62 



THE GENERAL FEATURES OF WINGS 



Fig. 46. — Wings of Thyridopleryx ephemercs- 

 formis; f, frenulum; / h, frenulum hook. 



base of the fore wing; but unlike the jugum it extends back above the base 

 of the hind wing and is clasped over an elevated part of the hind wing. 



This organ may be termed the 

 R,^^,^^'^''^ _ fibula or clasp. 



The fibula differs in different 

 insects in respect to the part of the 

 wing that enters into its composi- 

 tion. In some insects it is strictly 

 homologous with the jugum being 

 composed merely of the posterior 

 lobe of the wing; in other insects 

 it consists of the posterior lobe of 

 the wing and a part of the axillary 

 membrane. These two types are 

 illustrated by the fibulas of Rhya- 

 cophala and of Corydaliis respec- 

 tively. 



The fibula of Rhyacophila, a 

 generalized member of the order 

 Trichoptera, is a hatchet-shaped 

 lobe at the base of the wing (Fig. 49) . 

 It is joined to that part of the anal area lying in front of it by the axillary 

 furrow, which acts as a hinge. It is evident that the structure of the 

 wing is such that the fibula 

 is depressed with consider- 

 able force, for it is very diffi- 

 cult to mount a wing without 

 the fibula being folded under 

 it. As the longitudinal free 

 margin of the fibula is curved 

 down it seems probable that 

 the fibula clasps the anterior 

 tuberosity of the hind wing 

 and thus the two wings are 

 held together. 



Although the fibula of 

 Rhyacophila differs greatly 

 in appearance from the 

 jugum of the Hepialidae and 

 functions in a different man- 

 ner, the two are formed of 

 homologous parts of the wing. In Rhyacophila the axillary furrow is 

 immediately behind vein 3dAi (Fig. 49); in the Hepialidae there is a slit 



Fig. 47. — Wings of a hepirilid, seen from below. 



