296 Prof. H. Karsten on Rhynchoprion penetrans. 



place of rats, mice, and similar animals, which seek shelter there 

 from the rains, so long as the roof offers it to them ; and these 

 animals then serve for the preservation and increase of any pro- 

 geny of the Nigua that may have been left behind by the travellers; 

 hence it is that such places often particularly abound in Niguas, 

 which attack new-comers in great numbers, as I know from 

 personal experience. The same thing occurs, as indeed Rengger 

 relates, in the deserted houses of planters, in the rooms of which 

 the Niguas developed from the eggs left behind at first collect 

 in extraordinary numbers, but afterwards diminish again, and 

 finally disappear entirely, no doubt because these places are not 

 so convenient for the access and long residence even of the 

 smaller mammalia, so that the Fleas cannot increase, and con- 

 sequently at last die out. 



Rengger's statement that animals living in a wild state are 

 not attacked by the Nigua is consequently not correct, and has 

 already been refuted by several travellers. In Schmarda's rich 

 collection I found a Field-mouse from Cuenca, the tail and foot 

 of which harboured a great quantity of Niguas* (PI. VIII. 

 fig. 1). 



Swartz, Rengger, Humboldt, and other travellers report that 

 strangers are particularly attacked by the Sand-flea on their 

 arrival in America. It is true that new comers, to whom the 

 inconspicuous insect is unknown, have usually to suffer more 

 from it than others. During my residence in Venezuela I was 

 myself much plagued by it at first, whilst in the latter years of 

 my tropical travels in New Granada I was scarcely ever attacked 

 by a Nigua, although the Creoles at the same places complained 

 much of them. This, however, is favourable neither to Hum- 

 boldt's hypothesis of the delicate discrimination of the Nigua 

 between European and Creole blood, nor to Rengger's notion 

 that the human body loses some property which attracts the 

 Nigua. 



The fact that in newly arrived foreigners the Niguas collect 

 in greater numbers, acquire a larger growth within the skin, 

 and consequently cause more violent symptoms, admits of this 

 simple explanation : — The strangers do not notice the slight 

 tickling produced by the penetration of the animal into the skin, 

 as they do not understand the meaning of this slight pain ; and 

 the animal, after it has taken its place, causes no further incon- 

 venience, if the slightly inflamed spot of skin which it inhabits 



* Of these Niguas I removed two from the skin, in order to examine 

 their specific characters. I could find no perceptible difference from the 

 other individuals examined ; but I noticed the very remarkable fact, inex- 

 plicable by me, that in both of them all the legs were wanting up to the^ 

 trochanters. 



