October, '21] freeborx: anopheles occidentalis 417 



the western counterpart of the eastern quad rim aculaius Say it is undoubt- 

 edly the most important malaria carrier in the West, the above 

 explanation and charts are offered. 



The study of the chart of male collection establishes two points (1) 

 that hibernation, indicated by the absence of males starts just after the 

 middle of November and (2) the curx-es are strongly suggestive of only 

 two generations a year with a possibility of the earliest members of a 

 third overtaking the stragglers of a second during September with the 

 appearance of a possible fourth in October or November. In connection 

 with the first point, "hibernation" is used in a rather loose fashion as no 

 true donnanc}' occurs in the overwintering females. They frequently 

 change their places of refuge as shown by the fact that we were able to 

 make repeated complete collections beneath the bridge thruout the 

 winter. However, the\- show no inclination to feed in the open thru- 

 out the period, the earliest records of this being in January in unusually 

 warm weather. 



In February' of each year the females begin to em.erge from their over- 

 wintering refuges. In the year shown on the chart (1920) this emer- 

 gence was accomplished gradually, but on the following \'ear the emer- 

 gence was extrem,ely unifonn and striking. About February twentieth 

 communities in m.an}^ parts of California were deluged with enor- 

 mous swarmis of /I. occidentalis females which invaded houses, biting 

 viciously even in bright sunlight. By the last of the month, however, 

 they had practically disappeared. During this flight of emergence, the 

 area infested is ahr.ost always greater than at am^ other period of the 

 year, many districts being invaded in February by A. occidentalis that 

 are entirely free from them during the remainder of the year. The 

 only explanation at hand is that the unusual length of flight at this time 

 is a biological stimulus to ensure the spread of the species. Females 

 dissected during the winter and at the start of the emergence flight 

 contained round, half grown eggs in the ovaries but those taken later 

 showed fully developed eggs. Some unfed migrants given a blood meal 

 in the laboratory developed m.ature eggs in approximately 4S hours. 

 This migratory fhght from hibernation quarters on the part of the over- 

 wintering females is the official opening of the season for A . occidentalis 

 Eggs are deposited and the adult population falls off ver\- rapidly in 

 num.bers with adult mosquitoes practically absent in April. From the 

 information at hand, which is lim.ited to a single experiment and the 

 study of the chart of female occidentalis collections, it would appear that 

 the overwintering females lay but a single batch of eggs before death 

 overtakes them. Several migrant females were confined in the labora- 

 tory' in the spring of 1920, and gi\-en blood meals whenever they could be 



