THE THORACIC EXO-SKELETON. 195 



The Planta (PI. IX., Figs. 8 and 9) is a subquadrate plate, 

 triangular in the posterior tarsus, which articulates with a deep 

 emargination in the plantar edge of the last tarsal joint ; it 

 bears a long median setose spine, which projects between the 

 pulvilli. Its inner (upper) surface gives insertion to the strong 

 apodeme of the flexor tarsi muscle, and its lateral edges articu- 

 late with the stalks of the pulvilli. These edges exhibit a series 

 of parallel plications or ridges. 



The Pulvillus is a membranous, somewhat pyriform flattened 

 sac, the narrow neck of which is strengthened by a chitinous 

 ring articulating by syndesmosis with the edge of the planta, 

 end with the distal margin of the last tarsal joint. This ring 

 supports a fan of chitinized ridges, which radiate over the 

 dorsal surface of the pad (PL IX., Fig. 10). The pulvilli are 

 the cushions, by means of which flies and some other insects 

 climb windows or walkover the lower surface of glass, or other 

 smooth and polished bodies. 



With an oil immersion (^V) the pad is seen to be covered 

 on its under surface with papillae, arranged in very regular 

 rows, each papilla having a minute orifice at its extremity. 

 Towards the edges of the pad these papillae become longer, 

 and give place to long, hollow setae, with trumpet-shaped 

 orifices, which form a dense fringe projecting beyond the 

 edges of the pad. 



Henry Power [82] is the first author who mentions these 

 pads, and his description of the feet of the fly is so curious 

 that I shall give it in extenso. He says : 



' She (the fly) hath six legs, but goes only upon four ; the 

 two foremost she makes use of instead of hands, with which 

 you may often see her wipe her mouth and nose, and take up 

 anything to eat. The other four legs are cloven, and armed 

 with little clea's, or tallons (like a Catamount), by which she 

 layes hold on the rugosities and asperities of all bodies she 

 walks over, even to the supportance of herself, though with 

 her back downwards, and perpendicularly invers'd to the 

 Horizon. To which purpose also the wisdom of Nature hath 

 endued her with another singular Artifice, and that is a fuzzy 



