THE EXTERNAL ANATOMY OF INSECTS 



61 



The axillary excision. — In the wings of most Diptera and in the 

 wings of many other insects there is a notch in the inner margin of 

 the wing near its base (Fig. 71, ae), this is the axillary excision. 



The posterior lobe of the wing. — That part of the wing lying between 

 the axillary excision when it exists, and the axillary membrane is the 

 posterior lobe of the wing. The posterior lobe of the wing and an alula 

 are easily differentiated as the alula is margined by the axillary cord. 



The methods of uniting the two wings of each side. — It is obvious 

 that a provision for ensuring the synchronous action of the fore and 

 hind wings adds to their efficiency; it is as important that the two 

 pairs of wings should act as a unit as it is that the members of a boat's 

 crew should pull together. In many insects the synchronous action 

 of the wings is ensured by the fore wing overlapping the hind wing. 

 But in other insects special structures have been developed which 

 fasten together the two wings of each side. The different types of 

 these structures have received special names as follows: 



The hamuli. — With certain insects the costal margin of the hind 

 wings bears a row of hooks, which fasten into a fold on the inner 

 margin of the fore wings (Fig. 72) ; these hooks are named the hamuli. 



The frenulum and the frenulum hook. — In most moths there is a 

 strong spine-like organ or a 

 bunch of bristles borne by the 

 hind wing at the humeral 

 angle (Fig. 73,/); this is the 

 frenulum or little bridle. As a 

 rule the frenulum of the female 

 consists of several bristles ; that 

 of the male, of a single, strong, 

 spine-like organ. In the males 

 of certain moths, where the 

 frenulimi is highly developed, 

 there is a membranous fold on 

 the fore wing for recei\-ing the 

 end of the frenulum, this is the 

 frenulum hook (Fig. ^Sffh). 



The jugum. — In one family 

 of moths, the Hepialidse, the 

 posterior lobe of the fore wing 

 is a slender, finger-like organ 

 which is stiffened by a branch 

 of the third anal vein, and which projects beneath the costal margin 

 of the hind wing. As the greater part of the inner margin of the fore 



Fig- 73- — Wings of Thyridnpteryx ephemercB- 

 formis; f, frpnulum; fh, frenulum hook. 



