THE ORIGIN AND DISPERSAL OF A GROUP OF PRIMITIVE CADDISFLIES

Abstract

In many groups of insects there are two circumstances which invite investigation as to the origin and dispersal of the members of various phyletic lines. First, there are genera existing today which are of great geologic age, dating back at least to Cretaceous times. We have fossil evidence for the longevity of some of these genera (Carpenter et al., 1937), and excellent circumstantial evidence for many more. Secondly, there has been a survival to the present of a remarkable number of living fossils belonging to ancient branches of many lines, these survivors together forming an -almost steplike sequence into the past. In many cases the existing genus representing the base of a phyletic line may be different from the ancestor which began the line, but in such slight fashion that it is 'possible to reconstruct its past connections with startling clarity. By utilizing this second phenomenon with full realization of the first, it is possible to deduce not only zoogeographic dispersal patterns for many insect groups, but in large measure to correlate these with fairly definite ecological conditions in past eras. An example is outlined here, taken from the caddisflies or Trichoptera, an aquatic order of insects. The analysis of this example is of especial interest because it suggests an application of certain postulates of Cretaceous geography associated with the Middle Cretaceous Transgression.