THE HEAD CAPSULE. 125 



The revolution of the major axis takes place as a result of 

 the withdrawal of the frontal sac into the interior, and the 

 flattening of the posterior surface of the head capsule. During 

 these changes the proboscis becomes further developed, and is 

 gradually retracted within the head capsule. 



Fig. 25. — The Head and Proboscis of the mature nymph: /, lateral view; 

 2, lower surface ; j, lateral view of the proboscis : fs, frontal sac ; ep, epistome ; 

 a, lateral fold of rostrum ; b, clypeus ; c, palpigerous scale ; </, swelling between 

 meso- and prelabrum ; e, prelabrum ; /, discophore ; g, furca ; //, discal sclerite ; 

 /', cleft in the anterior margin of the oral sucker. 



It is generally stated that a lunula is characteristic of all 

 the cycloraphic Diptera ; in some, however, the frontal sac is 

 not withdrawn into the head, but becomes greatly reduced in 

 size, and remains external, or becomes obliterated by the great 

 development of the frontal sclerites. Kunckel d'Herculais 



