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. Basal-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 : Hydroi^hilus (ffi/drous of recent 

 authors) jnceits L., Europe ; H. ater Fabr., Paraguay. 

 Anacacna ovata Reiche, England. Berosics luridus L., and 

 B. signaticollis Charp., Brockenhurst. Laccohius ytenensis 

 Sharp, Brockenhurst. HelopJiorus aquaiicus L., Brocken- 

 hurst. Dactylosternum suhdcpressum Cast., Panama. 



Figs. 44-46a PI. XLIX. 



Hydrophilus piceus (PI. 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. Ivii. The[niedian lobe is well developed, mem- 

 branous, strengthened by three sclerites. A ring-like one (o) sup- 

 ports the median orifice, a thin rod-like one runs down the ventral 

 surface, and a large one (6) covers the dorsal surface ; the latter is 

 narrow at the tip and broadens out basally, where it extends into a 

 pair of median struts (vis), 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 (hp) 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 pn) 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 



