3136 



THE JOINTED ANIMALS 



adult, the mandibles being nearly always strong and toothed, and sometimes giving 

 off a tusk-like process in front of the head. At their transformation most May flies 

 do not change directly from the larval form into the imago, but first pass through 

 a stage, known as the subimago, in which they have their wings expanded, and 

 breathe through the spiracles like the perfect insect. In this form they are distin- 

 guished by the dullness of their integument, the shortness of the fore-legs and tail 

 bristles, and the less prominent and duller eyes. The subimago emerges from the 

 larval skin at the surface of the water, and, after standing awhile upon the water, 

 flies to a more convenient resting place. At the next molt, which soon fol- 

 lows, the perfect insect makes 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 snow-flakes, and at the end 

 of the 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 {Ephemera vulgata and E. danica) are, in the subimago stage, known 

 to anglers as "green drake" and "gray 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 trans- 

 formations 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 (Per- 

 lidce), forming the last 

 group of Pseudoneuroptera with 

 aquatic larvae, are narrow, elongated 

 insects of a flattened form, with 

 a good-sized head, rather long, 

 many-jointed antennas, and four 

 not very closely reticulated wings, 

 which shut horizontally over the 

 body when at rest. The abdomen 

 usually carries two long, multiartic- 

 ulated styles at the extremity. The 

 mouth organs are weakly developed 

 in the adult insects; the mandibles 

 COMMON STONE FLY (Perla bicaudata) . &nd maxillss are membranous; the 



Larva; 2. The fly escaping from the larval skin; 3. The 1-1 -,i 1 j 



perfect insect. maxillary palpi long, with slender 



Stone Flies 



