1 1 8 Flowers and their Unbidden Guests. 



lip is rolled inwards in a spiral of more than a circle, 

 and by the cohesion and sacculation of its parts is 

 converted into a pouch, the so-called " caloeolus," or 

 slipper. The tongue-like extremity of the lip is con- 

 cealed inside this pouch, and on it is a small depres- 

 sion which secretes nectar. The entrance to the pouch 

 is rendered so tortuous by the very peculiar curvature 

 of the lip, that it becomes simply impossible to most 

 insects to get at the nectar. Strong and heavy 

 humble-bees alone can rifle it ; and even they accom- 

 plish this, not by making their way into the pouch, but 

 by fixing themselves on the upper external surface of 

 the "calceolus." The impulse v given by the weight 

 of their bodies makes the lower lip descend, producing 

 a downward motion like that of a lower jaw; this 

 motion being regulated by a pair of strong elastic 

 bands, that extend from the pouch to the base of the 

 corolla. The tongue-like nectary, already described, of 

 course accompanies the pouch, of which it forms the 

 extremity, in this semi-circular gyration; and though 

 it was hitherto concealed inside the pouch immediately 

 below the narrow entrance, is now exposed at the 

 opening, and becomes readily accessible to the bee. 

 The humble-bees, however, cannot take advantage of 

 this without coming into contact with stigma and 

 pollen in due succession, and thus promoting inter- 

 crossing among these proterogynous flowers. Insects 

 that are without the requisite size or strength or 



