INOSITE. 



[ 429 ] 



INSECTS. 



I'NOSITE. — Or muscle-sugar, is a colour- 

 less crystalline substance, occurring- in tlie 

 muscular tissue of the heart, in tlie liver, 

 the lun^'-, kidneys, and brain ; and in plants, 

 in grape-juice, wine, and haricot-beans. 

 The crystals form rhombohedra, and are 

 soluble in water, but not in alcohol or ether. 

 INSECTS. — A class of invertebrate arti- 

 culate animals. 



Char. Head distinct, furnished with two 

 antennjE ; three pairs of legs ; respiratory 

 organs consisting of trachetB ; cutaneous 

 skeleton composed of chitine. 



Insects are distinguished from the Arach- 

 nida by the head being distinct from the 

 thorax, and the presence of antennae ; and 

 from the Crustacea by the respiratory organs 

 consisting of trachese. 



The body consists usually of thirteen seg- 

 ments or somites — one for the head, three 

 for the thorax, and nine for the abdomen, 

 the legs being attached to the second, third, 

 and fourth segments. 



The cutaneous skeleton or integument of 

 insects probably consists of three layers — an 

 outer epidermic, an intermediate pigment, 

 and an internal fibrous layer ; but consisting 

 as it does of chitine, it is very imperfectly 

 resolvable into its elementary components. 

 The epidermic layer often presents a distinct 

 cellular aspect (PI. 35. fig. 30 a), sometimes 

 the cells appearing as if flattened and over- 

 lapping (PL 35. fig. 30 c) and their free 

 margins fringed with minute hairs (fig. 30 6). 

 In other instances the epidermis appears 

 uniform and structureless. In its deeper 

 portion the epidermis is often strongly co- 

 loured by a resinous pigment, which is 

 removable by prolonged maceration in so- 

 lution of potash or in oil of turpentine. Be- 

 neath these imperfectly separable layers, is 

 another representing probably the cutis, and 

 consisting mostly of numerous secondary 

 layers made up of fibres, running ^^arallel or 

 interlacing, and leaving fissures and tubes 

 between them, sometimes presenting a stel- 

 late appeai'ance : these fibres may be sepa- 

 rated by maceration in caustic potash. 



The outer surface of the integument of 

 insects is usually furnished with processes 

 of various kinds, as tubercles, hairs, spines, 

 scales, &c. (.see Haies and Scales). The 

 inner surface also gives oft" processes, which 

 form a kind of internal skeleton, serving for 

 the attachment of muscles, &c. In sketch- 

 ing the various parts of which the skeleton 

 is composed, it must be understood that 

 they are not always equally distinct, and 



that upon their degree of development, form, 

 and general structure the characters of the 

 families, genera, and species are mainly 

 founded. 



The head (fig. 361 a) consists of an upper 

 anterior portion (PI. 33. fig. 1 d), the clypeus, 

 and an upper posterior portion (fig. 1 b), the 

 epicranium or vertex, which are sometimes 

 separated by a suture ; a posterior portion 

 or occiput (fig. 2 + ), by which the head is 

 articulated with the prothorax ; and a pos- 

 terior inferior portion (fig. 3 n), the gula. 



The eyes are situated upon the upper, an- 

 terior, or lateral parts of the head, and are 

 of two kinds, simple and compound. The 

 simple, called ocelli or stemmata (PI. 35. fig. 

 2 «; PI. 33. fig. 24 h), are usually from one 

 to three in number, but sometimes are 

 numerous in larvae ; they appear like shining 

 smooth specks (PI. 33. fig. 4), and usually 

 form a triangle behind or between the com- 

 pound eyes. They consist of an arched, 

 round, or elliptical cornea, behind which is 

 a conical or cylindrical lens, which is sur- 

 rounded by a layer of pigment of various 

 colours, resembling a choroid membrane, 

 and is in connexion with a filament of the 

 optic nerve. 



The two compound eyes (fig. 361 h) are 

 large, usually round or kidney - shaped 

 (PI. 33. figs. 1 c, 3 c), situated upon the 

 upper and outer part of the head, and are 

 sometimes so large (as in the Diptera, Li- 

 bellula, &c.) as almost or quite to touch 

 each other in front. They may be regarded 

 as composed of numerous simple eyes closely 

 aggregated ; their cornese vary in thickness, 

 are but slightly arched, quadrangular or 

 hexagonal in form, and in immediate con- 

 tact laterally. Hence the compound cor- 

 nea, when viewed from before or behind, 

 presents the appearance of a membrane with 

 numerous beautifuUv regular six- or four- 

 sided facets (PI. 33. figs. 5 a, b). The facets 

 are very variable in number ; but often many 

 thousands are present. They are occasion- 

 ally broader in fiont than behind, and are 

 sometimes doubly conve.x (as in the Lepi- 

 doptera), at others concavo-convex (in Li- 

 belhda, PI. 33. fig. 6 c); but usually the sur- 

 faces are parallel. The cornea possesses a 

 laminated structure. - 



Behind each cornea is a transparent cone 

 (PI. 33. fig. 6 *), representing a crystalline 

 lens, the apex of which is imbedded in a 

 transparent rod or pyramid lauiiuated in 

 structure, corresponding to a vitreous hu- 

 mour ; and this is probably continuous with 



