248 



THE MECHANICAL FUNCTIONS. 



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- 

 sistino" of .cylindrical filaments, terminated in a clubbed ex- 

 tremity; 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. 



capable of being- separated. The point, v, is directed a little upwards, and 

 is a little curved: the barbs, seen still more highly magnified at a, are about 

 six in number, and are placed on the under surface, and their points directed 

 backwards. At the base of the sting-, e, there is a semicircular dilatation 



apparently intended to prevent the in- 

 strument from being thrust too far out 

 of the sheath (seen separately at y,) in 

 which it moves: it has also a long ten- 

 don to v/liich the muscles are attached. 

 It is between these plates, when ap- 

 proximated, that the poison flows from 

 the orifice of the somewhat dilated ex- 

 tremity of the poison duct, D, which 

 comes from the anterior part of the 

 poison bag, b. This bag is of an oval 

 shape, and is not the organ which se- 

 cretes the poison, but merely a recep- 

 tacle for containing it: for it is conveyed 

 into this bladder by means of a long con- 

 voluted vessel, c, which receives it from 

 the secreting organs, s. These organs 

 consist of two somewhat dilated vessels 

 resembling caeca, 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 mus- 

 cles which move the sheath are distinct from those of the sting, and are at- 

 tached 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. Swam- 

 merdam 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.'' 



