THE WINGS OF INSECTS 405 



borne a greater or less number of parallel branches. This type of radial 

 sector is well-illustrated in the wings of Polystoechotes punctatus (Fig. 418). 



The transformation of a t\TDical dichotomously branched radial sector 

 into one that is pectinately branched is usually produced by a splitting 

 apart of veins R4 and R3 so that they arise separately from the supporting 

 stem of the pectinate veins thus formed. 



In the accompanying series of diagrams (Fig. 421), the first diagram 

 represents the manner of branching of the typical radius in which the radial 

 sector is dichotomously branched. The second diagram represents a radius 

 in which the radial sector has' become pectinate by the splitting apart of 

 veins Ri and R5, so that they arise separately from the supporting stem of 

 the pectinate vein thus formed. The third diagram represents a radial 

 sector in which vein R2 bears two accessory veins labeled R2a and R2b- 



The wings of Sisyra. — A figure of the wings of Sisyra flavicornis will be 

 furnished the student for study. Label the veins of both fore and hind 

 wings excepting the cross-veins and the marginal accessory veins. The 

 radial sector in these wings is an example of the simplest type of a pectinate 

 radial sector. 



A wing of Chauliodes. — A figure of a fore wing of a pupa of Chauliodes 

 will be fiu*nished the student for study. Label the tracheae. Note that the 

 forming cross-veins are not preceded by tracheae. 



In what important respect does the radial sector of this insect differ 

 from that of Sisyra. 



A wing of Corydalus. — A figure of a front wing of Corydalus conmtus will 

 be furnished the student for study. Label the veins. 



Compare the radial sector in the wings of Sisyra, Chauliodes, Corydalus, 

 and Polystcechotes. 



WINGS OF EPHEMERIDA 



Figure 422 represents the venation of a fore wing of a May-fly; this 

 figure is introduced here merely to facilitate the discussion of two features 

 of the wings of the Ephemerida ; we will not enter upon the study of the 

 homologies of the wing-veins of members of this order in this course. 



Concave and convex veins. — Examine the wings of a May-fly and note 

 that the wings are fan-like in form, due to a very regular series of corruga- 

 tions. Each wing-vein follows either the crest of a ridge or the bottom of a 

 furrow. A vein that follows the crest of a ridge is termed a convex vein, 

 and one that extends along the bottom of a furrow, a concave vein. In 

 Figure 422 the convex veins are marked with a plus sign and the concave 

 veins with a minus sign. 



Intercalary veins. — In the order Ephemerida the wings have been 

 specialized by addition; but in this order the added longitudinal veins arise 

 in a way that is very different from the manner in which the accessory veins 



