THE CANADIAN ENTOMOLOGIST 279 



verified the statements made by Williamson in Section DD of his 

 analysis, but, in fitting my model together, I found that the parts 

 mentioned by him here must necessarily be related in exactly the 

 manner described. 



As stated by Williamson in Section BB, "The female, by 

 drawing the hind lobe of the pronotum against the mesostigmal 

 laminae, prevents the escape of the male." In the case of Argia 

 moesta putrida, the result of this action is that the two pairs of 

 appendages ci the male are drawn together and it can readily be 

 seen by examining the figures that in such a position these appen- 

 dages are incapable of being shifted in any direction, and hence 

 escape of the male is impossible unless permitted by the female. 



The mutual adaptation of these structures in the two sexes is 

 so precise that it seems improbable that copulation could take 

 place between different species of Argia, even though very closely 

 related. 



Explanation of Plate VIII. 



Fig.l. — Position of the abdominal appendages of the male in 

 relation to the thorax of the female in copulation. 



Fig. 2. — Posterior view of end of male abdomen. 



Fig. 3. — Dorsal view of parts of the pronotum and mesoepis- 

 ternum of the female. 



Fig. 4. — Semi-diagrammatic combination of figures 2 and 3, 

 showing the relative positions of the parts in coupling. The parts 

 of the male are indicated by dotted lines. 



S. ap., superior appendage of male; i. ap., inferior appendage 

 of male; p. i. ap., posterior surface of same; v. i. ap., postero-ventral 

 surface of same; h. terminal hook of same; m. t., tubercle which 

 engages the cavity between the forks of the dorsal thoracic carina; 

 v. t., viscid lateral tubercle; m. p., middle lobe cf pronotum of fe- 

 male; h. p., hind lobe of pronotum of female; d. c, middorsal 

 thoracic carina; ms. 1., mesostigmal lamina of female; ms. t., meso- 

 ^pisternal tubercle of female. 



