38 THE LARVA OF THE BLOW- FLY. 



distal joint (Figs. 4 and 5). The distal portion is sub- 

 hemispherical and has two short conical sensory papillae at its 

 extremity, one above the other ; it contains a pair of sensory 

 organs, which terminate in the papillae. These closely resemble 

 the eyes of a leech, except in being devoid of pigment ; they are 

 apparently sensitive to light (see Sensory organs). 



The proximal portion is sub-cylindrical, and exhibits a very 

 remarkable half-disc on its ventral and outer surface, the 

 stomal disc (mihi), and in front of the stomal disc the orifice 

 of a sac, in which the great hook lies when retracted. 



FIG. 5. a, the head and mouth of the adult larva of the Blo\v-Fly seer, from the 

 ventral surface ; 3, Newport's segment ; 4, the prothoracic segment ; b, the 

 mouth more highly magnified (^ inch objective) ; /, the prestomal sclerite ; 

 d, the stomal disc ; m, aperture of the mouth ; //>, the labium ; c, transverse sec- 

 tion of the pseudo-tracheae of the stomal disc as seen with T V oil immersion lens. 



The real significance of these organs has been apparently 

 overlooked by Weismann and most subsequent writers, except 

 Macloskie* and perhaps Brauer [14, p. 32], who recognised that 

 the mouth parts of the larva correspond with the proboscis of 

 the imago, without entering into details. This is more remark- 

 able, as the development of the maxillae is easily traced from 

 the unmistakable maxillae of the embryo, from which they 

 scarcely differ, and these were correctly figured and described 

 by Weismann, who, however, entirely neglected the correspond- 



* See page 44. 



