DIPTERA. 411 



ORDER XI. DIPTERA. 



This order is distinguished by the presence of only a pair 

 of membranous extended wings affixed to the centre of the 

 sides of the thorax, and generally furnished near their in- 

 sertion with a pair of short clubbed organs, termed halteres 

 or balancers, respecting which, as analogically representing 

 the posterior wings of the four-winged insects, entomologists 

 are at variance. The mouth is provided with a sucker com- 

 posed of from two to six scaly lancet-like pieces, and inclosed 

 in a canal upon the upper sm-face of a fleshy proboscis, or 

 covered by one or two inarticvUated plates, which serve as a 

 sheath {Hippobosca). The body is composed, as in the other 

 six-legged insects, of three principal divisions, representing 

 the head, thorax, and abdomen. The ocelli, when present, 

 are generally three in number. The antennae exhibit vari- 

 ous degrees of developement, from the exceeding length in 

 the Macrocerce and Megistocerce to their scarcely visible size 

 in the Hippoboscce. In the Tipulidce we find these organs 

 composed of numerous articulations. In the Tabanidce and 



A, Antennae of Tipula— d, of Tabanus— c, of Musca. 



Beridcs they are short, and the terminal articulations be- 

 come almost indistinct, so that in the Muscidce, &c., where 

 they are very short, and appear to be composed only of three 

 joints, having a terminal bristle, it is impossible not at once 



