FLIGHT OF INSECTS. 353 



Although the greater number of insects have 

 four wings, there are many, such as the common 

 house fly, and the gnat, which have only two. 

 These compose the order Diptera (Fig. 162). 

 In these insects we meet with two organs, con- 

 sisting of cylindrical filaments, terminated in a 

 clubbed extremity ; one arising from each side 

 of the thorax (as seen in the above figure), in 

 the situation in which the second pair of wings 

 originate in those insects that have four wings. 

 They are named the halteres, or poisers, from 

 their supposed use in balancing the body, or 

 adjusting with exactness the centre of gravity 

 when the insect is flying. Whatever may be 

 their real utility, they may still be regarded as 

 rudiments of a second pair of wings ; and they 

 afford, therefore, when thus viewed, a striking 

 instance of the operation of the tendency which 



conveyed into this bladder by means of a long convoluted 

 vessel, c, which receives it from the secreting organs, s. These 

 organs consist of two somewhat dilated vessels resembling cceca, 

 but which have each a slender secretory vessel extending 

 from them. The sting moves in a tubular sheath, V; which is 

 open at its base, and along its upper surface, as far as the part 

 where the sting is prevented from being thrust out any farther. 

 The muscles which move the sheath are distinct from those of 

 the sting, and are attached to an elongated and curved part on 

 each side of its base, and to an arched and moveable part which 

 is apparently articulated with it. Swammerdam has delineated 

 these parts as caeca in his dissection of the common hive bee, but 

 has not noticed the secretory vessels. The sting of the hive bee 

 resembles that of the Anthophora retusa." 



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