494 Mr. D. Sharp and Mr. F. Muir on the Comparative 
men at base. Lateral lobes narrow and bluntly pointed, the distal 
half bearing fine hairs along edge. JBasal-piece long and narrow. 
No differentiated internal sac. 
The aedeagus of the Gyrinidae is of the trilobe type with 
well-developed basal-piece, and they should not be placed 
with the Dytiscidae, but near to the Hydrophilidae. The 
comparatively simple trilobe form and undifferentiated 
internal sac indicate a form of low specialisation (accom- 
panied by extreme adaptive characters of the body). In- 
formation as to the mode of fertilisation in this family is 
very desirable. 
Family HYDROPHILIDAE. 
Forms examined: Hydrophilus (Hydrous of recent 
authors) piceus L., Europe; H. ater Fabr., Paraguay. 
Anacaena ovata Reiche, England. Berosus luridus L., and 
B. signaticollis Charp., Brockenhurst. Laccobius ytenensis 
Sharp, Brockenhurst. Helophorus aquaticus L., Brocken- 
hurst. Dactylosternum subdepressum Cast., Panama. 
Figs. 44-46a Pl. XLIX. 
Hydrophilus piceus (Pl. XLIX fig. 44). 
The aedeagus of this insect is the best known of any, as it has been 
figured and described by many writers. See especially Escherich, 
Zeitschr. Wiss. Zool. lvii. The’median lobe is well developed, mem- 
branous, strengthened by three sclerites. A ring-like one (a) sup- 
ports the median orifice, a thin rod-like one runs down the ventral 
surface, and a large one (b) covers the dorsal surface ; the latter is 
narrow at the tip and broadens out basally, where it extends into a 
pair of median struts (ms), a keel runs down the centre, bifurcates 
about the middle and the keels continue on to the median struts. 
The lateral lobes are broad at the base, where they meet both dorsally 
and ventrally and embrace the base of the median lobe; from the 
base they taper off to a point at the apex. ‘The basal-piece is 
formed by a large, shield-shaped sclerite (bp) with its lateral edges 
turned up, the dorsal side being membranous. When the muscles 
acting upon the median struts force the median lobe outwards, the 
fact of it being articulated to lateral lobes (at the point of articula- 
tion pa) causes it to turn dorsally upon that point, this at the same 
time forces the lateral lobes apart. This appears to be the action of 
all the trilobe types in which the lateral arms are free (not con- 
solidated together) and the median lobe is articulated to the lateral 
