28 [February, 



The median lobe is remarkably short and broad, though it extends 

 forwards as a pair of long calliper-like struts, which struts are even 

 longer in the anterior direction tlian the base of the lateral lobes. 

 The median lobe forms a sort of triangle between the lateral lobes, 

 from which in this species (M. aquaticus) it is very distinct ; but little 

 more than the struts being embraced by the lateral lobes. Its dorsal 

 sclerite is longitudinally depressed along the middle and terminates as 

 a blunt point extending not quite so far back as the tips of the lateral 

 lobes. The ventral aspect is strongly chitinisecl along the middle in a 

 rod-like manner, and extends forwards so as to form a sharp-pointed 

 process in the middle of the arch formed by the calliper-like struts ; 

 at the termination there is a fine transverse chitinisation with only an 

 obscure appearance of ring structure, though it forms one lip of the 

 median orifice. This structure is, as I have previously said, remarkable 

 for its simplicity and symmetry. 



There is no sac ; the ejaculatory duct runs through the aedeagus 

 to its termination, and by holding open the median orifice one can look 

 down the duct. A little distance before entering the median lobe the 

 duct has a dilatation which ceases considerably before the apex - the 

 duct becoming again quite slender — but this dilatation has in no way 

 the nature of a sac. It is surrounded by a very strong muscular coat 

 and cannot be everted. 



Although the above is a fairly complete account of the aedeagus 

 itself, yet if we wish to form a conception of it as a mechanism we 

 must take into consideration the parts adjoining it as well as the 

 musculature. 



The freedom of movement between the various parts of the 

 aedeagus is extremely slight. There is some flexibility at the junction 

 of the basal sclerite with the parts beyond it, and there is a small 

 movement, required of the upper and lower lips of the extremity of 

 the median lobe so as to open the median orifice, but the median lobe 

 itself has very little capacity of movement independent of the lateral 

 lobes ; it can indeed by pressure be diverted a little to one side or the 

 other, and an extremely small backwards and forwards motion can be 

 produced by drawing on, or by pushing out, the struts ; the lateral 

 lobes can be made to approximate a little at their tips, or to diverge a 

 little ; but all these movements are very light, and, except in the case 

 of the opening of the lips of the median orifice, appear to be due to 

 elasticity of the parts. 



