OF WASHINGTON, VOLUME XVII, 1915 59 



man for the boll weevil, or for the Gypsy moth, in one of these 

 biocenoses, and substitute pathogenic organisms and their vectors 

 for the parasites we would have a set of analogously interde- 

 pendent relations. There is one striking difference. In the case 

 of the parasites of the boll weevil the purpose of entomologists 

 is to increase their efficiency by adding links in the chain or 

 otherwise; while in the case of parasites of man the purpose is 

 to break up the relations so that the attacks of the parasites 

 against the host will be lessened. Notwithstanding this differ- 

 ence both efforts rest on the same foundation, that is, an inti- 

 mate knowledge of the complicated relations between interlock- 

 ing and inter-independent groups of animals. 



The time has not arrived for the classification of the conditions 

 under which insects may transmit diseases, as our knowledge is 

 being extended almost daily and unsuspected conditions or sets 

 of conditions are coming to light. For the present purpose, how- 

 ever, certain conditions which seem to be of importance in con- 

 nection with disease transmission by insects will be mentioned, 

 not so much in the way of a classification as an enumeration of 

 the modes involved. 



Undoubtedly the most important habit of insects which has a 

 bearing on disease transmission is that of sucking blood. This 

 is the basis for the transmission of the great majority of insect- 

 borne diseases. There are probably many complicated inter- 

 relations involved. Among them seems to be the habit of cer- 

 tain parasites of man and other animals, such as the species of 

 Filaria, to swarm in the peripheral blood during the time when 

 nocturnal insects are active and the host is least in a condition 

 to interfere with their attack. Of course some investigators 

 explain this phenomenon on purely physiological reasons, that 

 is, the supposed expansion of the sub-cutaneous capillaries when 

 sleep begins which may allow organisms too large to reach posi- 

 tions immediately under the skin during the day to do so during 

 the night. But we think it not too much to suppose, in view of 

 what is known of the adjustments between other organisms, that 

 the swarming in the peripheral blood is an adaptation to assist 

 in establishing the necessary connection between two interde- 

 pendent forms. 1 



Another consideration of importance in this connection is what 

 may be called domesticity. This is of great importance, as F. 

 Knab has pointed out, 2 in connection with such diseases as yel- 

 low fever, kala azar, and Chagas disease. It must be evident 



1 Manson, Patrick. Tropical diseases. A manual of the diseases of 

 warm countries, 1914, 673. 



- Knab, F. Journal oi Economic Entomology, Volume V, p. 196. 



