492 Mr. D. Sharp and Mr. F. Muir on the Comparative 
Family DYTISCIDAE. 
Forms examined: Dytiscus punctulatus Fabr., and D. 
marginalis L., England. Ilybius aenescens Th., England. 
Figs. 37 and 38 Pl. XLVIII. 
Dytiscus punctulatus (Pl. XLVI figs. 87 and 377). 
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. 6. 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 
undifferentiated. 
Dytiscus marginralis. 
This only differs in details from D. punctulatus, the median lobe 
is expanded into a small flattened disc at apex ; the lateral lobes are 
longer. 
Tlybius aenescens (Pl. 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 with short stout sense-hairs, the rest of inner surface 
with long fine hairs. Basal-piece (bp) 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 Dytiscus. 
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 - 
