In a natural population in England the maximum production of parthenogenetic eggs per female was found towards the end of April. There were two periods of sexual reproduction, the first lasting only 2 weeks in May; the second lasting from the middle of Sept. to the end of Nov. The head spine was longest in spring when food was abundant and was short in the summer when food was scarcer. The tail spine increased in length with increasing temperature, showing an increase in summer, when the head spine decreased. Samples for comparison were taken from Greenland and Italy. The Greenland samples emphasized the correlation between low nutritional level and the reduction of the head spine. The Italian sample indicates that temperatures higher than those found in England may cause some reduction in the length of the tail spine.