350 



Mr. H. Ling Roth's observations on 





the nymphs are hatched out after 141 to 160 days' incuba- 

 tion, and a further 32% fairly evenly distributed in the 

 next 30 days (161 to 190 days), after which there is a 

 considerable drop. 



The variation in the number of days required for incuba- 

 tion by eggs dropped during 24 consecutive hours by any 

 one individual insect is great, the range varying from 6 to 

 92 days. I am unable to account for this. I do not think 

 it to be due to unsuitable, i. e. to too low a temperature, as 

 the nymphs and insects thrive well at 15° C. (59° F.). Nor 

 do I think it due to want of moisture, for of 71 unhatched 

 eggs examined from 421-580 days after having been 

 dropped, in 67 (= 94%) the contents were still quite 

 moist. It is, generally speaking, the last nymph to emerge 

 which makes the big jump in the range. I have not made 

 any experiment to ascertain whether the extreme range is 

 due to dryness, but H. H. P. and H. C. Severin have shown 

 that dryness at the time of hatching with Diapheromera 

 femorata, Say, has a marked retarding effect on the 

 emergence of the nymph from the egg (Jour. Econ. Ent. 

 1910, p. 481). 



Table II. — Range of Incubation of Eggs dropped in 24 Hours 

 by Individual Insects. 



* Only a portion of those observed are given, but sufficient to give 

 a general idea of the results. 



