MOLLUSCA. , 525 



which they are covered, and the form of the vacant spaces, 

 exhibit great regularity in the individuals of the same spe- 

 cies. The manner in which the membranaceous wings are 

 folded up, wlien at rest, is various. In some they are fold- 

 ed longitudinally, in others transversely, and in others 

 obliquely. Each of these arrangements prevails throughout 

 extensive groups. In the Diptera there is, under or be- 

 hind each wing, a stalk terminating in a small knob. These 

 are termed halteres, or poizers^ and are considered as the 

 rudiments of the second pair of wings. Between each poi- 

 zer and the base of the wing, one or two spoon-like scales 

 are found, termed winglets. They have likewise been ob- 

 served in a coleopterous insect, Dytiscus marg'malis. In 

 some of the diptera they are absent. The use of these ap- 

 pendices of the wings has not been satisfactorily determin- 

 ed. The muscles which move tlie wings take their rise in 

 the breast, and are capable of executing their functions 

 with great celerity. The elytra perform no other motion 

 than elevation and depression, and serve merely to protect 

 the wings when at rest, not to assist them when flying. 



The characters employed in the classification of the pri- 

 mary divisions of insects are, in a great measure, derived 

 from the wings. Their presence or absence — their num- 

 ber and appendices — their texture and consistence, toge- 

 ther with their size, position, and manner of folding up, 

 yield marks which are of easy detection, and which expe- 

 rience has found to be permanent. 



The legs never fall short of six in number. The first 

 pair, when six only are present, take their rise under the 

 thorax, and the second and third pairs under the breast. 

 When more numerous, some of them are attached to the 

 abdomen. They consist of the live following paits, the 

 hip {coxa)^ the thigh {Jemur), the leg {tibia), the toe 

 {tarsus), and the claw {w/gui.s). 



