NO. 2 HONEY BEE SNODGRASS 49 



muscle-bearing areas of the ventral sclerotization are continuous with 

 the episternal areas, as in the honey bee, but in some forms, as in 

 the tenthredinoid Arge, the two are separated by distinct sutural lines. 

 The pterothorficic endosternnm. — The sternal areas of the meso- 

 thorax and the metathorax support together a large endoskeletal 

 structure similar to the endosternum of the prothorax, except that the 

 sides are produced into tzvo pairs of divergent arms (fig. i6 H, 

 Endst) and the entire structure arises from the median inflections 

 of the two sterna. The pterothoracic endosternuum, therefore, appears 

 to be composed of two ordinary Y-shaped sternal furcae united at 

 their bases, which latter are stretched out anteriorly and posteriorly 

 in the median ridges of the supporting sternal plates. The supra- 

 neural bridge, however, spans the space between the bases of the 

 anterior arms, and thus appears to belong to the mesothoracic com- 

 ponent only. The long anterior, or mesothoracic, arms (vS'^2) extend 

 laterally and upward to the sides of the thorax, where each arm ends 

 in a thin, forward-curved expansion that gives attachment to a large 

 group of short muscle fibers (/p) arising on the posterior epimeral 

 region and on the small pleural apophysis of the mesopleuron (fig. 

 17 G). The shorter posterior, or metathoracic, arms (fig. 16 H, 

 SAz) go laterally and posteriorly over the bases of the second legs, 

 just beyond which each arm unites solidly with a thick ridge (o) fol- 

 lowing the external suture {rn) between the metapleuron and the 

 propodeum (fig. 15 'in). The pterothoracic endosternum gives attach- 

 ment to the ventral intersegmental muscles between the prothorax 

 and the mesothorax (fig. 22 C, ^2, j8), and between the metathorax 

 and the second abdominal segment (fig. 27 E, 118, up), to muscles 

 of the second and third legs (figs. 22 C, 24 A), to the muscles of the 

 axillary levers of the mesothoracic wings, and to the tergosternal 

 muscles of the metathorax (fig. 21 F, py, g8, pp). 



IV. THE WINGS 



The movements of an insect's wings fall into two distinct categories : 

 first, there are the movements of flight when the wings are extended, 

 and second, the movements by which the wings are turned posteriorly 

 over the body when at rest, or again brought into the extended posi- 

 tion preliminary to flight. The action of the wing muscles that pro- 

 duce the movements of flight depends chiefly on structural adapta- 

 tions in the skeletal parts of the thorax; the efifect of the muscles 

 concerned in the flexion and extension of the wings, on the other 

 hand, is mainly dependent on structural details in the wing bases. 



