NO. 2 HONEY BEE SNODGRASS 79 



known as the flexor of the clazvs. The direct effect of the muscle pull, 

 however, is exerted on the base of the pretarsus, and, unless the 

 claws interfere, it flexes the pretarsus on the end of the tarsus (fig. 

 25 F) and brings the arolium into a horizontal position. 



The arolium is an organ that enables the bee to cling to smooth 

 surfaces on which the claws cannot hold. When the arolium is in use 

 as an adhesive organ the claws are turned proximally with their points 

 directed outward, and the arolium is horizontally extended, its lateral 

 walls spread out flat, its under surface (fig. 25 G) pressed against 

 the support. The extension and flattening of the arolium has been 

 explained by Arnhart (1923; see Snodgrass, 1925) as caused by a 

 secretion liquid forced into the foot from a gland in the end of the 

 tarsus; but a study of the mechanism of the pretarsus shows that 

 the action of the arolium including its changes in shape can be 

 produced entirely by mechanical means. 



With a live bee it may be observed that when the flexing claws 

 encounter on a smooth surface nothing to restrain them or to give 

 them a hold, they turn helplessly upward beneath the end of the 

 tarsus with their points sprawling outward. The tension of the 

 contracting pretarsal muscle is then exerted on the base of the pre- 

 tarsus, which turns abruptly downward bringing the arolium into a 

 horizontal position, after which the arolium extends, spreads out 

 flat, and clings to the support. If the foot of a dead bee is in the 

 position shown at F of figure 25, an upward pressure on the retracted 

 planta {pin) extends the pretarsus until the planta comes into the 

 same horizontal plane as the arolium (Ar), and now, with continued 

 pressure, the arolium itself automatically unfolds and spreads out 

 with its flattened under surface downward. This action of the arolium 

 can be induced by manipulation of a detached pretarsus, in which 

 there can be no possibility of liquid pressure from within the tarsus. 

 The same kind of action, moreover, can be illustrated with a piece of 

 paper cut and folded into the form of a scoop (I) ; a vertical com- 

 pression of the base of the scoop spreads the sides widely apart (J), 

 giving thus a close imitation of the partly expanded arolium (H). 



The similar response of the arolium to pressure against the planta is 

 evidently caused in the living bee by the downward pressure of the 

 tarsus, which flattens the pretarsus and compresses the arolium 

 against the support beneath it. The manubrial sclerite of the pretarsus 

 (fig. 25 E, H, mn), however, representing the handle of the paper 

 scoop (I, J), plays an important part in the expansion of the arolium. 

 With the extension of the foot, the manubrium turns backward on the 

 end of the tarsus and hence pushes on the upper edge of the aroliar 

 6 



