INS 



INS 



size, from the time they have been depo- 

 sited by ihe parent till they are hatched ; 

 those of the tenthredo, however, and of 

 same others, are observed to increase in. 

 bulk. At first there is nothing to be per- 

 ceived in the eg-gs of insects but a watery 

 fluid ; after some little time, an obscure 

 point is observable in the centre, which, 

 according to Swammerdam, is not the in- 

 sect itself, but only its head, which first 

 acquires consistence and colour : and the 

 same author alleges, that insects do not 

 increase in bulk in the egg 1 , but that their 

 parts only acquire shape and consistence. 

 Under the shell of the egg, there is a thin 

 and very delicate pellicle, in which the 

 insect is enveloped, which may be com- 

 pared to the chorion and amnios, which 

 surround the foetus in quadrupeds. The 

 little insect remains in the egg till the 

 fluids are dissipated, and till its limbs have 

 acquired strength to break the egg, and 

 make its escape ; the different species of 

 insects remain inclosed in the egg for very 

 different periods ; some continue inclosed 

 only a few days, others remain for seve- 

 ral months. The eggs of many insects re- 

 main without being 1 hatched during the 

 whole winter, and the young insects do 

 not come forth from them till the season 

 at which the leaves of the vegetables, on 

 which ihey feed, begin to expand. When 

 the insects are ready to break their pri- 

 son, they commonly attempt to pierce the 

 shell with their teeth, and form a circular 

 hole, through which they put forth first 

 one leg, and then another, till they ex- 

 tricate themselves entirely. 



Insects are by far the most numerous 

 class of animals : about eleven thousand 

 species have been described by Gmelin, 

 in the last edition of the " System of Na- 

 ture :" a great many more have been de- 

 scribed by other naturalists since the pub- 

 lication of that work ; and a very consi- 

 derable number are to be met with in the 

 cabinets of the curious, which have not as 

 yet been described by any author. In 

 those parts of the world which we are 

 best acquainted with, we may easily sup- 

 pose that many species of insects exist, 

 which have hitherto escaped notice. The 

 minuteness of some insects makes them 

 easily overlooked ; the agility of others 

 renders the catching of them difficult ; 

 the retired situations which many of them 

 haunt favour their concealment. In the 

 unexplored parts of America, Africa, and 

 Asia, many thousand species must exist 

 utterly unknown to naturalists : all these 

 circumstances render it very probable, 

 that not one half of the insects which ex- 

 ist in the world have hitherto been de- 



scribed. In order to exhibit the propor- 

 tion they bear to plants, it may be proper 

 to remark, that, as inhabitants of England, 

 eight thousand species have been already 

 described, and only three thousand plants. 



Insects afford nourishment to a great 

 number of the superior animals: many of 

 the fishes, reptiles, and birds, draw the 

 principal part of their sustenance from 

 that source. The immense swarms of dif- 

 ferent species of crab, which abound in 

 every sea, directly or indirectly form the 

 principal part of the food of the cod, had- 

 dock, herring, and a great variety of 

 fishes. The snake, lizard, frog, and many 

 other reptiles, feed both on land and aqua- 

 tic insects. Gallinaceous fowls, and many 

 of the small birds, &c. feed on insects. 

 Swallows, indeed, feed entirely on wing- 

 ed insects. They afford food, likewise, 

 to many of the mammalia, viz. to many 

 species of the bat, to the ant-eater, &c. 

 and even to man himself. Many species 

 of crab, viz. lobster, common crab, shrimp, 

 prawn, land-crab, &c. are reckoned deli- 

 cacies. The larvae of some coleopterous 

 insects and locusts form part of the food 

 of man. Insects, likewise, by consuming 

 decayed animal and vegetable matter, 

 which, if left to undergo the putrefactive 

 process on the surface of the ground, 

 might taint the atmosphere with pestilen- 

 tial vapours, preserve the air pure for the 

 respiration of man and other animals. 



INSERTION, in anatomy, the close 

 conjunction of the vessels, tendons, fibres, 

 and membranes of the body, with some 

 other parts. 



INSOLATION, in chemistry, a term 

 sometimes made use of to denote that 

 exposure to the sun, which is made in or- 

 der to promote tne chemical action of one 

 substance upon another: one of the most 

 striking experiments of this kind is that 

 of the exposure of vegetables, as fresh- 

 gathered cabbage-leaves, in a glass jar of 

 water, to the rays of the sun, by the ac- 

 tion of which a large quantity of pure ox- 

 ygen gas is obtained. 



INSOLUBILITY, in chemistry. The 

 insolubility of a substance in a fluid, which 

 is the medium of chemical action, has an 

 influence on that action somewhat simi- 

 lar to that of cohesion, and is nothing but 

 a modification of it, in relation to the fluid 

 in which it is exerted. If substances in 

 their liquid state be made to act on each 

 other, their action will meet with little 

 foreign resistance, and will be, in a great 

 measure, proportioned to their affinity 

 and quantity ; but if one of them be solid, 

 and be farther insoluble in the fluid, 

 which is the medium of action, the insolu- 



