IMPORTANCE OF INSECTS. 341 



much used in the preparation of sealing-wax and varnishes. In all 

 these cases it is only the female insects that yield the colouring 

 matter. 



Of the secretions peculiar to insects silk may well be regarded 

 as the most valuable, since it has become nearly as essential to our 

 own purposes as it is to the economy of the producers of it. The 

 vessels which secrete it consist of two tubes ; these unite at the ex- 

 tremity, and open into a small perforated filiform organ, placed 

 between the palpi on the under lip j this is termed the spinnaret, 

 and the size of its aperture determines the thickness of the thread. 

 The fluid, before it comes in contact with the air, is viscous and trans- 

 parent in the young larva, but thick and opaque in the mature ones. 

 It is found, by chemical analysis, to be chiefly composed of Bombic acid, 

 a gumrny matter, a portion of a substance resembling wax, and a little 

 colouring-matter. It may be placed in boiling water without under- 

 going any change ; the strongest acids are required to dissolve it ; and 

 it has never yet been imitated artificially. More than 500,000 of human 

 beings derive their sole support from the culture and manufacture of 

 silk ; and upwards of 200,000^. may be said to be annually circulated 

 by the silk- worm. Then we have large sums of money changing hands 

 from the labours of the useful little bee j tons weight of honey and wax 

 are yearly consumed ; England pays more than 50,0001. for foreign 

 honey and wax, besides her own produce. A great variety of scents, 

 which from their agreeable odour are much used in perfumery, are 

 manufactures from insects. The Spanish Fly is absolutely indispen- 

 sable in the treatment of certain forms of disease; and that invaluable 

 agent, chloroform, was first produced from Formic acid ; this acid 

 was discovered in the Formic-ant, from which it derives its name. 

 Then there are the Gall-nuts, produced by a small fly, so useful in 

 dyeing, ink-making, &c. 



" Much more extensive and important than any of the foregoing, 

 but, as less palpable, even more disregarded, are the general uses of 

 insect existence. Disease, engendered of corruption in substances 

 animal and vegetable, would defy all the precautions of man, unless 

 these were aided by scavenger-insects, those myriads of flies and car- 

 rion beetles, whose perpetual labours, even in our tempered climate 

 but infinitely more so in warmer regions are essentially important to 

 cleanliness and health. 



A use of this nature, and one performed perhaps to an extent we 

 little think of, is the purification of standing waters by the innumer- 

 able insects which usually inhabit them. We have witnessed ample 



