182 OUR HOUSEHOLD INSECTS 



muscles are not attached to the wings themselves, but 

 run from one part of the walls of the thorax across to 

 the opposite wall, so that the greater part of the vibra- 

 tion of the wings is produced by alternating changes in 

 the shape of the thorax. 



.fJifc 



FIG. 57. Right Winglet of Bluebottle (Calliphora). 



At the base of each wing is a double membranous scale, 

 the alula, or "winglet" (Fig. 57); each of its divisions 

 has a rounded and thickened outer edge, and the membrane 

 is extremely closely covered with minute hairs similar to 

 those on the wings, the larger part having in addition 

 some long flexible hairs arranged pretty regularly but 

 not closely, in rows. The free edge of each is also very 

 closely fringed with delicate hairs. When the wings are 

 stretched out for flight, these scales form a continuation 

 of their area as far as the centre of the hind margin of 

 the thorax; hence the inner scale has, as shown in the 

 accompanying figure, a sloping edge where it fits under 

 the side of the triangular termination of the thorax. But 

 when the wings are closed, the outer scale is folded over 

 the inner side along their line of junction, as well as 

 under the wing itself, so that they lie one upon another 

 like the leaves of a book. It is impossible to say what 



