66 AN INTRODUCTION TO ENTOMOLOGY. 



fly, a common representative of this family found under stones in 

 the beds of streams at Ithaca. 



" May-fly nymphs mostly feed upon either mud or minute aquatic 

 vegetation, such as covered stones and the larger plants ; but (judg- 

 ing by their mandibles and maxillae) some must be predaceous. 

 Many of them live in concealment in the banks or under stones in 

 the bed of streams, rivers, and lakes ; others ramble openly amongst 

 water-weeds and swim with celerity." (EATON.) The nymphs of 

 May-flies undergo many moults ; Cloeon, according to the observations 

 of Lubbock, sheds its skin twenty-one times. In this insects there 

 are no traces of wings until the ninth state ; then the hinder lateral 

 angles of dorsal aspect of the meso- and metathorax become slightly 

 produced. With each successive moult these prolongations become 

 more marked, until in the twentieth state, the last passed under 

 water, the rudimentary wings cover half or the whole of the third 

 abdominal segment. The wing-cases are not inverted (see page 6). 



Most members of this family exhibit a remarkable peculiarity in 

 their development. After the insect leaves the water and has ap- 

 parently assumed the adult form, that is, after direct respiration 

 through the spiracles has been established, and the wings have be- 

 come fully expanded, it moults again. This is the only instance in 

 which insects with fully developed wings cast their skin. The term 

 subimago is applied to this stage between the nymph and the final 

 form of the insect. With some species the duration of the sub- 

 imago stage is only a few minutes ; the insect moults on leaving the 

 water ; flies a short distance; and moults again. In others this stage 

 lasts twenty-four hours or more. 



The adults of the Ephemeridae eat nothing. Not only is the 

 period of existence in this state so brief that food in addition to 

 that taken in the nymph state is unnecessary, but the imperfect con- 

 dition of the mouth precludes the taking of nourishment. With 

 many species of May-flies there is great uniformity in the date of 

 maturing of the individuals. Thus immense swarms of them will 

 leave the water at about the same time, and in the course of a few 

 days pass away ; this being the only appearance of the species until 

 another generation has been developed. The great swarms of 

 " Lake-flies " (Ephemera simulans], which appear along our northern 

 lakes about the third week in July, afford good illustrations of this 

 peculiarity. And an instance is on record where May-flies thronged 

 to the gas-burners in a store and actually quenched the flames. 



The Ephemeridae are regarded as the lowest living representatives 



