Vi INTRODUCTION. 
the above-quoted Italian authors from various parts of South America, and to charac- 
terize some new types from Mexico and Central America. ‘The materials which I had 
at my disposal were derived from various sources. During a stay of nearly five years 
in various parts of Guatemala, I tried to make myself acquainted with the Acarid- 
fauna of that country, by studying, as far as the unsettled life of a medical practitioner 
would allow, the living forms. I had then with me a Hartnack microscope of but 
moderate powers (objectives 4 and 7), the lenses of which became damaged by the long 
influence of the excessive humidity of the climate of the Costa Grande. Moreover, I 
laboured under an almost total want of modern literature on Acarids, having only some 
of the works of the older writers with me. These unfavourable circumstances will 
account to some extent for the differences the reader may find in the execution of the 
drawings and the descriptions of some species, several of which are too delicate for 
preservation in alcohol, the only method then within my reach. Many of my drawings, 
especially of Gamaside, have been entirely omitted from the present memoir, as being 
too incomplete to allow a comparison with the European forms; and even amongst 
those which I have admitted there are some which I should have liked to revise again 
from the specimens themselves. 
Fortunately, this lack of preserved materials has in numerous cases been made up by 
dried specimens obtained by other naturalists who have travelled in Central America. 
The fact that other collectors have frequently fallen in with the same species as myself 
shows that my researches, though far from complete, were sufficient at least to give an 
idea of the composition of the Acarid fauna of a tropical country, and to enable me to 
come to some general conclusions as to the geographical distribution cf the various 
groups. 
The most striking fact elicited by the study of the Acarid fauna of Central America 
is undoubtedly the great similarity between the types with which we are familiar in 
the temperate regions north of the European Alps and those occurring in the gloomy 
shadow of the tropical forest. Under the bark and in the fissures of putrefying tree- 
trunks in the tropics we meet with some minute Gamaside which only by a close 
microscopical examination can be distinguished from European species living under 
similar conditions. Upon various beetles of the families Copride and Passalide are 
found, with others, such well-known forms as Gamasus (Holostaspis) marginatus and 
G. coleoptratorum, species already described by authors of the past century. Among 
the fallen leaves and in the decaying fruits of Theobroma, Lucuma, and other tropical 
