HARDWICKE'S SCIENCE-GOSSIP. 



149 



lips, — namely, from the points in which the bands 

 separate which go to form the teeth of the second 

 row. This will be clear by reference to fig. 1 01, where 

 the points//' are those from which one of the teeth 

 of the third row has its divided origin, it, where the 

 bands unite and become flattened out ; after which 

 the tooth is soon terminated as in the teeth of the 

 other rows, but still thinner, sharper, paler in colour, 

 and more delicate in outline, so as frequently, with 

 every care and the best appliances, to be difficult to 

 observe. 



The explanations I have given are I think sufficient, 



trachea; become gradually changed as they approach 

 toward the teeth ; that each tooth of the third row is 

 but a simple development of the nearest rings of the 

 adjacent pseudo-trachea; ; the teeth of the second row 

 but a further development of those of the third ; and 

 finally, the strongly-marked simple strap-shaped teeth 

 of the first row are but further and final develop- 

 ments of those of the second. 



It now only remains that I should suggest a plan 

 by which any ordinary operator with but little skill 

 may prepare slides that will afford distinct views of 

 the teeth as well as of the pseudo-tracheae. 



Fig. 101. Teeth of the Blow-fly (mag.), showing teeth springing to different positions. 



and it will appear that the arrangement of the three 

 rows of teeth is such as to permit their superposition 

 with the flattest possible surface, and consistent with 

 the greatest steadiness of position. 



By contrasting the structure of the broken rings 

 of the pseudo-tracheae at about the centre of one of 

 the lines with those at the termination of the line, 

 and near to the teeth, and comparing these latter 

 with the teeth themselves, I think it will appear that, 

 after all, the teeth are but developments of the 

 pseudo-tracheae, and that the transition from the one 

 form of structure to the other is by no means abrupt, 

 but that, on the contrary, the rings of the pseudo- 



Press the head of a blow-fly gently between the 

 finger and thumb so as to cause the proboscis to shoot 

 out and the lips to expand, and then, with a fine pair 

 of scissors, cut off the lips when expanded close to 

 the mentum, and place them in a watch-glass contain- 

 ing three or four drops of water. To insure success, 

 proceed thus in three or four instances, until there are 

 several examples of the lips in the watch - glass. 

 Place the watch-glass with its contents in a good 

 light and upon a slip of white paper. Add to the 

 water about two large drops of spirits of wine and 

 two or three drops of strong liquor ammonise ; mix 

 with a camel-hair pencil, and watch the action of 



