414 THE WINGS OF INSECTS 



composed of two or more veins or sections of veins joined end to end with 

 no indication of the point of union; such veins are termed serial veins. 

 There are two serial veins in the fore wing of Odontaulacus editus; one of 

 these consists of a part of the radial sector and the radial cross-vein; the 

 other, of another part of the radial sector and the radio-medial cross-vein. 

 The former is designated as vein r 8c R^; the latter as vein r-m & R^. The 

 sign & is used in these designations instead of + , as the latter is used to 

 indicate compound veins formed by the coalescence of veins side b}^ side. 



Complete the lettering of the veins in the figure of the wing of Odontau- 

 lacus editus. 



The reduction in the number of the wing-veins by the coalescence of 

 adjacent veins. — This method of specialization by reduction is well-shown 

 in many Hymenoptera of which the following is an example. 



The fore wing of an Ichneumon-fly. — ^A figure of a fore wing of an Ichneu- 

 mon-fly, Exetastes fascipennis, will be provided for study. Compare your 

 figures of the wings of Sirex, Odontaulacus and an Ichneumon-fly and deter- 

 mine the composition of that part of the thickened costal margin of the wing 

 of an Ichneumon-fly that extends from the base of the wing to the stigma ; 

 letter this vein. 



Additional examples of specialization by reduction. — Continue the study 

 of the fore wing of an Ichneumon-fly. 



The Ichneimion-flies belong to a series of families, the parasitic Hymen- 

 optera, in which vein R5 is usually lost. In Odontaulacus editus, which also 

 belongs to this series of families, there is a small vestige of this vein retained; 

 but in the Ichneumon-flies it is lost completely. With this information in 

 mind proceed to letter the veins in the figure of a fore wing of an Ichneumon- 

 fly. 



A wing of a braconid. — A figure of a fore wing of a braconid, RJiogas 

 parasiticus, will be provided for study. 



Letter the veins in this wing. 



Compare the figures of a wing of an Ichneumon-fly and a wing of a 

 braconid and state in what features each wing is more generalized than 

 the other. 



THE TRACHEATION OF THE WINGS OF NYMPHS AND OF PUPJE 



It has been found that, in the course of the development of the wings of 

 the more generalized insects, the tracheas which traverse the principal 

 veins are developed before the veins appear, and that later the veins are 

 developed about these trachese. It is evident, therefore, that much light 

 can be thrown upon questions regarding the homologies of wing-veins by 

 studies of the tracheae which precede them; and the following suggestions 

 are given to aid students who wish to make such studies. 



If a living pupa or nymph be placed in formol (4%) the tissues of the 

 wings will be rendered translucent in a short time. In the case of very 



