Serological reactions and fermentation tests have been employed in the present investigation as a means of differentiating various strains of herpetomonads from one another as well as from leishmanias. The twelve strains of herpetomonads isolated from insects and plants all proved to be serologically unrelated to any of the leishmanias, and were distinguishable from them by the manner in which they affected various carbohydrates.

Three of the strains of herpetomonads tested had been isolated from milkweeds (Asclepias syriaca and A. niveaAsclepias syriaca and A. nivea) and four from bugs which feed on the latices of these plants (Asclepias syriaca and A. nivea) and four from bugs which feed on the latices of these plants (Oncopeltus fasciatus, Oncopeltus sp.? from Peru, and from Peru, and Lygæus kalmii). When tested for their serological and carbohydrate-fermenting properties, however, the seven strains proved to be of two kinds only, one represented by the strain first isolated from ). When tested for their serological and carbohydrate-fermenting properties, however, the seven strains proved to be of two kinds only, one represented by the strain first isolated from Oncopeltus fasciatus and hence named and hence named H. oncofelti, the other by , the other by H. lygæorum, so named because it was first isolated from Lygæus kalmii. Serologically there was a certain degree of group reaction among the flagellates of these two types, but in their action upon carbohydrates they were entirely different, . Serologically there was a certain degree of group reaction among the flagellates of these two types, but in their action upon carbohydrates they were entirely different, H. oncopelti splitting thirteen carbohydrates, H. lygæorum only three.

Three strains of herpetomonads isolated from flies proved to be distinct both in serological properties and in their action upon carbohydrates. One, derived from the house fly, and called H. muscidarum, was able to ferment most of the carbohydrates tested, including lactose which was not affected by any of the other strains. The other two, isolated from bluebottle flies, behaved much the same as the leishmanias with regard to carbohydrate fermentation, attacking five of the same sugars. One of them fermented galactose in addition, the other both galactose and inulin.

Two strains from mosquitoes (Anopheles and Culex) behaved identically in serological reactions and also in fermentation tests. They are regarded as one species and have been named H. culicidarum. This organism ferments thirteen sugars, including amygdalin which no other organism of the series attacks.

One of the most striking phenomena observed was the entire lack of fermentative faculty on the part of Herpetomonas ctenocephali and Trypanosoma rotatorium. Neither of these organisms was affected by any of the immune sera prepared with other flagellates.

The serological specificity of Leishmania tropica, L. brasiliensis, and L. donovani, and the close relation between L. donovani and L. infantum were confirmed in the present study. These organisms could not, however, be differentiated by fermentation tests.

The data presented suggest that the biological characteristics of flagellates of the Herpetomonas group may be utilized with advantage for identification of a species which occurs in different environments and for separation of different species when they are found in the same environment. If the leishmania parasites pass the flagellated or herpetomonad stage of their life history in some invertebrate host, it may be possible by tests of the sort described to distinguish them from the non-pathogenic herpetomonads which are so widely distributed among insects and plants.

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