THE LIFE HISTORY OF INSECTS 299 



3. Number the offspring consecutively, according to age, and note which 

 developed into winged and which into wingless forms. 



4. Note the age at which each of these individuals begins reproduction. It 

 might be suggested that when the first of this generation begins reproducing, 

 it is best to remove the young in order to prevent confusion of the generations. 



Exercise 45. Make a chart from your above notes, giving the number of the 

 individual, whether winged or wingless, date of birth, date of maturity (when 

 first young is produced). 



Exercise 46. Notes on the rapidity of growth. Isolate some newly born 

 individuals, noting the date and hour of birth. Watch these carefully, and note 

 the date and the hour that molting occurs. The cast skin will usually be found 

 near the young aphids, which begin feeding soon after molting. Those indi- 

 viduals just having molted will be found to be the lightest in color, but the cast 

 skin should be taken as the only proof that the insect has molted. As soon as 

 these individuals begin to reproduce, tabulate your above notes, giving the 

 number of hours between each molt for each individual. 



Exercise 47. Write up a detailed set of notes describing one wingless in- 

 dividual after each molt, up to and including the adult form, noting all the 

 changes which may occur. 



Exercise 48. Write up a detailed set of notes, similar to the above, for the 

 winged form. 



Exercise 49. Mount a wingless individual in the alcohol-glycerin solution 

 and make a drawing of the dorsal view. (The aphids should first be dipped in 95 

 per cent alcohol, and may then be mounted directly in the glycerin solution.) 



Exercise 50. Mount a winged individual in the alcohol-glycerin solution 

 and make a drawing of the dorsal view. 



SECTION XVIII. LIFE HISTORY OF THE DRAGON-FLY 



Material. It will be quite impractical for a class in elementary entomology 

 to try to trace the complete life history of the dragon-fly, but this form will 

 give the student a good idea of the habits and structure of aquatic nymphs. 

 The dragon-flies deposit their eggs on water plants, and as soon as these hatch, 

 the young nymphs settle to the bottom of the pond and may be found, at almost 

 any time of the year, crawling about in decaying vegetation or other sediment. 

 The easiest way to secure them is to rake out the sediment from the quiet pools 

 of a stream, or from the edge of ponds, with an ordinary garden rake. The 

 nymphs, together with a small amount of sediment, should be placed in an 

 aquarium until ready for observation. This applies especially to material collected 

 in the fall, as it will be difficult for each individual student to provide food and 

 suitable conditions for the nymphs that he may have under his observation. 



W 7 hen this work is undertaken by a class, each student should be provided 

 with a glass dish containing three or four of the largest-sized nymphs. As it is 

 necessary to feed the nymphs on other aquatic insects, it might be better not 

 to collect the material until early spring. 



