Let us look at a picture of the May-fly larva.
You see it has a head, a thorax to which is attached the six legs and the rudimentary wings, and an abdomen, all distinctly separated from each other.
The dobson has a head, but no thorax.
The body behind the head is divided into segments that all look very much alike, and there is a pair of legs attached to each of the first three segments.
The dobson eats other larvæ that it chews up with its strong jaws.
It lives almost three years in the larval state, so you see it has plenty of time in which to grow. Of course it moults. It is usually to be found under stones in swift, running water. Those two pairs of hooks at the tip of its body are its anchors.
It clasps them about a bit of stone or a stick that is firmly lodged, and then it can bid defiance to the swirling stream.
Ned wonders why it is always found hiding under stones.
Listen to John, he says fishes are very fond of dobsons, and that is why they hide away.