HOW INSECTS DIRECT THEIR FLIGHT. 115 



There are a few insects in which the motor and steering functions 

 are united, the flying-muscles being attached to the wings. These in- 

 sects fly gracefully, like birds ; still, the abdomen is very long, flexible, 

 and greatly augments the ease and variety of motion ; this is specially 

 apparent in the Agrions the dragon-fly, for example. It is probable 

 that the Lepidoptera (butterflies, moths) should be ranged in the same 

 category, for the movement of their wings is something like that of 

 birds ; the anatomy of their thoracic muscles, however, has not yet been 

 completely analyzed. 



In the Hymenoptera (bees, wasps, etc.) are found the first indica- 

 tions of separate functions of translation and direction. The wings, 

 having acquired a very perfect automatism by which the axis of sus- 

 pension has become permanently fixed, are solely devoted to the func- 

 tion of movement. The abdomen has become pediculated and exceed- 

 ingly mobile. As it bends up or straightens out, the center of gravity 

 is carried forward or backward. The Cynips (gall-flies, etc.) and Ich- 

 neumonides (insects that prey upon the eggs of other insects) offer 

 extreme examples. If the abdomen be prevented from moving, the 

 animal can still fly, but can not direct its course. Moreover, the pos- 

 terior legs of these insects are long, as in the Polistes and Megachiles, 

 and this also aids in displacing the center of gravity. 



In the Orthoptera (crickets, grasshoppers, etc.) the abdomen is but 

 slightly movable, and the steering power is almost wholly in the hind- 

 feet ; but, as these are already differentiated for an equally important 

 function, that of jumping, they lend themselves with a bad grace to 

 the former function, and so the hopping insects fly very badly. 



Thus far we have seen the two pairs of wings devoted to the motor 

 function. We come now to a class of insects in which the functional 

 adaptation is not secured through organs performing other functions 

 and lending themselves to extra duties, but where the adaptation is 

 secured through its own proper organs one of the pairs of wings, in- 

 deed, which, diverted from the motor function, has become a steering 

 apparatus. 



In the group of Coleopters (beetles), only one pair of wings, the 

 posterior, serve to sustain the insect ; the surface of support, therefore, 

 is considerably diminished, and the flight is heavy or clumsy. But 

 this disadvantage is compensated by the greater extent of surface 

 afforded by the non-modified pair. This surface, indeed, is so great 

 that the insect is obliged to fold it up during rest. The abdomen of 

 the Coleoptera is. stiffly fixed to the thorax, and consequently is but 

 slightly movable ; but this quality is unnecessary, since a special organ, 

 the elytrum (wing-case), has power to displace the center of gravity. 

 Raised up over the thorax during flight, the elytrum forms a little 

 swaying mass above the center of gravity, and the slightest motion of 

 this mass affects the balance of the insect. Remove the wing-cases, 

 and the insect still flies, but has no power to direct its motion, which 



