EXTERNAL ANATOMY OF INSECTS. 695 



orbicular shield, and thickly set with minute peduncu- 

 lated suckers a . The structure varies however in dif- 

 ferent species. Thus in D. limbatus the shield is trian- 

 gular with the smaller suckers at the base, and two rows 

 of larger oblong ones, concave but not umbilicated, at the 

 apex ; in another Brazilian undescribed species {D. ob- 

 ovatus K. MS.) the shield is oblong and quite covered 

 with suckers like those last mentioned; mD.sulcatus (Aci- 

 lius Leach) almost the whole plate is occupied by a very 

 large sucker, above which, at some distance in the inner 

 side, are two smaller ones, while the extremity of the 

 shield is covered by minute ones elevated on long foot- 

 stalks: the central umbilicated elevation of the large one, 

 which nearly fills its cavity, is in this species beautifully 

 radiated. The male of Colymbetes transversalis has also 

 an orbicular shield, but the suckers are much less strongly 

 marked. The use of this organ has been before suffi- 

 ciently explained b . 



A few words will be necessary upon the folding of the 

 legs in repose. When insects walk, the thigh is usually 

 in an ascending position, rising above the horizontal line, 

 the tibia forming with it rather an obtuse angle, and the 

 tarsus nearly a right one with the tibia; but in the My- 

 riapods, as far as I can unravel their swift many-footed 

 motions, these angles in walking do not take place ; in 

 repose however, in many insects, the coxa forms an angle 

 with the thigh below the horizontal line and with the tibia 

 above it, and the tibia and tarsus continue in the same 

 line, and point downwards nearly vertically ; in others, 

 as in the Tetramerous beetles, the last-mentioned joints 



» Philos. Trans. 1816. t. xx. /. 4, 1 1. b See above, p. 305—. 



