610 Mr. D. Sharp and Mr. F. Muir on the Comparative 



appears to be an organ admirably adapted for this purpose, 

 and its occurrence and reoccurrence in so many isolated 

 forms is, to say the least, highly suggestive. Even in cases 

 vifhere there is no true flagellum, it may well be the case 

 that the functional orifice of the male (not to be confounded 

 with our " median orifice") is applied to the orifice of the 

 spermatheca. See on this point our figures 58 and 63. 



Certainty as to this point can only be obtained by 

 repeated observations of the genital tube during its func- 

 tional activity, and as to this we have been able to make 

 but few observations. 



In Bhagonycha fulva $ the sac is large and rounded, 

 with three pairs of diverticula along the posterior surface, 

 and a large patch of strong spines on the ventral side 

 (fig. 237a, a) ; the duct opens between the most dorsal pair 

 of diverticula. During copulation this sac distends the 

 uterus to its own size, and the patch of spines covers the 

 entrance to the oviducts. The abundance of this species 

 would make it a convenient form to work out all the 

 details of copulation on. 



Unfortunately the process of killing the insects causes 

 the muscles that actuate the internal sac to relax or con- 

 tract, and so the exact relations of the sac and the female 

 parts are never fully revealed. The shape of the female 

 parts does not exactly correspond to the shape of the male 

 sac and all its diverticula, etc., but there is a co-relation- 

 ship between them, and apparently they always take up 

 the same position in any one species. Besides the direct 

 evidence as to the importance of the internal sac and its 

 evagination during coition there is the great mass of 

 indirect evidence afforded by the complex armatures 

 that are developed upon them, especially at the apex. 

 In Fissodes Hopkins * calls this armature the " seminal 

 valve," but in the various examples of the different 

 families that we have examined the armature does not 

 function as a valve. In cases where there is no differenti- 

 ated internal sac it is difficult to state how much of the 

 duct is evaginated, but judging by observations made on 

 certain Hydrophilidae a large amount is turned out. The 

 evagination is done, at any rate in part, by blood pressure, 

 and the invagination by the contraction of muscles attached 

 to certain points on the internal sac and to the median 

 lobe. 



* U.S. Dept. Agr. Technical Series, No. 20, part I, 1911. 



