16 PREVENTIVE AND REMEDIAL WORK AGAINST MOSQUITOES. 



Some attention has been paid to the subject of the size of the mesh 

 of screens with especial reference to the yellow-fever mosquito. 

 Working party No. 2 of the Public Health and Marine-Hospital 

 Service, at Veracruz, conducted a few experiments to determine 

 the question of the size of the mesh. Their experiments were con- 

 ducted by placing screens with a varying number of meshes to the 

 inch over breeding jars and putting bananas, sirup, and other food 

 on the other side so as to tempt the hungry mosquitoes to pass 

 through. The fruit and other food was placed in a jar which was 

 inverted over the mosquito-breeding jar, and a piece of gauze or net- 

 ting was inserted between the two jars so that the mosquitoes would 

 have to pass through the meshes in order to appear in the upper jar. 

 As a result it was found that both males and females passed through 

 a netting containing 16 strands or 15 meshes to the inch, but could 

 not pass 20 strands or 19 meshes to the inch. It therefore became 

 evident to these observers that the large-meshed mosquito bars ordi- 

 narily used in Veracruz would not offer proper protection and that 

 window screening must also be of a finer wire than is sometimes 

 employed. 



Goeldi refers to this screen question, both in regard to the yellow- 

 fever mosquito and to the common rain-water-barrel mosquito, in 

 connection with some very interesting observations about the range 

 of variation in the size of the individuals of the same species, a fact 

 which is frequently noticed with other insects but to which special 

 attention has not been called elsewhere with mosquitoes. 



Frequently I have observed, both in Stegomyia fasciata and in Culex fatigans, 

 alongside of individuals of normal stature individuals very much smaller — veritable 

 dwarfs. This observation may be made on specimens captured in freedom as well 

 as on those in captivity, in this last case the phenomenon repeating itself rather 

 frequently. There are sometimes born individuals, both males and females, so 

 small that they easily pass through the mesh of wire gauze much closer than the tnesh 

 of "Grassi's gauze" which to-day is produced on a large scale in Italy with a view to 

 the prophylaxis against the Anopheles 9,nd malaria (Grassi, himself, recommends a 

 gauze that shall not have less than nine meshes in IJ centimeters of distance, which 

 corresponds to little linear squares 1.7 mm. to the side). The government of the 

 State of Para imported for my experiments from Italy under this name a gauze which 

 had but six threads to 1^ centimeters of linear extension, corresponding to squares of 

 2| mm. along one side. I refer particularly to this last brand, which I consider suffi- 

 cient as a rule for application to hospitals to impede the invasion of mosquitoes from 

 the outside, but which I found, nevertheless, insufficient for the walls of my cages 

 destined for experiments on mosquitoes like Stegomyia fasciata and Culex fatigans in 

 captivity. 



In general, the phenomena of macrosomia and microsomia in plants and animals 

 are related directly with greater or less abundant nutrition, and I do not believe that 

 the quoted dwarf race of Stegomyia and Culex is to be explained in any other way 

 than by a sparse alimentation and a delayed development in the larval stage. On 

 this point I have at hand experiments in proof: Larvae reared in clear water — that 

 is to say, relatively poor in assimilable substances— gave me imagos of small stature. 

 Furthermore, it is yet to be shown that I am deceived in my opinion that the 



