NO. 2 HONEY BEE— SNODGRASS 59 



lateral margins of the back plates of the wing-bearing segments 

 causes the wings, in the manner of a pump handle, but involving the 

 interaction of many structural details, to turn upward, and an eleva- 

 tion of the notal margins tilts the wings downward. The up-and- 

 down movements produced in this way, however, are not sufficient for 

 progressive flight ; in addition the wings must have a partial rotary 

 movement on their long axes accompanied by a forward and rearward 

 motion, by which the anterior margins are turned downward and 

 forward during the downstroke, and reversed during the upstroke. 

 This compound motion gives a sculling movement to each wing, but 

 the result of the wings on opposite sides of the body acting together 

 is that a current of air is driven backward, and the insect propelled 

 forward. In the bee the two wings of each side act in unison as a 

 single pair of flight organs because of their connection with each 

 other, but the unity of action is further secured by a reduction of the 

 metathoracic musculature and its subordination to the action of the 

 highly developed mesothoracic muscles, which latter thus become the 

 chief motor elements for both pairs of wings. 



The cavity of the pterothorax is largely occupied by the huge dorso- 

 ventral and dorsal longitudinal muscles of the mesothorax (fig. i6 A, 

 C). The thick pillars of dorsoventral muscles {/S) have their upper 

 attachments (A) on the lateral parts of the anterior plate of the 

 mesonotum (B), and go downward and backward (C) to their lower 

 attachments on the ventral and lower lateral walls of the pleuro- 

 sternal region of the mesothorax. The longitudinal muscles (//) are 

 attached anteriorly on the prephragma and the median area of the 

 anterior notal plate (B), and extend posteriorly through the meta- 

 thorax into the posterior end of the propodeum, where they are at- 

 tached on the postphragma (2Ph). Though the postphragma of the 

 bee is extended through the metathorax far back into the propodeum , 

 to accommodate the great length of the mesothoracic dorsal muscles, 

 it is morphologically an ingrowth between the mesonotum and the 

 metanotum. A pair of small fan-shaped muscles (C, pd), arising on a 

 median process of the posterior end of the propodeal tergum (IT) 

 and spreading to their insertions on the rear surface of the phragma, 

 must represent, therefore, the usual longitudinal dorsal muscles of 

 the metathorax with their posterior attachments transposed backward 

 on the propodeum. A pair of short external dorsal muscles of the 

 mesothorax (D, yo) traverses the scutellar area of the mesonotum. 



The vertical component of the upstroke of the fore wings, resulting 

 from a depression of the lateral margins of the mesonotum, is effected 

 principally by the great dorsoventral muscles (fig. i6 C, y^), but 



