40 GN THE ANATOMY OF THE i'L^ . 



The dorsal plate of the fourth segment, the facial plate, bears the antennas 

 at its superior or curved margin, which corresponds with a concavity in the 

 anterior margin of the occipito-frontal plate ; inferiorly it is continuous 

 with the proboscis ; its inferior angles are united to the cheeks, and the 

 remainder of its margin is continuous with the membrane already referred 

 to, which attaches it loosely to the occipito-frontal plate. 



When the perfect fly emerges from the pupa, a large pro- 

 tuberance occupies the front of the head immediately above the an- 

 tennse. It consists of a membrane, by the dilatation of which the 

 insect bursts off the anterior extremity of the pupa case ; when 

 fully dilated it is larger than the head, which is as yet retained 

 within the pupa ; but as soon as the latter escapes, this protuber- 

 ance rapidly collapses and is withdrawn into it. Until the integu- 

 ments are thoroughly hardened it may be made to protrude 

 from the head by gentle pressure, but is not afterwards 

 visible externally ; by pressing back the facial plate, however, 

 a fissure is opened between it and the occipito-frontal plate and 

 cheeks, by which the cavity formed by the retraction of the 

 membrane communicates freely with the external air. The 

 membrane is continuous with the integument at the margins of 

 the fissure ; it assumes a bright yellow color in the mature insect, 

 and is thrown into numerous plications ; its connections are best 

 seen from the interior of the head. The extent of its surface is 

 very great, and is much increased by being closely covered with 

 papillse. It has probably three distinct functions : first, it serves 

 to break off the anterior extremity of the pupa case ; secondly, it is 

 undoubtedly connected with the humming of the insect, the 

 facial plate being thrown into rapid vibrations during the 

 emission of sound; and lastly, it seems to receive numerous nerve 

 filaments from the antennal nerves, and is probably an accessory 

 organ of smell. From its position I propose the term frontal 

 sac to designate it. 



