Thursday, September 16, 2004

Mom, come quick! My caterpillar's head just fell off!

After a very long and hectic day, I found a few spare moments around sunset to take some pictures of my happily growing caterpillars. Just about all they've done since the last pictures I posted is eat, and it shows:

Third instar Manduca on a tomato stem
Third instar Manduca sexta on the defoliated stem of a tomato plant.

It's hard to believe that just last week (a mere 168 hours ago) they were hatching out of their little eggs and taking their first bite of a tomato leaf. Now they're seasoned tomato leaf-devouring pros who are consuming their own mass in leaves daily. They're growing up so quickly!

The caterpillar pictured above is in its third instar, meaning that it's molted twice since it hatched. It's near the end of its third instar, which you can tell by comparing the size of its head capsule to its body. Hardened portions of a caterpillar's body (its head capsule) can't expand in size within an instar; the caterpillars have to molt if they want to get a bigger head. However, soft portions of a caterpillar's body can grow in size, so as a caterpillar grows within an instar their head capsule gets proportionally smaller than their body. One method to determine the instar a caterpillar is in, without closely monitoring its development, is to measure the animal's head capsule width.

When it comes time to molt, the caterpillar doesn't bother with trying to expand its old head capsule. It grows an entirely new head capsule behind the old one, digests away all the material contained within the old head capsule, and then just tosses the old capsule aside.

Third to fourth instar mold of M. sexta
A M. sexta undergoing its third to fourth instar molt. Note that the head capsule appears clearer than normal and has been pushed forward and down.

This picture shows a M. sexta that is in the middle of its third molt (between the third and fourth instars). To see the molt in progress, compare this picture with the prior one in the post. The head capsule is more translucent than normal, due to the digestion of contents within. The head capsule is also starting to be pushed off the caterpillar's body (it's further forward and lower than normal) as the new capsule grows in behind it. About one of the only things that the caterpillar hasn't digested off the old capsule are its mandibles - they're the two little black spots that you can see at the bottom of the old head capsule.

1 comment:

Radagast said...

Importing comments:

aww, the poor things..
they would be so much happier over in the rain forest education site-
September 19, 2004, 4:28:14 PM PDT – Like – Reply

Well, insect systems are somewhat different from human systems in this regard. Insects don't breathe through their mouths/heads, so that's not a problem. Their nervous systems are indeed much more distributed than ours, though they still do have integrating centers in their heads. And you are correct, one of the primary functions of the head is indeed the digestive "widgets" (mandibles, associated food-sensing structures, entry point to the gut, etc.)

I'm not well versed in how neuronal development and control is affected by molting. I'll have to look that up and get back to you ... I'm curious myself.
September 17, 2004, 11:25:29 PM PDT – Like – Reply

Semantic Compositions
I want to be 100% sure I'm not misunderstanding something here. Aside from the digestive widgets, is there anything actually essential to the caterpillar's continued survival inside their heads? A brain? Breathing organs of any sort? Or is the head largely distinguished by being at an end and having mandibles? If the latter isn't the case, how does the caterpillar transfer control from one head to the other?
September 16, 2004, 11:55:15 PM PDT