183 



own country. Only with some doubt do I now enter territories of human know- 

 ledge which have nothing whatever to do with limnology and only very little with 

 zoology generally; we must now turn our attention to agricultural history, to 

 the great changes in agriculture which our country has been subject to in the for- 

 mer century. I beg Prof. Dr. JOH. STEENSTRUP to accept my best thanks for the help 

 he has given me upon this point. It is a well-known fact that in the eighteenth 

 century, and still in the first quarter of the nineteenth, the swine were driven to 

 the woods where they lived on mast; special swine stables were hardly known. 

 Horses and cattle lived the greater part of the year out of doors, many of them 

 the socalled "Udgangsog" (jades) the whole of the year. At that time Danish agri- 

 culture was based upon cereal culture; a little after the middle of the nineteenth 

 century, after the establishment of cooperative dairies, the stress w r as laid upon the 

 greatest possible production of bacon, meat, milk and butter. Then it was no more 

 possible to let the farm animals live almost their whole life out of doors. Year 

 after year the stables became better and better; larger and larger; the number of 

 animals continually greater; the time in which the animals lived out of doors 

 continually shorter; the hogs were almost all their lives bound to the hogpens, 

 the cattle was often kept in the stables the whole year round or, apart from a 

 very short summertime, driven into the stables before night. The stables were bet- 

 ter and better lighted, during the last century often by electric light. 



It will be clearly understood that this change in Danish agriculture must of 

 course be of the greatest significance to the Anophelines; for during a great part 

 of their flying time the large mammalia year after year disappeared 

 from their flying areas. But not only the domestic animals disappeared, man 

 himself by no means lived as much in the fields as hitherto; the agricultural ma- 

 chines make the extensive use of manual working power superflous. Where formerly 

 at harvest time long rows of harvesters and harvest women could be seen in the 

 fields for more than three weeks we now only see a few selfbinders going their 

 way round over the fields. 



But simultaneously with the disappearance of all warm blooded animals from 

 the fields, in our warm well-lighted stables we created refuges which acted upon 

 the mosquitoes as large thermostats spread in many, many thousands 

 all over the country; the odour from all the large animals streaming out on 

 the still evenings through open doors and windows, the heat which thermically 

 attracted the mosquitoes, and the light which attracted them phototactically, con- 

 verted the stables for our Anophelins, which were on the wing in the evening in 

 search of prey, into thermical and lighted traps by which the mosquitoes 

 instinctively governed their flight. Inclosed inside the stables they found 

 all that they wanted for their life: plenty of food, a suitable temperature, darkness 

 and no draught; only the conditions for mating and egg-laying processes were 

 wanting, but apart from them life in the stables was really possible for flying in- 

 sects. Arrived in the stables and, owing to the much stronger odour and greater 



