MAYFLIES, DUNS AND SPINNERS 229 



their eggs gradually part at a time in one of the 

 following ways. The female either alights on the 

 water to wash off the eggs which have issued from 

 the oviducts during her flight, or else she creeps down 

 into the water, enclosed within a film of air with 

 wings collapsed so as to overlie the abdomen and, with 

 her setae close together, to lay the eggs in rounded 

 patches on the underside of stones in a single layer 

 evenly spread. She then floats up to the surface, 

 where her wings are suddenly unfolded and erected and 

 either flies away, or if her setae have become wet 

 she is detained by them, and is drowned on the sur- 

 face of the water. He adds that "in some instances, 

 however, the female dies under water beside her eggs." 



The eggs hatch in due course, and the nymph, 

 which emerges from the eggs, 

 is in shape very like the adult 

 insect, but it has invariably three ciliated caudal setae or 

 tails, and the mouth organs are fully developed, as at 

 this stage it requires a considerable quantity of food. 

 The nymphs live, some on minute aquatic vegetation 

 and some on mud, but judging from their powerful 

 mandibles some species are probably carnivorous. 



They all have tracheal branchiae of various shapes, 

 which are arranged in pairs on seven or fewer of the 

 foremost segments of the abdomen ; and their function 

 is considered to be the change of carbonic acid, intro- 

 duced into the air contained within the tracheal system 

 from the fluid which serves as blood, for oxygen held 

 in solution in the surrounding water. 



