4 6 



ENTOMOLOGY 



Iron. Diptera have, in place of the hind wings, a pair of clubbed threads, 

 known as balancers, or halteres, and male Coccidae have on each side a 

 bristle that hooks into a pocket on the wing and serves to support the 

 latter. In many muscid flies a doubly lobed membranous squama occurs 

 at the base of the wing. 



In Hymenoptera the front and hind wings of the same side are held 

 together by a row of hooks (hamuli); these are situated on the costal 

 margin of the hind wing and clutch a rod-like fold of the fore wing. In 

 very many moths, the two wings are enabled to act as one by means of a 

 frenulum, consisting of a spine or a bunch of bristles near the base of the 

 hind wing, which, in some forms, engage a membranous loop on the fore 

 wing. 



Venation, or Neuration. A wing is divided by its veins, or ner- 

 vures, into spaces, or cells. The distribution of the veins is of great sys- 

 tematic importance, 

 but, unfortunately, 

 the homologies of the 

 veins in the different 

 orders of insects have 

 not been fixed until 

 recently, so that no 

 little confusion has 

 existed upon the sub- 

 ject. For example, 

 the term discal cell, 

 used in descriptions of Lepidoptera, Diptera, Trichoptera and Psocidae, 

 has in no two of these groups been applied to the same cell. The 

 admirable work of Comstock and Needham, however, seems to settle 

 this disputed subject. By a study of the tracheae which precede and, in a 

 broad way, determine the positions of the veins, these authors have ar- 

 rived at a primitive type of tracheation (Fig. 67) to which the more com- 

 plex types of tracheation and venation may be referred. 



In general, the following principal longitudinal veins may be distin- 

 guished, in the following order : costa, subcosta, radius, media, cubitus, and 

 anal (Figs. 67-71). 



The costa (C) strengthens the front margin, of the wing and is essen- 

 tially unbranched. 



The subcosta (Sc) is close behind the costa and is unbranched in the 

 imagines of many orders in which there are few wing veins, though it is 

 typically a forked vein. 



M4 



3d A 2dA 



IstA 



Cu2 



FIG. 67. Hypothetical type of venation. A , anal vein; C, 

 costa; CM, cubitus; M , media; R, radius; Sc, subcosta. Figs. 

 67-71 after COMSTOCK and NEEDHAM. 



