478 



COMPARATIVE ANATOMY 



CHAP. 



between the various organs and even the component parts of organs, 

 breaking up into the finest capillaries. The essential structure of the 

 tracheae is everywhere the same, whether we examine a principal trunk 

 or a fine terminal branch. The tracheal tube (Fig. 337) is lined inter- 

 nally with a chitinous intima, which is a continuation of the chitinous 

 cuticle of the body, and, like the latter, is 

 shed during ecdysis. The intima is crenu- 

 lated spirally. This chitinous spiral chiefly 

 serves for holding the tracheae open. The 

 intima is clothed with an outer layer of cells, 

 the boundaries of which are often difficult 

 to distinguish. This layer represents the 

 epithelium which secretes the intima, and 

 is a continuation of the outer hypodermis. 

 The outer apertures of the tracheal system, 

 the so-called stigmata, are wonderfully 

 varied in their arrangement. They some- 

 times lie openly on the surface, sometimes 

 are partly concealed by cuticular folds, or 

 again are covered by the elytra {Coleoptera). 

 Set£e often project over the aperture from 

 its edges, and these may be elegantly 

 branched or feathered, so as to represent a 

 kind of fish-trap apparatus, which prevents 

 the entrance of foreign bodies, dust, etc., 

 with the air. The outer aperture is often 

 the chitinous intima (cc), in which long and slit-like ; in Other cases, on the con- 

 (T'ira" to° *'"' '^"^^ *i"<'kening tr&xj, it is large and has a sieve -like mem- 

 brane stretched over it. Beneath each 

 stigma, at the beginning of each principal tracheal trunk, there is a 

 closing apparatus which we cannot here more minutely describe ; by 

 means of this apparatus, into which nerves enter, the trachea can be 

 completely closed towards the exterior. The air in the trachese is 

 chiefly renewed by the respiratory movements of the abdomen (in the 

 Hexapoda) ; these movements are caused by the contractions of the 

 muscular fibres which run dorso-ventrally. By the contraction of the 

 abdomen (exspiration) the tracheae are compressed and the air driven 

 out through the stigmata, or, if the stigmata are closed, forced into 

 the tracheal system of the thorax, head, and extremities. By the 

 expansion of the abdomen (inspiration) new air from without again 

 enters the tracheae, the elastic spiral of the tracheal tube playing an 

 important part in its expansion. 



We are justified in assuming, that the tracheal system of the 

 Antennata originally consisted of as many pairs of isolated tracheal 

 bundles and stigmata as there were body segments (excluding the 

 anal segment). Reduction, however, has everywhere taken jjlace, first 

 of all at the anterier and posterior ends of the body, so that in some 



Fio. 337.— Structure or a tra- 

 chea. Diagrammatic ; portions of 

 the epitlielium and chitinous intima 

 removed in order to show tlie 

 structure, hy, Tracheal epitlielium 

 = tracheal hypodermis = matri.x of 



