BIOLOGY OF MAYFLIES OF THE GENUS BAETIS. 



'It is evident that the cycle started October 28, 1918, was completed in the 

 laboratory in the same length of time as that of the previous cycle extending 

 from May-October in the creek; but that the corresponding cycle (Oct. 28) in 

 the creek was completed three months later. 



A summary of the corresponding yearly cycles (Table B) and the con- 

 tinuous cycles (Table C) for the years 1918-1920 inclusive, together with the 

 average monthly temperatures, shows a striking ratio between the length of cycle 

 and the temperature. One six month cycle confined entirely to the warm months, 

 is followed by two nine month cycles extending through the winter. Two years 

 are necessary to complete three continuous cycles (Fig. 3). There are three 

 periods of emergence during a year. 



Four females captured during a mating flight October 28, 1918, were found 

 to be parasitized. Each imago contained a single nematode coiled up in the body 

 cavity (Fig. 16). The hosts appeared natural in flight, and it was not until the 

 specimens were preserved that the presence of the parasite was observed. The 

 abdomen, normally conspicuously filled with eggs, seemed to contain nothing 

 except the nematode. 



B. PYGMAEA AND B. PRONPINQUUS. 



Associated with nymphs of B. posticatus are nymphs of B. pygmaea and 

 B. propinquus. An examination of a series of reared specimens of these three 

 species fails to reveal specific characters which are valid at any given instar. 

 Mature and fully colored nymphs may be separated by the relative characters 

 given in Table D. A microscopic examination of the mouth-parts shows slight 

 variations in the form of the labial palp (Figs. 13, 14, 15), and in the maxilla 

 (Figs. 17, 18, 19). 



The venation of the hind wing of the male (Figs. 4, 5, 6) and female, and 

 the genitalia of the male (Figs. 1, 2, 12), furnish definite specific characters in 

 the imago. There is also a variation in the relative length of the first and second 

 leg in the males of these species (Figs. 9, 10, 11). 



The oviposition of B. pygmaea is described by Morgan in 1911 (3) ; the 

 mating flight of a species of Baetis by the same author in 1913 (4). 



The life cycle of B. pygmaea reared in the creek extending from August, 

 1919 to August, 1920; that of B. propinquus from September, 1919 to August, 

 1920. Change of residence prevented further rearing experiments to determine 

 the number of stadia in these two species, and whether there are overlapping cycles. 



LITERATURE CITED 



(1) Clemens, W. A. Canad. Entomol., 45 :340-34l. 1913. 



(2) Eaton, A. E. Monogr. Recent Ephemeridae. 1885, p. 169. 



(3) Morgan, A. H. Ann. Ent. Soc. Amer., 4:116. 1911. 



(4) Morgan, A. H. Ann. Ent. Soc. Amer., 6:393. 1913. 



(5) Needham, J. G. Bull. No. 47, N. Y. State Mus. 1901, p. 423. 



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