210 AN INTRODUCTION TO ENTOMOLOGY 



BB. Wings developed internally. 

 C. Fore wings greatly thickened. 



D. Fore wings modified so as to serve as covers of the posterior wings 



COLEOPTERA 



DD. Fore wings reduced to slender, leathery, club-shaped appendages 



Strepsiptera 



CC. The two pairs of wings similar in texture. 



D. With the tendency to develop accessory veins retained. .Mecoptera 

 DD. With the tendency to develop accessory veins lost. 



E. The most characteristic method of reduction of the wing-veins 

 of the preanal area being by coalescence outward. 

 F. Anal veins of the fore wings tending to coalesce at the tip. Wings 



usually clothed with hairs Trichoptera 



FF. Anal veins of the fore wings not tending to coalesce at the 



tip. Wings clothed with scales Lepidoptera 



EE. The most characteristic method of reduction of the wing-veins 

 of the preanal area being by coalescence from the margin of the 

 wing inward. 



F. With only one pair of wings Diptera 



FF. With two pairs of wings Hymenoptera 



The sequence in which the orders of insects are discussed in the 

 following chapters has been determined by the above table. This 

 sequence, lil^e all linear arrangements of groups of organisms, is more 

 or less arbitrary. Thus while there is an effort to place first the more 

 generalized orders and later those that are more specialized, the 

 putting together of orders exhibiting the same type of specialization 

 results in some cases in the placing of comparatively generalized 

 forms after those that are obviously more highly specialized. The 

 position of the Plecoptera is an illustration of this. The insects of 

 this order are evidently more generalized than, for example, the 

 Neuroptera or the Odonata, which are placed earlier in the linear 

 series. 



The comparatively high position assigned to the Plecoptera is, 

 however, only apparent. A reference to the table will show that the 

 orders of insects are grouped in two series, "A" and "AA". Under 

 "A" are placed those orders in which the wings are specialized by 

 addition in the preanal area, and under "AA" those orders in which the 

 wings are specialized by reduction in the preanal area. Each of these 

 series includes some quite generalized insects and others that are 

 highly specialized. The completion of the discussion of the first series 

 before taking up the second series results in the generalized members 

 of the second series following the highly specialized members of the 

 first series. 



The more generalized members of these two series, the Orthoptera 

 of the first series and the Plecoptera of the second series, are probably 

 more closely allied to each other than is either of these orders to the 

 more specialized orders of the series in which it is placed; the two 

 series arose from a common starting point, the Palaeodictyoptera, but 

 have widely diverged in the course of their development. 



An even more striking illustration of the difficulty of indicating 

 the relative ranks of orders by the use of a single linear series is the 

 position of the Isoptera in the above table. This order is a very 



