the out-of-focus video–it was impossible to see what I was getting…)

Now, I ask you:  How can anyone see stuff like this and not just love insects?

 

 

Most people who have ever tried to grow tomatoes have certainly run into hornworms. These caterpillars are the larvae of hawk moths.  Strangely enough, the hornworm most commonly found on tomatoes is not the tomato hornworm, Manduca quinquemaculatus (Haworth), but the tobacco hornworm, M. sexta (L.).

As common and large as this pest is, many people never see (or never know they’ve seen) the adult moths.  They are nocturnal and do their very best to look like tree bark when resting during the day.

Tobacco hornworm larvae can get rather large: the size of an adult’s finger is not unusual.  Owing to their great camouflage, people ofter don’t even see these gigantic critters on their plants until they notice a lot of missing leaves, or they see the suddenly obvious caterpillars looking like this:

So what are those cute little white tufts?  No, they are not decorations the caterpillar has chosen, they are cocoons.  Each cocoon has been spun by a tiny Cotesia congregata wasp larva which hatched from an  egg laid inside this rather unfortunate hornworm.  The hundreds of eggs in each caterpillar hatch in a few days and the larvae then feed on the caterpillar from the inside for a couple of weeks, causing its ultimate demise.

This killing of the host, BTW, is the difference between a parasite and a parasitoid: a parasite feeds on another organism but does not usually threaten the host’s existence (think fleas or lice–annoying, but not inherently deadly).  Conversely, a parasitoid feeds on its host, and in so doing, kills it–this is the case with these tiny wasps.

In a move straight from Alien, the mature larvae chew their way out of the caterpillar, through its integument (“skin”), which brings us to the photo and video I took on 15 Sep 2015 when I found this caterpillar on our tomatoes.  In the photo, we can see:

• The heads of several larvae just having chewed their way through the integument
• Several larvae about 1/4 the way out
• At least one larva ca. 1/2 way out
• Fully emerged larvae
• Larvae spinning silken cocoons
• Cocoons already thick enough that we can no longer see the larvae within

Click on photo to enlarge it & sharpen its text

And, just for fun, here’s a video I made using my son’s microscope.  You can see a larva’s head just emerging through the integument, and next to that one, a larva busily making its silken cocoon.

When all is said and done, we shall have a very dead hornworm and a whole bunch of adult Cotesia wasps: