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 setse. 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. Each vas 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 May-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 Hving forms. In 

 some cases (CcBnis 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 



■^^ 



Fig. 353. — Fore wing of Chirotonetes alhomanicatus . 



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 intercalary 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 



