This caterpillar was in the act of loyally carrying out a law inflicted upon him by Nature--a law purposely inflicted upon him to get him into trouble--a law which was a trap; in pursuance of this law he made the proper preparations for turning himself into a night-moth; that is to say, he dug a little trench, a little grave, and then stretched himself out in it on his stomach and partially buried himself--then Nature was ready for him.

She blew the spores of a peculiar fungus through the air with a purpose.

Some of them fell into a crease in the back of the caterpillar's neck, and began to sprout and grow--for there was soil there--he had not washed his neck.

The roots forced themselves down into the worm's person, and rearward along through its body, sucking up the creature's juices for sap; the worm slowly died, and turned to wood.

And here he was now, a wooden caterpillar, with every detail of his former physique delicately and exactly preserved and perpetuated, and with that stem standing up out of him for his monument--monument commemorative of his own loyalty and of Nature's unfair return for it.
Nature is always acting like that.