1938 ] 
Lines of Descent of Insects 
165 
THE INTERRELATIONSHIPS AND LINES OF 
DESCENT OF LIVING INSECTS 
By G. C. Crampton, 
Massachusetts State College, Amherst, Mass. 
The lines of descent shown in the accompanying phylo- 
genetic tree should be interpreted as though the figure were 
a three dimensional one (as is indicated by the basal attach- 
ments of the branches of the tree), since the usual method 
of portraying the lines of descent in the form of a dicho- 
tomously branching tree, drawn in one plane, does not bring 
out the fact that several lines of descent may converge upon 
a common ancestry, and does not indicate the complicated 
interrelationships of these lines of descent at all accurately. 
In fact, sections of cones made up of converging lines would 
better illustrate the fact that some lines of descent inter- 
grade “horizontally” as well as “vertically,” but the method 
of illustrating the interrelationships of the lines of descent 
shown in the accompanying figure will serve well enough 
for all practical purposes, if the figure is interpreted as a 
three dimensional one. 
The hypothetical “Protomalacostraca” shown at the base 
of the phylogenetic tree, represent the extinct common an- 
cestors of the higher Crustacea (such as the Tanaidacea, 
Mysidacea, Anaspidacea, etc.) insects and “myriopods”. 
The character of the head, with its sessile eyes, the mono- 
condylar mandibles, with their differentiated incisor and 
projecting molar regions, the large paragnaths, the slender 
multiarticulate, cerci-like uropods, and other feature of 
the Tanaidacea (such as Tanais, Apseudes, Leptochelia, 
etc.) are strikingly suggestive of the precursors of similar 
structures in the Machiloid insects, and the ancestors of 
the Tanaidacea (represented by the hypothetical “Proto- 
tanaidacea in the diagram) must have been extremely closely 
related to the more direct ancestors of the Hexapoda (rep- 
resented in the diagram by the hypothetical “Protohexa- 
