THE CATERPILLAR. 25 



degree, having, when extended, a Y-shaped form, 

 but when not required for defence entirely retract- 

 ed within the body, leaving only a transverse slit 

 to indicate its point of emission [see Fig. 28]. In all 

 other butterflies the organ is found upon the under 

 surface of the body between the lower lip and the 

 front legs, and its identity with the osmateria has 

 therefore escaped attention. In this place it is 

 never developed as a Y-shaped organ, but is pro- 

 truded as a conical or hemispher- 

 ical bladder of greater or less size 

 [Fig. 33], or, in the blues and their 

 allies, as a lenticular disk. It is 

 found also in many other Lepidop- 



FIG. 33. a, front por- 



tera ; whether developing exter- tinn of the caterpillar 



of Danais Plexippus, 



nally upon the upper or under 

 surface of the body, it is always, 

 excepting in the anomalous group 



,.-.., TT -,.., the transverse slit at 



to which the blues and similar up, x 2. 

 butterflies belong, emitted from a transverse slit 

 across the segment. Its use, when developed on 

 the upper surface, is unquestionably as a scent- 

 organ, a weapon of defence against its foes, for 

 the stench it emits in some species is simply in- 

 sufferable ; when developed beneath, the purpose 

 of this organ is problematical ; some have sup- 

 posed that it secreted a fluid for the lubrication 

 of food just before eating, but this seems exceed- 

 ingly doubtful. 



