440 NEUROPTERA chap. 



pair of oviducts. The male has neither vesiculae seminales, acces- 

 sory glands, nor ductus ejaculatorius. The testes are elongate 

 sacs, whose extremities are prolonged backwards forming the vasa 

 deferentia ; these open separately at the extremity of the body, 

 each on a separate intromittent projection of more or less complex 

 character, the two organs being, however, connected by means of 

 the ninth ventral plate, of which they are, according to Palmen, 

 appendages. We should remark that this authority considers 

 Heptagenia to form, to some extent, an exception as regards the 

 structures of the female ; while Polymitarcys is in the male sex 

 strongly aberrant, as the two vasa deferentia, instead of being 

 approximately straight, are bent inwards at right angles near 

 their extremities so as to meet, and form in the middle a common 

 cavity, which then again becomes double to pass into the pair of 

 intromittent organs. 



According to the views of Exner and others, the compound 

 eyes of Insects are chiefly organs for the perception of movement ; 

 if this view be correct, movements such as those made during 

 the dances of may-flies may, by the number of the separate eyes, by 

 their ciurved surfaces and innumerable facets, be multiplied and 

 correlated in a manner of which our own sense of sight allows 

 us to form no conception. We can see on a summer's evening how 

 beautifully and gracefully a crowd of may-flies dance, and we may 

 well believe that to the marvellous ocular organs of the flies them- 

 selves (Fig. 274) these movements form a veritable ballet. We 

 have pointed out that by this dancing the peculiarly formed aliment- 

 ary canal becomes distended, and may now add that Palmen and 

 Fritze believe that the unique structure of the reproductive organs 

 is also correlated with the other anatomical peculiarities, the con- 

 tents of the sexual glands being driven along the simple and 

 direct ducts by the expansion of the balloon-like stomach. During 

 these dances the momentary conjugation of the sexes occurs, 

 and immediately thereafter the female, according to Eaton, 

 resorts to the waters appropriate for the deposition of her eggs. 

 As regards this, Eaton says : ^ " Some short-lived species discharge 

 the contents of their ovaries completely en masse, and the pair 

 of fusiform or subcylindrical egg -clusters laid upon the water 

 rapidly disintegrate, so as to let the eggs sink broadcast upon 

 the river-bed. The less perishable species extrude their eggs 

 1 Tr. Linn, Soc. 2nd ser. Zool. iii. 1883, p. 11. 



