66 



APPLIED ENTOMOLOGY 



outgrowths of the body wall. Into them pass tracheal trunks which 

 branch again and again so that only their own walls and those of the gill 

 itself separate the air in the tracheae from that in the water outside, and 

 so thin are these layers that the oxygen in the water can pass through 

 them into the tracheae, and carbonic acid gas pass out. 



These insects add to their list of peculiarities also, the fact that after 

 becoming full grown and being able to fly, they molt once more, even a 

 thin layer over the final wings being shed. $ub;>>M^ 



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FIG. 38. Adult May-fly (Hexagenia varidbilis Eaton) showing the long cerci. N.atural 



size. (From Folsom.) 



From these statements the group may be characterized thus: 

 Inserts having as adults delicate bodies and usually four wings, the 

 front pair much larger than the others (which are sometimes absent), and 

 generally with many cross-veins: end of the abdomen with two or three long, 

 caudal filaments composed of many segments: reproductive organs with two 

 openings to the exterior: mouth parts of the chewing type but practically 

 rudimentary: nymphs living in water and with an incomplete metamor- 

 phosis, the final molt coming after the wings have become fully developed. 

 May-flies are most abundant near streams and lakes, as their nymphs 

 live in the water. The fully mature nymphs leave the water, usually 

 in greatest numbers about sunset, and suddenly molting, extend their 

 wings and fly off, but as above stated usually molt again within a few 

 hours. As their flight generally begins about dusk and as they are 

 strongly attracted to lights, they are often seen in multitudes around 

 street lights during the evenings. 



