THE THORAX AND ITS APPENDAGES. 63 



brane, to which it is attached between the humeral angle of the wing 

 base and the edge of the notum. The tegulaj are present in most in- 

 sects, generally on the base of each wing, but they usually have the 

 form of small inconspicuous hairy pads, as hhown in the diagram 

 (fig. 6, Tg). In the flies, moths, butterflies, and Hymenoptera, 

 however, the tegulae of the front wings develop into large conspicu- 

 ous scales overlapping the humeral angles of the bases of these 

 wings. 



The motion of the wing in flight consists of both an up-and-down 

 movement and a forward-and-backward movement, which two com- 

 bined cause the tip of the wing to describe a figure-eight course if 

 the insect is held stationary. Corresponding with these four move- 

 ments are four sets of muscles. In the dragonflies nearly all of the 

 wing muscles are inserted directly upon the base of the wing itself, 

 but in other insects, excepting possibly the mayflies, the principal 

 muscles are inserted upon the thoracic walls and move the wing 

 secondarily. In the lower insects, such as the grasshoppers, crickets, 

 stoneflies, net-winged flies, etc., the two wing-bearing segments are 

 about equal in their development and each is provided with a full 

 equipment of muscles. In these insects the wings work together by 

 coordination of their muscles, although each pair constitutes a sepa- 

 rate mechanism. In such insects, however, as the true flies and the 

 wasps and bees the metathorax, as we have seen in the case of the 

 bee, is greatly reduced, and what is left of it is solidly attached to 

 the mesothorax. In the flies the hind wings are reduced to a ])air 

 of knobbed stalks having no function as organs of flight, while in 

 the bees the hind wings, which are very small, are attached to the 

 front wings by a series of booklets on their anterior margins (fig. 

 25 D, mc) which grasp a posterior marginal thickening of the 

 front wings. Moreover, when we examine the interior of the bee's 

 thorax we find that the muscles of the metathorax are greatly 

 reduced or partly obliterated and that the great mesothoracic mus- 

 cles serve for the movement of both wings, thus assuring a perfect 

 synchrony in their action. Hence, it is clear that the union and 

 consolidation of the thoracic segments in the higher insects is for 

 the purpose of unifying the action of the wings. 



The muscles of flight in the bee may be very easily studied by cutting 

 the thorax of a drone into lateral halves. The cavity of the thorax 

 is occupied almost entirely by three great masses of muscles. One 

 of these is longitudinal, median, and dorsal (fig. 27, LMcl^), extend- 

 ing from the mesoscutum {Sctr.) and the small prephragma {Aph) 

 to the large mesothoracic postphragma (Pph^). A small set of 

 muscles (LMcl^) then connects the posterior surface of this phragma 

 with the lower edge of the propodeum (IT). On each side of the 



