5D0 THE MICROSCOPE. 



an insect's tarsus. They are not attacked to the fifth tarsal 

 joint of a Dipterous insect ; neither are they attached to 

 the fifth tarsal joint of a Hymenopterous insect, but to the 

 terminal sucker, which again, in this great order, is a 

 sixth tarsal joint, membranous, flexible, elastic in the 

 highest degree, retractile to almost its fullest extent within 

 the fifth tarsal joint — a joint modified to an extraordinary 

 degree for special purposes. 



" The plantula of Lucanus, with its pair of minute claws, 

 at once occurred as a case strictly in point. The ungues 

 are hairs modified for special purposes ; and they have the 

 structure of true hairs. The sustentacula of Epeira, the 

 analogous structures on the entire under surface of the 

 last tarsal joints in Pholcus, the condition of the parts in 

 the hind limbs of Notonecta, in both its mature and earlier 

 conditions, as well as in Sarcoptes, Psoroptes, and some 

 other Acari, all contribute to the proof of this fact. The 

 various orders of insects have, for the most part, each their 

 own type of foot. Thus there is the Coleopterous type, the 

 Hymenopterous type, the Dipterous type, the Homop- 

 terous type, &c. ; and so very distinctive, that in critical 

 instances they will sometimes serve at once to show 

 to which order an insect should be referred. Thus, 

 amongst all the Diptera, I have as yet met with but one 

 subdivision which presents an exception to the structure 

 described. This exception is furnished by the Tipulidce, 

 which have the Hymenopterous foot. With hardly an 

 exception, then, I believe the form of foot described will 

 be found universal amongst the Diptera, and will be found 

 amongst the members of this order alone. It may be 

 desirable to add a few words on the best plan of conduct- 

 ing observations on these parts. Their action should be 

 studied in living insects under the influence of chloroform, 

 careful notes taken of appearances, and accurate drawings 

 made. It is of the greatest advantage to preserve carefully 

 all the parts examined : for this purpose Deane's medium 

 or glycerine jelly suits very well; some of the delicate 

 preparations, however, can only be kept satisfactorily in a 

 solution of chloride of zinc. The old plan of soaking in 

 caustic potash, crushing, washing, putting into spirits of 

 Mine (<>r pressing and drying first), and then into turpen- 



