  A caterpillar tree is a connected, acyclic, graph in which all vertices are
either a member of a central path, or joined to that central path by a single
edge. In other words, caterpillar trees are the class of trees which become
path graphs after removing all leaves. In 1973, F. Harary and A.J. Schwenk
provided two proofs found in [1] which show that the number of non-isomorphic
caterpillars with N vertices is given by the formula $2^{N-4} + \ 2^{ \lfloor
\frac{N - 4}{2}\rfloor}$, where $\lfloor \ \rfloor$ denotes the floor function.
The first proof follows from a special case of an application of P\'{o}lya's
Enumeration theorem on graphs with integer-weighted vertices. The second proof
proceeds through an appropriate edge labelling of the caterpillars. The proof
presented here owes much of its insight to the first two, but has the benefit
of utilizing a natural labelling for the caterpillars. We will proceed by
labelling the vertices of the caterpillars with integer-weights, followed by an
application of the orbit-counting theorem.
