284 PTILOTA: IMAGO EXTERNAL ANATOMY. 



in a horizontal direction, where they are extended laterally, or 

 in a vertical position, where they do not extend laterally (as 

 in the burying beetles, Necrophorus). Ordinarily, as their 

 name imports, they cover the wings ; but in some beetles the 

 wings are not thus defended, owing to the small or irregular 

 form of the wing-covers. In the Brachelytra, however, in 

 which the elytra are of the least size, and the wings large, 

 they possess their ordinary function, the wings being very 

 much folded. In some beetles the wings are entirely want- 

 ing ; and here the elytra are firmly soldered together, thus 

 becoming a defence for the abdomen, the upper surface of 

 which is, in such cases, soft and membranous. 



(e) Halteres (see figs. 126 and 127, z z). These organs, 

 which are usually termed balancers or poisers, are a pair of 

 short threads arising behind the base of each of the wings in 

 the Diptera (to which order they are exclusively confined), 

 and terminated by a small oval or triangular knob. These 

 organs are capable of a very rapid motion ; and it has been 

 usually supposed that by their beating upon the winglets 

 (alulae), they produce the humming noise so distinctly heard 

 in this group of insects during flight. That they do thus 

 beat upon the winglets, I have already observed ; but it ap- 

 pears doubtful whether this action be the cause of tin- sound 

 in question, because those Diptera which do not, either by 

 nature or artificially, possess winglets, make a buzzing noise. 

 The same is also the case with the bees and other Hymeno- 

 ptera. Other naturalists have supposed the noise to originate 

 in the motion of the wings themselves, or (with more proba- 

 bility) in the rushing of the air through a spiracle near tlie 

 base of the halteres ; in which case, the sole use of the hal- 

 teres will be as poisers to keep the body steady in flight. 

 Thus Schelver cut off the halteres from a fly, and found that 

 it could no longer fly ; hence (and from other circumstances) 

 lie considered that the halteres are connected with respira- 



