Purple crow butterfly : Hair pencils

Hair pencils is the biggest feature of the male Euploeini, its function except for releasing nine sex pheromone. In addition expelling the effects of enemies, but some Euploeini after being captured, and not necessary to stretch the hair pencils（Parantica sita niphonica Moore, Salatura genutia genutia Cramer, Tirumala limniace limniace Cramer……), this moment by fragrant scales constitued the slit, is another obvious maleness characteristics, but Parantica sita niphonica Moore and Idea leuconoe and has no slit, such may judge by directly observing the coupling slit the end of the abdomen .
 

Mating Signals
 

At the abdominal tip of the male milkweed butterflies are special "hair pencils" which, the butterfly everts when mating to secrete a powerful pheromone called danaidone. In addition to their mating function, the hair pencils can be used to drive off enemies. The males also have on their fore and hind wings androconia scent scales used to win the favors of a female.
     
 

The male synthesized danaidone is an important precursor substance derived from the flower and fruit tissue of various pyrrolizidine alkaloid (PA) rich plants, including the Eupatorium formosanum and Eupatorium tashiroi of the Asteraceae family; and the Setaria viridis and Messerschmidia argentea of the Boraginaceae family. For this reason, males visit these flowers more frequently than do females, a phenomenon that biologists call "pharmacophagy".
 

Reference:

◎臺灣冬天的蝴蝶谷 = Migration and overwinter aggregations of nine danaine butterfly species in Taiwan / 王效岳,李俊延著. 出版項 臺北市 : 省博物館, 民86[1997].  

◎紫斑蝶小檔案http://www.freeway.gov.tw/UserFiles/File/Technical/紫斑蝶小檔案.pdf