STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 439 



hsemocoel of tlie oral lobes is well developed. This supports 

 the view set forth by Kraepeliii, and with which I agree that 

 the inflation of the oral lobes is due to the blood. I consider 

 that the extension of the proboscis is due to the inflation of 

 the tracheal sacs of the head. The proboscis having been pro- 

 truded the oral lobes are then diverged by the contraction of 

 the retractor muscles of the furca and discal sclerites, and dis- 

 tended by the inrush of blood which keeps them turgid, and 

 causes the openings into the pseudo-tracheal channels to 

 remain open. 



The Labial Salivary Glands (figs. 19 and 1, Ih.sL). — 

 These salivary glands lie in the haustellum at the base of the 

 oral lobes. The glands, which are spherical in shape, are com- 

 posed of a large number of gland cells somewhat triangular 

 in shape. Each gland cell is 40 fx in size, and possesses a 

 large nucleus (12 ju), and internal to this a permanent circular 

 vacuole {vac), which is 16 /x in size, and is lined by a thin 

 chitinous intima. The duct of each gland cell opens into the 

 side of the vacuole (od.). The ducts {ic.d.) are intracellular, 

 and run from the centre of the gland, some of them uniting, 

 to form a number of fine ducts on the ventral sides of the discal 

 sclerites, which unite and open into the oral pits by a median 

 pair of pores. Kraepelin, in his description of the proboscis 

 of the blowfly, described the labial glands and their ducts 

 (but not their histology) of that insect, his description being 

 similar to the condition I find in M. domestica. Lowne, 

 however, states that in the blowfly he traced the ducts of the 

 gland cells through the oral lobes to the apertures of the 

 gustatory papillae, which he regarded therefore as the aper- 

 tures of the labial salivary glands. 



The secretion of the labial salivary gland serves to keep 

 the surface of the oral lobes moist. 



VI. Summary. 



1. The exoskeleton of the head capsule and of the pharynx 

 is described in detail; the relations of the parts in the terms 



