THE MUSCLES OF THE WINGS 159 



angles, which when pulling the wing sidewise, bring about in union with the 

 other muscles a screw-like turning of the wings. 



While in dragon-flies all the muscles which are principally influential in mov- 

 ing the wing are directly attached to it, and thus evidently assert their strength 

 most advantageously, the case is essentially different with all other insects. 

 Here, as has already been superficially mentioned above, the entire set of 

 muscles affecting the wing is analyzed into two parts of which the smaller 

 only is usually directly joined to the wings, while the movement is indirectly 

 influenced by the remainder (Graber). 



In the dragon-fly the two wings are "brought into correlative action by means 

 of a lever of unusual length existing amongst the chitinous pieces in the body 

 wall at the base of the wings (Fig. 175, 6). The wing-muscles are large ; 

 according to von Lendent'eld there are three elevator, five depressor, and one 

 abductor muscles to each wing. He describes the wing-movements as the 

 results of the correlative action of numerous muscles and ligaments, and of a 

 great number of chitinous pieces connected in a jointed manner" (Sharp). 



If again we take the longitudinal section of the thoracic cavity of gnats in 

 Fig. 171, we shall perceive a compactly closed system of muscular bars 

 intersecting each other almost at right angles and interlaced with a tangled mass 

 of trachere, some of which muscles extend (Inn) longitudinally, that is from the 

 front to the back, while others (6-r) stretch out in a vertical direction, that 

 is between the plates of the abdomen and back. 



In order that we may more easily comprehend this important muscular 

 apparatus we will illustrate the thoracic cavity of insects by an elastic steel 

 ring (Fig. 172), to which we may affix artificial wings. If this ring be pressed 

 together from above downward, along the line rs, thus imitating the pulling 

 of the vertical or lateral thoracic muscles, then the wings on both sides spring 

 up. This is to be explained by the fact that through this manipulation a press- 

 ure is exerted on the lifting power arm of the wings. If, on the other hand, the 

 ring be compressed on the sides (ml), which is the same thing as if the longi- 

 tudinal muscles contracted the thorax from before backward, and thus arched it 

 more, then the wings are lowered. 



Agrioninse, according to Kolbe, can fly with the fore pair of wings or with the 

 hind pair almost as well as with both pairs together. Also the wings of these 

 insects can be cut off before the middle of their length without injuring their 

 power of flight. Butterflies, Catocalae, and Bombycidse fly after the removal of 

 the hind wings. Also the balancers of the Diptera must be useful in flying, 

 since their removal lessens the power of flight. 



Chabrier regarded the under sides of the shell-like extended wing-covers of 

 the beetles as wind-catchers, which, seized by wind currents, carry the insect 

 through the air. We may also consider the wing-covers as regulators of the 

 centre of gravity of flight. 



The observations of insects made by Poujade (Ann. Soc. Ent., France, 1887, 

 p. 197) during flight teaches us, says Kolbe, that in respect to the movement 

 during flight of both pairs of wings, they may be divided into two categories : 



1. Into those where both pairs of wings (together), either united, and also 

 when separated from each other, perform flight. Such are the Libellulidse, 

 Perlidse, Sialidse, Hemerobidse, Mymeleonidse, Acridiidte, Locustidse, Blattidae, 

 Termitidse, etc. 



2. Into those whose fore and hind wings act together like one wing, since 

 they are connected by hooks (hamuli), as in certain Hymenoptera, or are at- 

 tached in other ways. Here belong Hymenoptera, Lepidoptera, Trichoptera, 

 Cicadidse, Psocidse, etc. 



