ON THE UTILITY OF THE NATURAL SCIENCES. 213 



The department of natural history which relates to insects is less con- 

 spicuous than botany ; but it is not less important in its bearings on 

 the interests of society. Naturalists have already characterised about 

 80,000 species of insects, and their continued investigations are still 

 adding to the number. The crustaceans (crabs), arachnidans (spiders), 

 annelidans (worms), &c, &c, in all about 130,000 species, were once in- 

 cluded in the department of entomology. 



Insects are the scavengers of nature : in pursuit of food,, they remove 

 from the surface vast quantities of decaying and putrescent matters, 

 which, if left undisturbed, might so contaminate the air as to render it 

 poisonous to the inhabitants of the earth. Everywhere they constitute 

 a large part of the food of birds and fishes ; and in some countries 

 certain species are eaten by man. 



Various insects are known to be injurious to the grains and grasses 

 of our fields ; to the fruit trees of our orchards, and to plants in 

 conservatories. It is known that some species prey upon others ; and 

 that those which feed on the various aphides, or plant-lice, may be em- 

 ployed to relieve us from the ravages of such destroyers. 



A knowledge of the habits, mode of life, and of the food of the 

 various kinds, leads to means of escaping the injuries which many of 

 them inflict, and of fostering those which are useful to man. Doubtless 

 there are some persons who are not aware that most insects pass through 

 four stages of existence : 1, the egg ; 2, the caterpillar ; 3, the chry- 

 salis, and 4, the butterfly, or imago. In order to guard against the 

 ravages of insects, it is necessary to know the stage of existence during 

 which they are most injurious, and" also to be able to recognise 

 the different shapes under which they appear. Without a thorough 

 knowledge of the phenomena of insect metamorphosis, it is vain to 

 attempt to control their increase. The information of Linnaeus on this 

 point enabled him to teach his countrymen to destroy an insect, the 

 Cantliaris navalis, which had cost the Swedish Government many 

 thousand pounds a year, by its ravages on the timber of one dock-yard 

 only. After its metamorphosis, and the season when the fly laid its 

 eggs were known, all its ravages were stopped by immersing the timber 

 in water during that period. 



In 1817, the late Mr. Thomas Say described the "Hessian Fly" Ceci- 

 domyia destructor, which commits great ravages on growing wheat ; and 

 at the same time pointed out the Ceraphron destructor, which probably 

 restrains the increase of the first. It deposits its eggs in the bodies of 

 the larva? of the Hessian Fly, and the young when they escape from 

 the egg, feed upon the larvse till it dies. 



During the year 1849, Miss Morris, of Germantown, discovered that 

 the Tomicus liminaris of Mr. Say, is a destroyer of the peach tree. It 

 bores through the bark and feeds upon the living portion. She has 

 stated also that the Boris tripunctatus, of Mr. Say is one of the des- 

 troyers of the potato, in the stem of which its larva? undergo their meta- 

 morphosis. And Mr. S, S. Haldeman ascertained that the Hylesinut 



