406 



NEUROPTERA 



CHAP. 



Perlidae, while those of the male (Fig. 258, A) are mere useless 

 rudiments. Morton has pointed out that in Scotland more 

 than one species of Taeniopteryx occasionally produces micro- 

 pterous males, and he associates this phenomenon with the early 



time of their appearance 

 " almost in winter." J In 

 Nemourct trifasciata this 

 reduction of the wings 

 takes another but equally 

 curious form ; the hind 

 wings of the male being 

 long enough to cover the 

 body, while the anterior 

 pair are reduced to mere 

 rudiments. 



The phenomena of mi- 



FIG. 258. Isogenus nubecula, Loch Tanna. A, Cropterism ill Perlidae are 

 Male ; A', wings of male more magnified ; B, -\\Q\\ worthy of more de- 

 wings of female. . J 



tailed investigation. Mr. 



Morton informs the writer that the male of Perla 'maxima (Fig. 

 251) in North Britain has the wings so short that they cannot 

 be of any use as organs of flight. In Central Europe the wings are 

 ample, as shown in our figure. In Perla cephalotes the male is 

 short-winged in both Britain and Central Europe ; of the male of 

 Dictyopteryx microcephala only the micropterous form is known to 

 exist. In Isogenus nubecula (Fig. 258) it appears that the 

 wings of the female are always more ample than those of the 

 male of the same locality, and that local micropterism affects 

 the two sexes unequally. Within the Arctic circle this Insect 

 is usually of the Scotch form, though the male there occasionally 

 has more ample wings. 



It has been observed that in some Perlidae the eggs, after 

 they have been extruded, are carried about by the female ; for 

 what reason is not at all known. They are said to be enclosed 

 in a membranous capsule at the apex of the abdomen. The 

 number of eggs deposited is sometimes very large, amounting to 

 five or six thousand, and they are often of very minute size. 



About twenty-four species of Perlidae occur in Britain. 2 The 



1 Entom. Month. Mag. xxix. 1893, p. 249. 



- No satisfactory systematic work of a general character on British Perlidae 



