J. D. Tothili — The Ancestry of Insects. 



377 



recent aquatic insects such as Sialis and Sisyra there are serially 

 arranged jointed appendages functioning as gills, but all such 

 structures in recent insects are shown by embryology to be 

 secondarily acquired. It seems fairly certain that the similar 

 structures in Stenodictya are secondarily derived and that 

 insects were originally terrestrial ; in this case the structures 

 would have no phylogenetic significance. 



Returning to our problem, it may be of interest to construct 

 from the available data a hypothetical ancestor for the Ptery- 



Fig. 2. 



Fig. 2. A. Stenodictya (Palaeodictyoptera), one of the most generalized 

 of all Pterygogenea. (After Handlirsch.) 



B. Larva of Stenodictya. The abdominal appendages are probably 

 secondary. (After Handlirsch.) 



gogenea. As wings seem to have arisen in the Palseodictyoptera 

 the ancestor would have no wings and consequently the thoracic 

 segments would be no larger than the abdominal. The result 

 would be an animal with head and trunk, the latter with 14 

 segments (fig. 3). The habit must have been predaceous 

 which would imply a short straight alimentary tract. 



Turning to the structure of the nervous system, one of the 

 most conservative of morphological structures and consequently 

 one of the most useful for phylogenetic purposes, embryology 

 of recent forms (Viallanes, Wheeler) shows that there are six 

 neuromers in the head, a brain triad and a gnathal triad, and 

 that a pair of ganglia connected by commissures follow in each 

 segment. Cephalization would be less marked than in recent 

 insects. The nervous system is indicated in the diagram 

 (fig. 3, B). 



