34 NATURAL HISTORY OF AQUATIC INSECTS CH. 



The compound eyes arc each divided into two 

 separate portions, the socket of the antenna being' 

 interposed. Since one mass of facets is above and 

 the other beneath, it has been ingeniously conjectured 

 that one serves for vision in air and the other in water. 

 This conjecture has never been tested with the care 

 necessary to confirm or refute it. I once endeavoured 

 to determine by direct observation whether the lower 

 patch is actually submerged or not, but found that 

 the capillary curves about the head and body render 

 it very difficult to decide where the water-line comes. 

 These curves must greatly obscure or at least limit 

 vision by the lower lenses. The occurrence of divided 

 eyes in some Dragon-flies, in Adelotopus, a Carabid 

 beetle, which is found under the bark of trees,^ as 

 well as in many Dipterous Insects which never 

 approach the surface of water," must be accounted 

 for in some other way. The antenna has a very 

 unusual shape. The basal joint is small, the second 

 larger, the third much bigger, looking like a second 

 antenna outside the other. The remaining part of 

 the shaft is club-shaped and indistinctly jointed. 

 The tip is provided with pits and hairs, and is 

 probably highly sensitive. In the Dytiscidae the 

 antennae, as I learn from Dr. Sharp, want the usual 

 sensory structures and are always wet. The peculiar 

 form of the Gyrinus-antenna is probably a means of 

 keeping it dry.^ The Insect-antenna, when highly 



1 Dr. Sharp, Ent. Month. Mag. V. p. 52. 



2 Osten Sacken, Characters of the three Divisions of Diptera^ 

 p. 447. 



^ " In some aquatic Beetles [Gyrinus, Parnus) they [the 



