40 THE LARVA OF THE BLOW-FLY. 



larva (Fig. 6, mx) will assist the reader in attaining a clear 

 idea of their relations. 



The Mouth presents a triangular orifice between the proximal 

 joints of the maxillae ; the apex of the triangle is in front and 

 its base is formed by the labium. 



The Stomal Disc exhibits a nearly semicircular surface, grooved 

 by a number of radiating dichotomously-dividing channels, 

 similar to the pseudo-tracheae of the proboscis of the imago, 

 but without ring-like thickenings (Fig. 5, c). 



It is of considerable morphological interest, because the 

 principal imaginal disc from which the proboscis of the fly is 

 developed is formed by an invagination of its hypoderm. 



The channels on its surface open into a cup-shaped cavity on 

 the oral edge of the disc or by a single vessel, which is formed 

 by the union of those of the posterior third of the disc, into the 

 external angle of the mouth. These channels apparently serve 

 to distribute the salivary secretion over the food and also to 

 conduct the food into the mouth. They always contain a con- 

 siderable quantity of grumous fatty material, and their contents 

 are blackened intensely by osmic acid. 



The Great Hooks are uncinate processes of the cuticular layer 

 of the integument, and when at rest are retracted within 

 special cavities in the maxillae (Fig. 8, j, h), which do not com- 

 municate with the mouth. They are the retractile claws of 

 the maxilla, and resemble the claws on the thoracic feet of 

 some larvae. In the recently hatched larva the great hooks are 

 very small straight chitinous rods, the distal extremities of which 

 are bent at right angles (Figs. 7 and 9), and like the claws 

 on other feet are shed and renewed at each ecdysis. Before the 

 second moult the new and old hooks are occasionally seen 

 side by side, a fact recorded by Weismann. 



Brauer [14, p. 33] calls these hooks maxillae, but also speaks 

 of them as mandibles, and does not assert their morphological 

 identity with either ; Menzbier* supposed them to lie when re- 

 tracted within the mouth, and regarded them as indurations of 



* Menzbier, M., ' Uber das Kopfskelet und Mundwerkzeuge der Zwei- 

 flugler.' Bull. Soc. Imp. Nat., Moscow, t. 55, 1880. 



