684 



CLASS IV. INSECTA. 



taking no food ; short antennae ; four membranous, closely reticu- 

 lated wings, the hinder pair being much smaller than the fore- 

 wings and at times absent ; the end of the abdomen bears three 

 or two very long cerci ; larvae aquatic ; an active winged sub- 

 imago stage precedes the imago. 



These delicate insects known as May-flies spend almost all 

 their life in a larval state, assuming their final stage but for a 

 few hours and only for the purposes of reproduction. In the 

 adult the mouth and its appendages have atrophied. The an- 

 tennae are short and with but few segments. The eyes are 



large and complex, especially in 

 the male, sometimes divided into 

 two separate organs on each side, so 

 that together with the three ocelli 

 there are seven visual organs. The 

 mesothorax, in correlation with the 

 large fore-wings, is very bulky. The 

 anterior legs are as a rule longer 

 than the others, in some males 

 very long. The abdomen has ten 

 segments. The alimentary canal of 

 the imago is very capacious and 

 has thin, extensile walls : within it 

 is stored air, the amount of which 

 can apparently be controlled by the 

 insect, and thus it acts as a balloon. 

 The reproductive organs are the 

 most primitive known amongst in- 

 sects. Both male and female ducts 



FIG. Wl.-Ephemera^vulgata. Af anal are pa i re d and devoid of aCCCSSOry 



glands or diverticula ; they open 



on the seventh abdominal sternite. The short life of the adult 

 May-fly, often only a few hours, is spent in an aerial dance, during 

 which the males fertilize the females, which then resort to the 

 water to lay their eggs. Some in a fever of hurry deposit the 

 contents of their ovaries in one mass which disintegrates into 

 eggs as it sinks through the water, others more leisurely wash the 

 extruded eggs from their abdomen by occasional dips in the stream 

 or even creep below the water and deposit them under stones. 

 The eggs are very numerous and may take half a year to hatch. 



