STEPS IN SPECIALIZATION 119 



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 special- 

 ized 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. 



In discussing the steps in the specialization of wings several features in 

 the development and perfecting of these organs are to be considered; these 

 may be treated under separate heads, as follows: 



(a) THE DEVELOPMENT OF THE WINGS 



(i.) The development of wings externally. 



This is the more primitive method of development of wings and is 

 characteristic of the more generalized orders of insects, those in which the 

 metamorphosis is either of the type known as gradual or of the type known 

 as incomplete. 



(2) The development of wings internally. 



This is an acquired method of development of wings and is characteristic 

 of the more highly specialized orders of insects, those in which the type 

 of metamorphosis is that known as complete. 



It is not probable that the orders in which the metamorphosis is com- 

 plete constitute a monophylitic group ; the indications being that this type 

 of metamorphosis has arisen independently in different parts of the insect 

 series. 



(6) THE BASAL CONNECTIONS OF THE TRACHEA THAT PRECEDE 

 THE WING-VEINS 



(i.) The costo-radial and the cubito-anal groups of tracheae distinct, 

 i. e. with no transverse basal trachea. Correlated with this condition, the 

 medial trachea is a member of the costo-radial group. 



(2.) The costo-radial and the cubito-anal groups of tracheae united by 

 a transverse basal trachea. Correlated with this are the successive stages 

 in the migration of the base of the medial trachea, along the transverse 

 basal trachea, from the costo-radial group of trachea to the cubito-anal 

 group. 



(c) THE CROSS-VEINS 



(i.) The areas of the wings between the longitudinal veins traversed 

 by an irregular network of veins. 



(2.) With numerous regular cross-veins. 



(3.) With the cross-veins greatly reduced in number. 



