MONITORS 223 



filings instead of sand it puts some of them into the statocyst 

 where, under the influence of gravity, they press upon the 

 sensory cells at the bottom. If a strong magnet is held above 

 the animal the filings are lifted from the bottom of the 

 statocysts, and the animal turns upon its back so that they 

 again press upon the cells at the bottom ; the animal, adapted 

 to respond to gravity, is completely misled by the trick. 



Most insects have two pairs of functional wings, the front 

 and hind, but the flies, comprising the order Diptera, have 

 only one, the front. The hind pair of wings in the flies is 

 represented by two little rods shaped like drumsticks that 

 project from the side of the body just behind the wings. When 

 a fly is flying, the little rods, known as halteres, vibrate at a 

 speed so high that they produce a gyroscopic effect which 



HALTERE 



Fig. 20. A mosquito {Anopheles) at rest with the hind feet raised from the 

 perch. The haltere, or modified hind wing, is visible below the base of the 

 front wing. 



enables the fly to orientate itself and keep on an even keel. 

 They are proprioceptors that monitor the animal's flight. 

 The common house-fly, that can alight upside down on the 

 ceiling and is so agile at dodging when you try to swat it, 

 must have a very complex nervous mechanism for co-ordinat- 

 ing its movements in which the halteres play an important 

 part. The apparently erratic movements led some wag to 

 wise-crack, "Time flies. We cannot — their flight is too 

 irregular." 



Other proprioceptive senses are served by a complex of 

 sensations from different kinds of sense-organs and chemical 



