98 



SATURDAY LECTURES. 



with me, but they bear a general resemblance to those of 

 the flesh-fly. (Fig. 9.) 



Fig- 9- — Sarcophaga san-accnia- : a, larva, b, pupa, c, fly, the hair lines show, 

 ing average natural lengths ; d, enlarged head and first joint of larva, showing 

 curved hooks, lower lip {g), and prothoracic spiracles; e, end of body of same- 

 showing stigmata (/) and pro-legs and vent ; h, tarsal claws of fly with protect- 

 ing pads ; /, antenna of same — enlarged. (After Riley.) 



You will notice that the larva tapers toward the head, 

 which is retractile, and armed with two black hooks. It is 

 blunt and truncate behind, and breathes principally through 

 a pair of spiracles situated on this truncate, anal end. 



AVhilo I must pass over the complicated mouth, the stereo- 

 scopic eyes, with their 4,000 facets, and many curious details 

 in the structure of the fly, it will be well to dissipate two 

 quite widespread popular errors in regard to it. Most of 

 the old authors tell us that the fly walks on the ceiling, and 

 in other Avays defies the laws of gravitation, by suction. You 

 will notice from the figure of the foot of the flesh-fly (and 

 that of the house-fly has a similar construction) that it ends 

 in a pair of strong hooks and a pair of pads or cushions, 

 technically termed pulvilli. These were supposed to act as 

 sucking disks by creating a vacuum, on the same principle 

 as does the piece of circular leather used by school-bo3's for 

 a similar purpose. In reality, how^ever, these pads are beset 

 with innumerable knobbed hairs, which are kept moist by 

 an exuding fluid. In short, we have here a repetition, after 

 a fashion, of the ambulacra! suckers of the star-fish. The 

 smoother the surface, the greater the adhesion of the digituli. 

 This you will at once understand by gently pressing or 

 drawing the moist finger ends over a glass window, or other 

 polished surface. The adhesion is very sensible, whereas, 



