214 ON THE UTILITY OF THE NATURAL SCIENCES. 



aculeatus, of Say, feeds upon the inner bark of the white ash, Fraxinus 

 acuminata. 



There was a time when a loathsome disease, which Van Helmont 

 tells us he contracted by shaking the gloved hand of a lady friend, was 

 treated by bleeding, purging, and sweating. Since his days, investiga- 

 tions have demonstrated that this malady depends upon the presence 

 of an acarus — the itch-insect — and is to be cured only by destroying the 

 animal. There are several affections attributed to the acarus tribe ; the 

 ulcers caused by the chigoe or jigger (Pulex penetrans), an insect which 

 is prone to burrow in human flesh, are familiarly spoken of by all who 

 "have visited the tropics. The harvest bug (Leptus autumnalis) buries 

 itself in the legs of labourers in the harvest field, producing intoler- 

 able itching and pain. In cases of plica polonica, an affection of the 

 scalp, we are told that millions of lice appear on the third day of the 

 disease. Numerous affections have been traced to the larvse of flies 

 deposited in the tissues of mammals ; a disease familiarly known as 

 the botts, which occurs in horses, sheep, and in man, has this origin. 

 Worms of several kinds are known to thrive in the human body, and to 

 produce disease. Indeed, the affections known to be caused by insects, 

 spiders, worms, &c, are so numerous, that Raspail has ventured to con- 

 struct a system of medicine based upon the animate origin of diseases. 

 In his work may be found a history of many affections, caused by the 

 presence of minute parasitic insects. 



Though many evils arise from insects, they are also the sources of 

 much that is good. The product of the silk-worm, the wax and honey 

 produced by the labour bees, are familiar to all. The gall-nut which 

 forms the basis of one of the most valuable articles known, I mean ink, 

 is due to the labors of an insect on a variety of oak. Gum ammoniac, 

 the varieties of lac (shell-lac, lac-lake, and stick-lac), have a similar ori- 

 gin. To various species of coccus we are indebted for several important 

 dyes, among the most conspicuous of which are the cochineal and 

 kermes. The varieties of cantharides afford blistering materials ; and 

 a tea made of bees has been recently found useful in a distressing disease. 



Within a short time, the unexpected presence of microscopic' insects 

 in certain regions has been supposed to show the course of the winds ; 

 when their habits and geographical distribution are fully ascertained, 

 their appearance in any unusual locality may be relied upon as an indi- 

 cation of the direction of atmospheric currents. 



A class of animals which exercises an influence of such extent, for 

 good as well as for evil, over the condition of man, is surely worthy of 

 his attentive study. 



Conchologists have described about 15,000 species of mollusks. 

 They afford food to man and other animals. The strata of the earth 

 records their antedeluvian existence, hence a knowledge of conchology 

 enables the geologist to recognise fossil shells, the presence of which 

 serves to characterise certain formations. 



Herpetologists have made us acquainted with about 2,000 species of 



