80 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IO3 



base, thus augmenting the effect of the pressure on the planta; 

 finally, when fully extended, the manubrium clamps down on the 

 arolium and holds it securely in the spread condition. There can be 

 little doubt, therefore, that the entire action of the arolium is a 

 mechanical result of the pull of the pretarsal muscle in conjunction 

 with pressure of the foot against the support when the claws fail to 

 take a hold on the latter. It has been said that the arolium adheres 

 to smooth surfaces by means of a sticky exudation from the spines 

 of the planta, but observation fails to reveal the presence of any such 

 adhesive. The upper surface of the arolium is covered with minute 

 hairs, but its under surface is almost entirely bare. 



VI. THE ABDOMINAL PETIOLE 



The outstanding feature in the external organization of the clisto- 

 gastrous Hymenoptera is the secondary consolidation of the fourth 

 body segment with the thorax, and the division of the body by a deep 

 constriction between the fourth and fifth segments, accompanied by 

 a narrowing and usually an elongation of the anterior end of the 

 fifth segment, or even of the entire segment, to form a petiole. Two 

 important mechanical advantages result from these alterations, which 

 take place during the pupal stage of development. First is the greatly 

 increased length that the dorsal wing muscles of the thorax are able 

 to acquire by a posterior extension of the mesothoracic postphragma 

 into the fourth segment (fig. i6 C) ; second is the high degree of 

 potential mobility conferred on the shortened abdomen. The first 

 advantage is self-evident, the second needs elucidation. 



In the more generalized insects the abdomen is broadly attached 

 to the thorax by its first segment, and the muscles that move the 

 abdomen as a whole are dorsal and ventral intersegmental muscles 

 between the metathorax and the abdomen. In winged insects, how- 

 ever, the dorsal intersegmental muscles of the metathorax become 

 functionally a part of the flight mechanism (fig. 26 A, a'3) by a 

 transfer of the ridge or phragma of their abdominal attachment to 

 the thorax. As a consequence, the dorsal metathoracic muscles of 

 winged insects have no motor effect on the abdomen. Ventral muscles 

 between the thorax and the abdomen are seldom strongly developed, 

 and they may be absent; when present they comprise generally one 

 pair or two pairs of slender muscles arising anteriorly on the meta- 

 sternal apophyses and attached posteriorly sometimes on the first ab- 

 dominal sternum (A), but more commonly for greater effectiveness 

 on the second (C) or even the third, fourth, or fifth sternal plate. 



