492 Mr. D. Sharp and Mr. F. Muir on the Comjmrative 



Family DYTISCIDAE. 



Forms examined : Dytiscus punctulatus Fabr., and D- 

 marginalis L., England. Ihjbins aenescens Th,, England. 

 Figs. 37 and 38 PI. XLVIII. 



Dytiscus imnctulatus (Fl. XLVIII figs. 37 and 37rr). 



Basal half of median lobe forms a tube, the distal half projecting 

 as four prongs, the dorsal one chitinous, the ventral and lateral ones 

 membranous (fig. 37a, a. b. b.). The dorsal half of lobe forming a 

 strong chitinous plate, broader and turned down in the middle (c) 

 and bearing hairs at the apex, the basal part being curved upward 

 and expanded ; the ventral half is membranous (m). Lateral lobes 

 large and broad, bearing hairs at the tip and attached to the base 

 of the median lobe on the dorsal side (pa). A thin strut (bp) broad 

 at the end where it supports the membrane between the bases of 

 the lateral lobe, on the ventral side, represents the basal-piece. 

 This functions as a lever to which the muscles for turning the 

 aedeagus are attached. When invaginated the aedeagus rests on its 

 side, but when evaginated it takes a turn and the dorsal becomes 

 ventral. Our figure shows it in its true dorso-ventral position. Sac 

 undiiferentiated. 



Dytiscus marginalis. 



This only differs in details from D. pundnlatus, the median lobe 

 is expanded into a small flattened disc at apex ; the lateral lobes are 

 longer. 



Ilyhius aenescens (Pi. XLVIII fig. 38). 



Median lobe consisting of a strong, curved, thin sclerite, broad- 

 ened at the base and turned down to form a short groove, the ventral 

 side of this groove being covered by a membranous tongue (a), thus 

 forming a very short tube where the undifferentiated sac opens. 

 Lateral lobes broad at base, flattened and slightly twisted at tips 

 and attached on dorsal side of the base ; the inner dorsal surface 

 being studded witli short stout sense-hairs, the rest of inner surface 

 with long fine hairs. Basal-piece (hj)) broad at the end where it 

 partly surrounds the base of the median lobe but narrow beyond. 



This appears to be a more perfect structure than the 

 aedeagus of Dytiscics. 



The three families, Dytiscidae, Haliplidae, and Pelo- 

 biidae, are closely allied as to the aedeagus, the median 

 lobe being on the same plan, and differing from Carabidae 



