260 CLASS VIII. 



which are usually filled with air by external openings (stigmata}. 

 These canals have three coats: an external, loose, transparent 

 membrane, in which fibres and scattered points (cell-nuclei) may 

 be distinguished ; a middle, composed of a flat, horny, sometimes 

 brown or yellow elastic thread rolled spirally : and an inner coat 

 which is composed of Mtine, a continuation of the external skin, 

 and is thrown off at every moult 1 . Through the elasticity of the 

 spiral thread the air-canals are duly kept open : its turns lie close 

 to each other, and so the appearance of rings is produced, as in the 

 wind-pipe of mammals (this the representation of the trachese of 

 Pediculus in SwAMMERDAM, Bill, natur. Tab. I. fig. VII. resem- 

 bles too closely) ; but the similarity is only in appearance ; there 

 are no absolute rings, but only the turns of a single uninterrupted 

 thread. Each branch, arising from a stem, has a new thread, 

 whether the branch proceeds laterally from the stem, or two branches 

 arise at the end of the stem ; this thread is finer than that of the 

 stem, and in the terminal branches is only visible when very 

 highly magnified. From being full of air, the canals, when 

 Insects are dissected under water, have a silvery splendour, and 

 present on account of the extreme fineness of their branching a 

 very beautiful appearance to the observer 2 . Usually the air-canals 

 divide, like arteries, into continually finer branches. In some 

 Insects however there spring from a large stem on every side 

 throughout a greater or less extent extremely fine and numerous 

 branches (as ex. gr. according to LEON DUFOUR, in Prionus, from 

 the double stem which lies between the last stigma of the thorax 

 and the first of the abdomen). In Nepa and Ranatra saccules are 

 seen in the cavity of the thorax, between which similar fin* 

 branches (retia mirabilia) of the air-canals lie, and which are sur- 

 rounded by a muscular coat 3 . Care must be taken to distinguish 

 these saccules from the sacculated dilatations of the air-canals them- 

 selves, which are met with in flying Insects in the last period q 



1 It has not been made out, as far as T know, whether the innermost membrane o 

 the air-tubes is present in those insects also which have no stigmata, but gill-plates, 

 the larvae of Ephemera, for instance. 



2 M. MALPIGHI, who first made use of the names of trachece and stigmata, says 

 " Tanta est fructificatio horum vasorum, tain mirce implicationes ut nilpulchrius conspic 

 possit." De Bombyce, p. 12. Opera om. Tom. II. Londini, 1687, fol. 



3 LEON DUFOUR, Rech. svr let IL'niiptires, p. 253. PI. xvnt. 



