INOMERIA. 



[ 355 ] 



INSECTS. 



the air-cells being injected from tlie trachea ; 

 the skin of the frog, and especially of the 

 triton, &c. 



BiBL. Berres, Anat. d. mikrosk. Gebild. 

 d. Menschl. Korp. ; Rusconi, Ann. des Sc. 

 nat. 2 ser. xvii. ; Tulk and Henfrey, Anat. 

 Manipul.; Doyere, Comptes Rend. 1841 j 

 Harting, Het Mikroskoop, ^-c, abstracts in 

 Ed. Monthly Journ. 1852; Robin, Du Mi- 

 croscope, ^-c; Quekett, on Injections ; Goad- 

 by, in Wythes' 'The Microscopist ,' 



INOMERIA, Kiitz.— A genus of Oscilla- 

 toriaceae (Confervoid Algae) with calcareously 

 hardened, incrustingfronds, growing on stones 

 in fresh water. The fronds are composed of 

 vertical, parallel, whip-shaped filaments, with 

 the sheaths obscm*e, connected together, and 

 decomposed into very slender fibrils above. 

 Kiitzing supposes his I. Roemeriana to be 

 synonymous with Hassall's Lithone?na crus- 

 taceum. 



BiBL. Kiitz. Spec. Alg. -p. 343, Icon. Phys. 

 ii. pi, 83 ; Hassall, Brit. Freshw. Algce, p. 266. 

 pi. 65. fig. 3. 



INSECTS. — A class of invertebrate arti- 

 culate animals. 



Char. Head distinct, furnished with two 

 antennae ; respu'atory organs consisting of 

 tracheae ; 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 tracheae. 



Most insects have three pairs of legs, and 

 the body consists usually of thirteen seg- 

 ments; one for the head, three for the 

 thorax, and nine for the abdomen, the legs 

 being attached to the second, third and 

 fom'th segments. But in some (MjTiapoda) 

 the segments of the body and the legs are 

 very numerous. 



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 re- 

 solvable into its elementary components. 

 The epidermic layer often presents a distinct 

 cellular aspect (PI. 28. fig. 30 a); sometimes 

 the cells appearing as if flattened and over- 

 lapping (PI. 28. fig. 30 c), and their free 

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

 In other instances, the epidermis appears 

 uniform and structureless. In its deeper 

 portion, the epidermis is often strongly im- 

 bued with a resinous pigment, which is 



removable by prolonged maceration in solu- 

 tion of potash or in oil of turpentine. 

 Beneath these imperfectly separable layers, 

 is another representing probably the cutis, 

 and consisting mostly of numerous second- 

 ary layers made up of fibres, running parallel 

 or interlacing, and leaving fissm'es between 

 them, sometimes presenting a stellate ap- 

 pearance : these fibres may be separated by 

 maceration in caustic potash. 



The outer surface of the integument of 

 insects is usually fiu-nished with processes of 

 vai'ious kinds, as tubercles, hairs, spines, 

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

 inner surface also gives off 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. 365 a) consists of an upper 

 anterior portion (PL 26. 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. 24-), 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. 28. fig. 

 2 a ; PI. 26. fig. 24 b), are usually from one to 

 three innumber, but sometimes very numerous 

 in larvae ; they appear like shining smooth 

 specks (PI. 26. fig. 4), and usually form a 

 triangle behind or between the compound 

 eyes. They consist of an arched, round, or 

 elliptical cornea, behind which is a conical 

 or cylindi'ical lens. The optic nerve forms a 

 cup-shaped expansion for the reception of 

 the lens, which is siurounded b}^ a layer of 

 pigment of various colom's, resembling a 

 choroid membrane. 



The compound eyes (fig. 365 b) are two, 

 large, usually round or kidney - shaped 

 (PI. 26. figs. 1 c, 3 c), situated upon the 

 upper and outer part of the head, and are 

 sometimes so large (as in the Diptera, 

 LibellulcB, &c.) as almost or quite to 

 touch each other in front. They may be 

 regarded as composed of numerous simple 

 eyes closely aggregated ; their corneae vary 

 in thickness, are but slightly arched, qua- 



2a2 



