LISTERA 107 



its pollen mass on the stigma. Darwin's son also watched 

 a bed of Listera at some distance, and brought home 

 insects with the pollen mass attached to the forehead. 

 He also noticed that a great number of spider-webs 

 were spread over the plants, as if the spiders were 

 aware how attractive the Listera was to insects. 

 Darwin also tells us of an observation that shows that 

 the insect world has tragedies in the midst of its 

 daily life. In speaking of how slight a touch is sufficient 

 to cause the rostellum to explode, he says: "I may 

 mention that I found an extremely minute Hymenop- 

 terous insect vainly struggling to escape, with its head 

 cemented by the hardened viscid matter to the crest of the 

 rostellum and to the tips of the pollinia. The insect was 

 not as large as one of the pollinia, and after causing the 

 explosion had not strength enough to remove them; it was 

 thus punished for attempting a work beyond its strength, 

 and perished miserably." 



The three species that are familiar to us may be recog- 

 nised by the shape and cutting of the lip. 



KEY TO THE SPECIES OF LISTERA 



Lip broadly wedge-shaped, two-lobed 



at the apex. I. Broad-Hpped Twayblade. 



Lip very narrow and strap-Hke. 



Lip two-cleft and twice as long 



as petals. 2. Heart-leaved Twayblade. 



Lip two-parted and four to eight 



times as long as petals. 3. Southern Twayblade. 



