GRTHOPTERA. 



171 



Stone-Flies. 



its appearance. The emergence of May-flies takes place at different periods during 

 summer and autumn, and that of any one species may last for several days in 

 succession. At this time they sometimes appear in countless numbers, as thick 

 in the air as snowflakes, and at the end of their brief existence leave their dead 

 bodies to cover the ground, or float in masses down the stream. 



Nearly fifty species of Ephemeridce are found in the British Islands. Two of 

 the commonest (Epheffwm vulgata and E. danicd) are, in the subimago stage, 

 known to anglers as "green drake," and "grey drake." They are four-winged 

 species, with a body from one-half to three-quarters of an inch in length, and 

 furnished at the end with three very long tails. The fore-legs are extremely long, 

 especially in the males, which sex is distinguished also by the much larger size of 

 its eyes. The larvae of E. vulgata burrow in the mud, or hide under stones, in 

 ponds and sluggish streams. They have rather long antennae, and the tusks of 

 their mandibles project a good way, and cross one another in front of the head. 

 They have six pairs of tracheal gills, which are turned up over the back, each gill 

 consisting of two narrow blades, united at the base, and fringed with hairs along 

 each side. The final transformations of the larvae occur about the end of May, or 

 early in June, at which time, on a fine evening, the winged insects may sometimes 

 be seen in hundreds, dancing in the air. 



The stone-flies (Perlidai), forming the last group of Pseudoneur- 

 optera with aquatic larvae, are narrow, elongated insects of a flattened 

 form, with a good-sized head, rather 



long, many pointed antennae, and four z*,^ 



not very closely reticulated wings, 

 which shut horizontally over the body 

 when at rest. The abdomen usually 

 carries two long, multiarticulatecl 

 styles at the extremity. The mouth- 

 organs are weakly developed in the 

 adult insects; the mandibles and 

 maxillae are membranous ; the maxil- 

 lary palpi long, with slender terminal 

 joints, and the labial palpi three- 

 jointed. The thorax is square or 

 oblong, with its three segments 

 'almost equally developed. The tarsi 

 are three-jointed, and have their claws 

 separated by a bilobed pad. The 

 species of this family are not numer- 

 ous, though some are almost worldwide in their distribution. The adults appear 

 about the same time as dragon-flies and alder-flies, and frequent nearly the same 

 places. Though they have large enough wings, they fly heavily, and not for any 

 considerable distance at a stretch, and are generally most active in the evening. 

 The female fastens her eggs loosely together, and drops them in masses as she flies 

 over water. The larvaa are mostly found in rapid streams, where they keep under 

 stones, or among broken pieces of wood, and live by preying actively upon the 



COMMON STONE-FLY (Perla bicaudata). 



1, Larva ; 2, The fly escaping from the larval skin ; 3, The 

 perfect insect. 



