6o SMITHSONIAN M 1SCP:LLANE0US COLLECTIONS VOL. IO3 



probably important adjuncts arc the large pleuronotal muscles arising 

 on the sides of the pleuron (fig. 17 E, 75) and attached on the ends of 

 the posterior scutal areas of the notum (fig. 19 F, 75). The wing 

 bases, hinged to the back by the first and fourth axillary sclerites, are 

 pressed downward proximal to the fulcral second axillaries by the 

 descending margins of the notum, and thereby the distal parts of 

 the wings are thrown upward. The vertical component of the down- 

 stroke, conversely, results from the contraction of the longitudinal 

 muscles, which restores and increases the curvature of the meso- 

 notum, and thus turns the wings down on the fulcral sclerites. 



In most insects the wing-bearing plates of the back are sufficiently 

 thin and flexible to respond by changes in shape to the alternating 

 pull of the vertical and longitudinal muscles. Such movements, how- 

 ever, are not possible in the thick and rigid mesonotum of the bee and 

 related Hymenoptera, and it is for this reason that the mesonotum is 

 cut by a transverse fissure (fig. 16 C, sf) into an anterior and a 

 posterior plate. The fissure widens on each side into an open cleft 

 (fig. 19 J, sf), the edges of which are connected by an infolded 

 membrane. The closed dorsal part of the fissure acts as a hinge be- 

 tween the two plates. Pressure on the top of the back depresses the 

 notal plates at the hinge line and widely opens the lateral clefts, the 

 anterior notal plate here moving forward, the posterior plate back- 

 ward. The divergent movement of the two plates, however, ac- 

 centuates the downward movement of their lower margins at the 

 points just before and behind the lateral clefts where the wings are 

 articulated to the back by the first and fourth axillary sclerites. This 

 action of the mesonotum is easily demonstrated by manipulation of a 

 dead bee, and could be well illustrated with half of a hollow rubber 

 ball having a meridional slit on each side. 



In the chalastogastrous Hymenoptera the posterolateral areas of 

 the mesoscutum form deep concavities of the scutal surface at the 

 sides of the elevated median area of the scutellum, and are separated 

 from the main part of the scutal plate before them by the lines of 

 abrupt deflections of the notal surface. These lines coincide with the 

 scutal clefts of the Clistogastra, and, in fact, each is split laterally 

 by a short cleft. The response of the relatively flat mesonotum of a 

 tenthredinid to gentle compression between its two ends is most 

 pronounced on the sides ; the anterior wing processes spread laterally 

 and move upward and posteriorly, the lateroposterior margins of the 

 anterior scutal areas at the lateral clefts folding backward above the 

 anterior edges of the posterolateral areas. In the densely sclerotic and 

 strongly convex mesonotum of the Clistogastra a much deeper splitting 



