312 



AN INTRODUCTION TO ENTOMOLOGY 



algse, and diatoms. The variations in the details of their habits are 

 described as follows by Dr. Needham ('i8). 



"A few, like Hexagenia, Ephemera, and Po/jmz7a/-cv5 are burrowers beneath the 

 bottom silt. A few like Canis and Ephemerella, are of sedentary habits and live 

 rather inactively on the bottom, and on silt-covered stems. Alany are active 

 climbers among green vegetation; such are Callibcetis and Blasturns; and some 

 of these can swim and dart about by means of synchronous strokes of tail and gills 

 with the swiftness of a minnow. The species of Leptophlebia love the beds of 

 i.-jOW-flowing streams, and all the flattened nymphs of the Heptageninse live in 

 swiftly moving water, and manifest various degrees of adaptation to withstanding 

 the wash of strong currents. The form is depressed, and margins of the head and 

 body are thin and flaring, and can be appressed closely to the stones to deflect the 

 current." 



There are two features of special interest in the structure of the 

 naiads of May -flies: first, the hypopharynx bears a pair of lateral 



lobes, which are believed to be 

 : . vestiges of paragnatha ; and sec- 



ond, the presence of accessory 

 circulatory organs in the cerci 

 and median caudal filament 

 'Fig. 3 57)- 



May-fiies exhibit a remark- 

 able peculiarity in their develop- 

 ment. After the insect leaves the 

 water and has apparently as- 

 sumed the adult form, that is, 

 after the wings have become fully 

 expanded, it molts again. These 

 are the only insects that molt af- 

 ter the}^ have attained functional 

 wings. The term 5«6wzag(9 is ap- 

 plied to the instar between the 

 naiad and the final form of the 

 insect, the adult. With some 



Fig. 357. — A, caudal end of abdomen of 

 Clo'eon dipterum: h, heart; a, acces- 

 sory circulatory organs. B, twer.ty- 

 sixth segment of a cercus: 0, orifice in 

 blood vessel. (After Zimmerman.) 



species the duration of the sub imago stage is only a few minutes; 

 the insect molts on leaving the water; flies a short distance; and 

 molts again. In others this stage lasts twenty-four hours or more. 

 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 shnulans, which appear along our northern lakes 

 about the third week of July, afford good illustration of this peculi- 

 aritv. 



Family EPHEMERID^ 



The May-Flies 



The oraer Ephemerida includes a single family, the Ephemeridas; 

 the characteristics of this family, therefore, are those of the order, 

 which are given above. 



