154 TEXT-BOOK OF ENTOMOLOGY 



of the wing-membrane, which is very extensive and complicated, and therefore 

 indicates the performance of an important function, perhaps of a kind of 

 balancing sense, and also that these same insect wings, with their delicate 

 membrane, are very easily affected by different outside agents, as, for instance, 

 warmth, currents of air, etc. 



Usually in their inactive or passive state the wings are held off 

 horizontally from the body during flight, and are laid upon the back 

 again when the insect alights ; but an exception occurs in most 

 butterflies and Neuroptera, among which the wing-joint allows only 

 one movement round the oblique and long axis of the wings. From 



FIG. 169. Anterior part of a Cicada for demonstrating the mechanism of the articulation of the 

 fore wing : a, articular head ; b, articular pan, frog, or cotyla ; g, elastic band ; c, d, e, system of 

 elastic rods ; r^, r t , 1st and 2d abdominal segments. Iff, hind wings. After Graber. 



this cause, too, the insects just mentioned can unfold their wings 

 suddenly. 



The transition of the wings from the active to the resting condi- 

 tion seems to be by way of a purely passive process, which, there- 

 fore, usually gives no trouble to the insect. The wing being 

 extended by the tractive power of the muscles, flies back, when this 

 ceases, to its former or resting posture by means of its natural elas- 

 ticity, like a spiral spring disturbed from its balance. The structure 

 of this spring joint is very different, however. 



It usually consists (Fig. 169) of two parts. The wing can move itself up and 

 down in a vertical plane by means of the forward joint, and at the same time can 

 rotate somewhat round its long axis, because the chitinous part mentioned above 

 is ground off after the fashion of a mandrel. 



The hinder joint, at a greater distance from the body, virtually consists of a 

 rounded piece (a) capitate towards the outside, and of a prettily hollowed socket 

 (ft) formed by the union of the thick ribs of the hind wings, which slides round 



