ACADEMY OF NATURAL SCIENCES. 7 



its bearings on the interests of society. Naturalists have already 

 characterized 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 included 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 putres- 

 cent 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 employed 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 chrysalis, 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 inju- 

 rious, and also to be able to recognize the different shapes under 

 which they appear. Without a thorough knowledge of the phe- 

 nomena 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 

 Cantharis 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," Oecidomyia destructor, which commits great ravages on 

 growing wheat ; and at the same time pointed out the Ceraphron 



* Smith. Introduction to Botany. 



