HOUSE FLIES AND BLUEBOTTLES 



183 



is the significance of these alulce. Though connected with 

 the wings, they can hardly have much influence upon 

 flight, and the peculiar arrangement of the double fold, 

 together with the remarkable profusion of hairs, seems 

 to suggest some other function. They reach their highest 

 development in the family Muscidce. 



A little distance beneath the larger fold of each alula, 

 and entirely overarched and concealed 

 by it, is an organ (Fig. 58) which is 

 highly characteristic of flies. It consists 

 of a slender stalk carrying a globular 

 expansion at its outer end, and near 

 the point of attachment of the stalk to 

 the thorax are three minute sets of 

 rows of papillae with hairs. These 

 stalked globes are called halteres, 

 balancers, or poisers. They are most 

 conspicuous in such flies as the daddy 

 long-legs, or crane flies, and in the 

 bluebottle are reduced to extremely 

 small dimensions, so that they are not 

 likely to be noticed at all unless care- 

 fully looked for. It is curious that the 

 development of the alulce is always in 

 inverse proportion to that of the hal- 

 teres. Though so minute, their struc- 

 ture is sufficiently elaborate to suggest 

 that they must be of considerable impor- 

 tance in the economy of the insect, and 

 many different functions have been more or less con- 

 jecturally assigned to them. Their names, as above, 

 indicate a notion once current that they helped the insect 

 to maintain its equilibrium during flight ; they have, 

 again, been considered to be organs of hearing, or to be 



FIG. 



Bluebottle (Calli- 

 pkora). Showing 

 Basal Papillae and 

 Nerve. 



