424 NORTH AMERICAN INDEX FOSSILS. 



them disappear. Thus in the Cicada wing (Fig. 1725) cell R^^ 

 has disappeared by the coalescence of veins R^ and R-. Such 

 disappearance can usually only be determined by a study of imma- 

 ture stages, or primitive types. In the strongly veined insect 

 wings, the simple cells are subdivided by numerous longitudinal 

 and cross veins, and the names of the original cells are then 

 applied to the corresponding areas of the much divided wing. 



A series of folds or furrows further exist on many wings. 

 These serve either for strengthening the wing, or are the result 

 of folding of the wing when at rest. The subcostal furrow lies 

 between costa and radius, with the subcostal vein at its bottom. 

 It is a strengthening furrow. Other furrows are: the anal fur- 

 row, usually between the cubitus and first anal vein (Fig. 1737), 

 and the nodal furrow, extending from the costa to the inner 

 margin. 



The wings themselves are frequently more or less modified. 

 Thus in the great order Coleoptera or beetles, the anterior wings 

 are replaced by, or modified into, a pair of horny sheaths or elytra 

 which close together over the back of the insect, concealing the 

 hind wings (Figs. 1747-49). According to some authors, these 

 are not the homologues of the front wings, but of the tegulce or 

 paraptera of other insects {i. e., the small sclerites at the base 

 of the wing so well developed in the Hemiptera). Comstock and 

 Needham however conclude from their study of the tracheation of 

 the elytra, that they are modified wings. In the Diptera (flies), 

 the posterior wings are wanting entirely while in the Strepsiptera, 

 the anterior wings are replaced by small appendages, the posterior 

 wings being large. 



The abdomen is typically composed of ten segments, though one 

 or more of the terminal segments is commonly much modified, 

 or even wholly withdrawn into the interior of the body. The an- 

 terior segment may be more intimately related to the metathorax 

 than to the rest of the abdomen, becoming a median segment. The 

 unequal enlargement of one of the segments as in the ants, may 

 also greatly modify the abdomen. Each segment is typically pro- 

 vided with a dorsal and a ventral plate, while a stigma or breath- 

 ing pore is generally present on each side. These latter may how- 

 ever be greatly reduced in number, or even disappear altogether. 



