EPHEMERIDA 



309 



visible segments; the eleventh segment, which bears the cerci, 

 is overlapped by the tenth (Fig. 351). The cerci are long, slender, 

 and many-jointed; and in some species there is a median caudal 

 filament, which resembles the cerci inform; these three organs, 

 the two cerci and the median caudal filament, are commonly referred 

 to as the caudal setae. In the male there is a pair of clasping 

 organs placed ventrally at the extremity of the tenth segment ; these 

 are usually two-, three-, or four-jointed and are termed the forceps- 

 limbs. Eachvas deferens and each oviduct has a separate opening; 

 in the male these openings are at the caudal end of the body; in 

 the female, between the seventh and eighth stemites. 



In some May -flies the compound eyes are divided; one part of 

 each, in such cases, is a day-eye, and the other a night-eye (seepage 



144)- 



As the adult IMa^'-fly takes no food, its alimentary canal is not 

 needed in this stage for purposes of digestion, and, instead of serving 

 this function, acts as a balloon, being inflated with air, 

 thus lessening the specific gravity of the body and aid- 

 ing in flight. 



In this order a marked cephalization of the flight 

 function has taken place, which has resulted in a great 

 reduction of the hind wings in all living forms. In 

 some cases {Ccenis et al.), this has gone so far that the 

 hind wings are wanting (Fig. 352); but at least one 

 pair of wings is present in all members of this order. 



When at rest, the wings are held upright; they are 

 never folded over the abdomen. No anal furrow has 

 been developed. A striking feature of the wings of May-flies is their 

 well-known corrugated or fan-like form, there being a remarkably 



Pig- 352.— 

 C cBnis , a 

 t w o- winged 

 May-fly. 



Fig. 353. — Fore wing of Chirotonetes albomanicatus. 



perfect alternation of so-called convex and concave veins. Correlated 

 with the development of the fan-like form of the wings has been the 

 development of intercala;ry veins, that is, veins that did not arise as 

 branches of the primitive veins, but were developed in each case as 

 a thickened fold, more or less nearly midway between two preexisting 



