INSECTS : THEIB FEET. 118 



rubbing the tarsi together is not to clean them, but the 

 pulviUi, for which they serve as brushes. Besides rubbing 

 the tarsi together, flies are often seen, while thus em- 

 ployed, to pass the two fore tarsi and the tibiae, with 

 sudden jerks, over the back of the head and eyes, and the 

 two hind tarsi and tibias over and under the wings, and 

 especially over their outer margins, and occasionally also 

 over the back of the abdomen. That one object of these 

 operations is to clean these parts from dust, I have no 

 doubt, as on powdering the flies with flour they thus 

 employ themselves, sometimes for ten minutes, in detach- 

 ing every part of it from their eyes, wings, and abdomen ; 

 but I am also inclined to believe that, in general, when 

 this passing of the legs over the back of the head and 

 outer margin of the wings takes place in connexion with 

 the ordinary rubbing of the tarsi together, as it usually 

 does, that the object is rather for the purpose of complet- 

 ing the entire cleansing of the tarsal brushes (for which 

 the row of strong hairs visible under a lens on the exterior 

 margin of the wings seems well adapted), so that they 

 may act more perfectly on the pulvilli. Here, too, it 

 should be noticed, in proof of the importance of all the 

 pulviUi being kept clean, that as the tarsi of the two 

 middle legs cannotbe applied to each other, flies are con- 

 stantly in the habit of rubbing one of these tarsi and its 

 pulvillus, sometimes between the two fore tarsi, and at 



other times between the two hind ones 



" Though the above observations, hastily made on the 

 spur of the occasion since beginning this note, seem to 

 prove that it is necessary the pulvilli of flies and of some 

 other insects should be kept free from moisture and dust 

 to enable them to ascend vertical polished surfaces, they 

 cannot be considered as wholly settling the question as to 

 the precise way in which these pulvilli, and those of in- 

 sects generally, act in effecting a similar mode of progres- 

 sion ; and my main reason for here giving these slight 



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