JMAV-FLIES 



439 



to I'almen ^ the divisions of the canal are separated by changes 

 in the direction of certain portions anterior to, and of others 

 posterior to, its central and greater part — the stomach — in such 

 a manner that the portions with diverted positions act as valves. 

 The stomach, in fact, forms in the interior of the body a delicate 

 capacious sac ; when movement tends to increase the capacity of 

 the body cavity then air enters into the stomachic sac by the 

 mouth orifice, but when muscular contractions result in pressure 

 on the sac they close the orifices of its extremities by the valve- 

 like structures we have mentioned above ; the result is, that as 

 complex movements of the body are made the stomach becomes 

 more and more distended by air. It was known even to the old 

 naturalists that the dancing may-fly is a sort of balloon, but they 

 were not acquainted with the exact mode of inflation. Pahnen 

 says that in addition to the valve-like arrangements we have 

 described, the entry to the canal is controlled by a circular muscle, 

 with which are connected radiating muscles attached to the walls 

 of the head. Palmen's views are adopted, and to a certain extent 

 confirmed, by Fritze,^ who has examined the alimentary canal of 

 the may-fly, and considers that though the normal parts of the 

 canal exist, the function is changed in the imago, in which the 

 canal serves as a sort of balloon, and aids the function of the 

 reproductive organs. The change in the canal takes place in an 

 anticipatory manner during the nymph and sub-imago stages. 



The sexual organs of Ephemeridae are remarkable for their 

 simplicity ; they are destitute of the accessory glands and diver- 

 ticula that, in some form or other, are present in most other 

 Insects. Still more remarkable is the fact that the ducts by 

 which they communicate with the exterior continue as a pair to the 

 extremity of the body, and do not, as in other Insects, unite into 

 a common duct. Thus in the female there is neither bursa copu- 

 latrix, receptaculum seminis, nor uterine portion of oviduct, and 

 there is no trace of an ovipositor ; the terminations of the ducts 

 are placed at the hind margin of the seventh ventral plate, just 

 in front of which they are connected by a fold of the integu- 

 ment. The ovary consists of a very large number of small egg- 

 tubes seated on one side of a sac, which forms their calyx, and 

 one of whose extremities is continued backwards as one of the 



1 Tiber vaarige Ausfuhrscjdnge, etc., Helsingfors, 1884, p. 53. 

 2 Ber. Ges. Freiburg, iv. p. 6 ; cf. J. B. Micr. Soc. 1889, p. 206. 



