92 University of California Publications, r Entomology 



is disturbed by the interpolation of the independent veins, 

 which crowd off those behind the primary; but these new veins 

 fall into the general scheme, so that instead of fiye alternating 

 veins there are seven. With the branching of the independents 

 the same process is repeated. All this time the veins have been 

 coming to lie more nearly in one plane, so that the alternation 

 is not so evident. This wing never exhibits the fixity and 

 regularity of alternation seen in the Ephemerida^, except at 

 the extreme base. 



Among the independents one may see quite distinctly a 

 feature which later in the Odonata comes to be very charac- 

 teristic. It is the tendency toward the production at regular 

 intervals along the outer part of the membrane of a series of 



pairs of long, un- 

 branched veins (Fig. 

 27). These can be 

 clearly distinguished 

 from the intervening 



FIG. 27. Diagram of the venation of the Prot- veiuS by the fact that 

 odonata. The short lines represent independents \\\q surface of the 

 without basal attachments. 



wing membrane be- 

 tween them is nearly perpendicular to the plane of the wing. 

 With their connecting cross veins they form a simple truss. 

 Some Ephemeridse exhibit the same tendency, and it is well 

 developed in most Odonata. 



The only other feature worthy of particular note in regard 

 to this venation is the evidence of a tendency to narrow some- 

 what the space between the primary and the first posterior 

 just before they are pushed apart by the independents. This 

 suggests that at this point arises the arculus, which is the most 

 prominent structural difference between the wings of the Prot- 

 odonata and the Odonata. 



The structures exhibited by the Protodonata are thus exactly 

 in accord with the supposed ancestral position of the group, 

 and clearly indicate, though they do not attain to, the venation 

 of the Odonata. 



ODONATA. 



There is no group of insects so nearly isolated from all other 

 existing forms as the Odonata. The whole thoracic organiza- 

 tion forms a special type. Only the Ephemerida^ possess in 

 common with it the peculiarities of hinge structure and muscle 



