Sentence:  The <WSD>rostellum</WSD> has an outer firm pouch, inside which is the viscid substance to which are firmly attached the caudicles of the pollinia. An insect entering the fl. probes the spur of the labellum and its back comes into contact with the <WSD>rostellum</WSD> and depresses the pouch, causing the viscid substance to adhere to the insect. 
Meaning of the word marked with <WSD> tags:  A projecting part of the column in the flower of an orchid that separates the male stamen from the female gynoecium.