308 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



pressure resulting in morphological change in the nerve pattern from 

 that source than from changes in the structures served by the nerves. 



Many apparent dififerences in nerve patterns may exist w^ithout in- 

 volving fundamental morphological differences. In one insect, two 

 groups of axones may adopt a common path for a greater distance, 

 or in a second, may separate at a point closer to the ganglion. In the 

 first case, a single definitive nerve results, vi^hile in the second, two 

 nerves will be found in a nerve-muscle relationship in which only a 

 single nerve exists in the first insect. A simple example of this may 

 be seen in the segmental nerve patterns of the pregenital abdominal 

 segments of Dissosteira (fig. 5 A) and Diapheromera (fig. 5 B). In 

 Dissosteira, two prominent nerve roots leave the ganglion, but in 

 Diapheromera, the same nerve elements are combined and leave the 

 ganglion as a single nerve. 



The problem of recognizing homology in nerves thus depends upon 

 the establishment of criteria of homology independent of these defini- 

 tive disguises. A second objective of this paper is therefore an ex- 

 amination of the nerve patterns of the thorax of Dissosteira, to dis- 

 cover, if possible, any such criteria of nerve homologies in the thorax. 

 The writer recognizes, of course, the fact that detailed studies of 

 many more forms are prerequisite to the assured establishing of the 

 scope of any such criteria. 



The choice of Dissosteira as the insect to be studied arose mainly 

 from the fact that Dr. R. E. Snodgrass has described its skeletal struc- 

 tures and musculature with clarity and precision. His 1929 paper, 

 "The Thoracic Mechanism of a Grasshopper, and its Antecedents," is 

 especially valuable. The groupings, designations, and numbers as- 

 signed to the muscles by Snodgrass are employed throughout this 

 paper. 



The considerable extent to which morphologically homologous 

 nerves may vary in appearance creates a difficult problem in nomen- 

 clature. Some workers have assigned individual names to the nerves 

 of a particular species, as in Korshelt's work on Dytiscus, but in most 

 instances such names are of very limited value in comparative studies, 

 Maki (1936) in describing the nervous system of Chauliodes, used an 

 elaborate system of roman numerals, lowercase letters, and arable 

 numerals. The roman numerals designate the "nerve roots" or largest 

 nerves as they leave the ganglion, and successive branchings from the 

 "root" are indicated by appropriate letters and numbers. This is at 

 least a workable system for tentative use in that it is sufficiently 

 flexible to accommodate losses or additions of branches without alter- 

 ing the main designations. 



