PHILIP A. MUNZ 31 



Table III 

 The Ratio of the Width of the Wing to the Length, in the 

 Pseudostigmatinae. 



Genus and Typical Species Ratio, as Decimal 



Megaloprepiis coeruleatus . 267 



Mlcrostigma exustiim .237 



Anoniisma ab)wrnie .204 



Pseiidostigma aberrans .177 



Mecistogaster uuirchali .148 



2. Progressive petiolation. — In each subfamily except the Agrion- 

 inae, where speciaHzation is along a different direction, there is a 

 ^'ery noticeable tendency toward greater petiolation; it has been 

 referred to constantly in the discussion on phylogeny and is used 

 often in the keys, offering as it does an easy index to the amount of 

 departure from the primitive. The Protoneurinae might be con- 

 sidered an exception, since in them, A becomes shorter and the 

 reduction process is a matter of retraction. But in both cases the 

 result is the narrowing of the wing near the base. Dr. Calvert 

 discusses petiolation from the paleontological standpoint and says, 

 "It seems justifiable to assume that the farther distad petiolation 

 has progressed, the more specialized the insect." 



Petiolation reaches its highest development in the Megapoda- 

 grioninae where in Chorismagrion (fig. 69) and Perilestes (fig. 70) 

 it extends to beyond the level of MA. 



3. Retraction of the nodus toward the wing base. — This is one of 

 the most common and evident occurrences in specialization and is 

 discussed in detail by both Professors Needham and Calvert. In 

 the Agrionidae the nodus is almost uni\'ersally at more than one- 

 third the distance from the base of the wing toward the tii), in the 

 Coenagrionidae at generally less. 



4. Tendency for M->, i^s, and Mz to arise nearer the tip of the wing 

 and further from the subnodus. — As Dr. Calvert remarks this out- 

 ward "migration may be real or apparent. Retraction of the 

 nodus with no change in position of the separation point of Mo 

 would result in a greater interval between nodus and M- and hence 

 an apparent movement of the latter toward the stigma, but no real 

 movement in respect to the, wing as a whole." His table for 



MEM. AM. ENT. SOC, 3. 



