10 



of CellfJien/is is usually accompanied by the male, who helps 

 her to escape when menaced by the open mouth of a hungry 

 fish. Several hundred eggs are often laid by a single female. 



The period of incubation varies with the season and also 

 independently of it. In midsummer, eggs of some species hatch 

 in from six to ten days, while others, laid in autumn, do not 

 hatch until the following spring. In the same lot of eggs the 

 period of incubation may vary, even in midsummer, from a 

 week to more than a month. 



The apparent abundance of nymph and imago is far from 

 corresponding, the difference in some cases being quite surpris- 

 ing. Of the nymph of Celifhemis eponina, we have secured only 

 a few examples; yet the imago is a familiar sight everywhere 

 about the Station. The species of Sj/)»prfrH))i are common and 

 familiar dragon-flies; but we have obtained only a few of the 

 nymphs. This genus probably breeds in swampy places, where 

 the vegetation is so dense, the water so shallow, and the mud 

 so deep as to make collecting very difficult. On the other hand, 

 Epicordidia princeps is abundant and widely distributed as a 

 nymph, but the imago is not commonly taken. Nymphs of 

 Macrotnia, Progowphun, and Hagenius are not at all rare in 

 streams ; the imagos are considered very rare or almost 

 unobtainable. In the Gomphidre are numerous similar exam- 

 ples. This discrepancy may be due either to the swift, high, or 

 prolonged flight of the imago, or to the shortness of its life ; 

 but in some cases it is almost inconceivable how the imagos 

 can vanish so completely as they do. 



Walsh ('63, p. 239) makes some remarkable statements as 

 to the relative proportion of the sexes in (iompJius. In some 

 species he found four males to every female, and in another 

 two or three females to each male, and he asserts that this is 

 the case in freshly emerged material. Mr. Needham is of the 

 opinion that in nymphs generally there is no notable excess of 

 either sex, but that in the imago an excess of males may occur 

 because of the destruction of the females by flshes in species the 

 females of which oviposit unattended by the males. It is prob- 



